More Important Topics of Cylinder

The aurora is a glow observed in the night sky usually in the polar zone. It is also known as "northern lights" or "aurora borealis," Latin for "northern dawn" since (in Europe especially) it often appears as a reddish glow on the northern horizon, as if the sun were rising from an unusual direction.

Aurora borealis most often occurs from September to October and March to April. Its southern counterpart

"aurora australis," has similar properties, so scientists prefer "polar aurora" (or "aurora polaris"). Dr Tony Phillips ( NASA Spaceweather.com ) collects and publishes images of Auroral displays from both hemispheres.

Aurora is now known to be caused by electrons of typical energy of 1-15 keV, i.e. the energy obtained by the electrons passing through a voltage difference of 1000-15,000 volts.

The light is produced when they collide with atoms of the upper atmosphere, typically at altitudes of 80-150 km. It tends to be dominated by emissions of atomic oxygen--the greenish line at 5577 A and (especially with electrons of lower energy and higher altitude) the dark-red line at 6300 A. Both

these represent "forbidden" transitions of atomic oxygen from energy levels which (in absence of collisions) persist for a long time, accounting for the slow brightening and fading (0.5-1 sec) of auroral rays.

Many other lines can also be observed, especially those of molecular nitrogen, and these vary much faster, revealing the true dynamic nature of the aurora.

Aurora can also be observed in the ultra-violet (UV) light, a very good way of observing it from space (but not from ground--the atmosphere absorbs UV). The "Polar" spacecraft even observed it in X-rays. The image is very rough, but precipitation of high-energy electrons can be identified.

Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs," at others ("active aurora") they evolve and change constantly.

Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by the Earth's magnetic field, Indeed, satellites show auroral electrons to be guided by magnetic field lines, spiraling around them while moving earthwards.

The curtains often show folds called "striations." When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of perspective.

In 1741 Hiorter and Celsius first noticed other evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated.

Kristian Birkeland (1903) deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named "auroral electrojets." (see also Birkeland currents).

Still another evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881) established that aurora appeared mainly in the "auroral zone," a ring-shaped region of approx. radius 2500 km around the magnetic pole of the Earth, not its geographic one. It was hardly ever seen near that pole itself.

The instantaneous distribution of aurora ("auroral oval," Yasha Feldstein 1963) is slightly different, centered about 3-5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest equatorward around midnight.

The presence of a radiation belt had been theorized prior to the Space Age and the belt's presence was confirmed by the Explorer I on January 31, 1958 and Explorer III

missions, under Doctor James Van Allen. The trapped radiation was first mapped out by Explorer IV and Pioneer III.

Qualitatively, it is very useful to view this belt as consisting of two belts around Earth, the inner radiation belt and the outer radiation belt. The particles are distributed such that the inner belt consists mostly of protons while the outer belt consists mostly of electrons. Within these belts are particles capable of

penetrating about 1 g/cm2 (2) of shielding (e.g., 1 millimetre of lead). The term Van Allen Belts refers specifically to the radiation belts surrounding

Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts. The atmosphere limits the belts' particles to regions above 200-1000 km (1), while the belts do not extend past 7 Earth radii RE (1). The belts are confined to an area which extends about 65° (1) from the celestial equator.

along warm and cold fronts (frontal lift)

where air flows up the side of a mountain and cools as it

rises higher into the atmosphere (orographic lift)

by the convection caused by the warming of a surface by insolation (diurnal heating)

when warm air blows over a colder surface such as a cool body of water.

2. Clouds can be formed when two air masses below saturation point mix. Examples are: our breath on a cold day, aircraft contrails and Arctic sea smoke.

3. The air stays the same temperature but absorbs more water vapor into it until it reaches saturation point.

The water in a typical cloud can have a mass of up to several million tonnes. The volume of a cloud is correspondingly high and the net density of the relatively warm air holding the droplets is low enough that air currents below and within the cloud are capable of keeping it suspended.

Conditions inside a cloud are not static: water droplets are constantly forming and re-evaporating. A typical cloud droplet has a radius on the order of 1 x 10-5 m and a terminal velocity of about 1-2 cm/s. This gives these droplets plenty of time to re-evaporate as they fall into the warmer air beneath the cloud.

Most water droplets are formed when water vapor condenses around a condensation nucleus, a tiny particle of smoke, dust, ash or salt. In supersaturated conditions, water droplets may act as condensation nuclei.

The growth of water droplets around these nuclei in supersaturated conditions is given by the Mason equation.

Water droplets large enough to fall to the ground are produced in two ways. The most important means is through the Bergeron Process, theorized by Tor Bergeron, in which supercooled water droplets and ice crystals in a cloud interact to produce the rapid growth of ice crystals; these crystals precipitate from the cloud and melt as they fall. This process typically takes place in clouds with tops cooler than -15°C.

The second most important process is the collision and wake capture process, occurring in clouds with warmer tops, in which the collision of rising and falling water droplets produces larger and larger droplets, which are eventually heavy enough to overcome air currents in the cloud and the updraft beneath it and fall as rain.

As a droplet falls through the smaller droplets which surround it, it produces a "wake" which draws some of the smaller droplets into collisions, perpetuating the process. This method of raindrop production is the primary mechanism in low stratiform clouds and small cumulus clouds in trade winds and tropical regions and produces raindrops of several millimeters diameter.

The actual form of cloud created depends on the strength of the uplift and on air stability. In unstable conditions convection dominates, creating vertically developed clouds. Stable air produces horizontally homogeneous clouds.

Frontal uplift creates various cloud forms depending on the composition of the front (ana-type or kata-type warm or cold front). Orographic uplift also creates variable cloud forms depending on air stability, although cap cloud and wave clouds are specific to orographic clouds.

Cirrus

These form above 16,500 feet (5,000 m), in the cold region of the troposphere. They are denoted by the prefix cirro- or cirrus. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent.

cirrus

cirrus castellanus

cirrus radiatus

cirrus uncinus

cirrus fibratus

cirrus spissatus

cirrus intortus

cirrus vertebratus

cirrus floccus

cirrus duplicatus

cirrus with mammatus

cirrus kelvin-helmholtz

Isolated cirrus clouds often indicate a stable situation and do not bring precipitation.

Cirrocumulus

Abbreviation: Cc cirrocumulus undulatus

cirrocumulus castellanus

cirrocumulus floccus

cirrocumulus lenticularis

cirrocumulus lacunosus

cirrocumulus with mammatus

Cirrocumulus clouds are often associated with a front but do cause precipitation.

Cirrostratus

Abbreviation: Cs cirrostratus duplicatus

cirrostratus nebulosus

cirrostratus fibratus

cirrostratus undulatus

Cirrostratus clouds are often translucent and do not bring precipitation

Contrail

Aircraft engines emit water vapour into the atmosphere, and this vapour is then frozen into ice crystals. These are known as condensation trails (contrails).

Medium-level clouds

Altostratus

Abbreviation: As

altostratus undulatus

altostratus radiatus

altostratus lenticularis

altostratus duplicatus

altostratus translucidus

altostratus opacus

altostratus mammatus

altostratus praecipitatio

Altostratus is usually associated with a weather front and can bring rain or snow.

Altocumulus

altocumulus duplicatus

altocumulus undulatus

altocumulus stratiformis

altocumulus castellanus

altocumulus radiatus

altocumulus floccus

altocumulus lacunosus

altocumulus translucidus

altocumulus perlucidus

altocumulus opacus

altocumulus with mammatus

altocumulus virga

Altocumulus is not usually associated with a front but can still bring rain or snow.

Nimbostratus

Abbreviation: Ns

nimbostratus opacus

nimbostratus pannus

nimbostratus praecipitatio

nimbostratus virga

nimbostratus floccus

Nimbostratus tend to bring constant precipitation Low-level clouds

Stratocumlus

Abbreviation: Sc

stratocumulus opacus

stratocumulus undulatus

stratocumulus castellanus

stratocumulus floccus

stratocumulus lenticularis

stratocumulus radiatus

stratocumulus lacunosus

stratocumulus duplicatus

stratocumulus translucidus

stratocumulus perlucidus

stratocumulus mammatus

stratocumulus praecipitatio

Stratocumulus can produce rain or drizzle

Stratus

Abbreviation: St

stratus opacus

stratus nebulosus

stratus translucidus

stratus undulatus

stratus lenticularis

stratus fractus

stratus praecipitatio

Stratus can often produce drizzle

Cumulus

Abbreviation: Cu

cumulus humilis

cumulus fractus

cumulus mediocris

cumulus congestus

orographic

cumulus radiatus

cumulus praecipitatio

arcus

tuba

pileus

velum

pannus

Cumulus is sometimes called fair weather cloud but can develop into more stormy conditions

Vertically developed Clouds

Cumulusnimbus

capillatus

calvus

incus

pileus

spissatus

mammatus

arcus

shelf

scud

roll

praecipitatio

tuba

velum

pannus

Cumulonimbus is the cloud of storms and rain or showers

Other clouds

Nacreous cloud (mother of pearl). A thin cloud seen most often between sunset and sunrise and is between 12 to 18 miles (19 to 29 km) high.

Noctilucent cloud. A thin cloud seen most often between sunset and sunrise and is 32 to 35 miles (51 to 56 km) high.

Snow is commonly formed when water vapor undergoes deposition high in the atmosphere at a temperature of less than 0°C (32°F), and then falls to the ground.

A snowflake always has six lines of symmetry, which arises from the hexagonal crystal structure of ordinary ice (known as ice Ih) along its 'basal' plane.

There are, broadly, two possible explanations for the symmetry of snowflakes. Firstly, there could be

communication (information transfer) between the arms, such that growth in each arm affects the growth in each other arm. Surface tension or phonons are among the ways that such communication could occur. The other explanation, which appears to be the prevalent view, is that the arms of a snowflake grow independently in an environment that is believed to be rapidly varying in temperature, humidity and so on.

This environment is believed to be relatively spatially homogenous on the scale of a single flake, leading to the arms growing to a high level of visual similarity by responding in identical ways to identical conditions, much in the same way that unrelated trees respond to environmental changes by growing near-identical sets of tree rings.

The difference in the environment in scales larger than a snowflake leads to the observed lack of correlation between the shapes of different snowflakes.

However, the concept that no two snowflakes are alike is not necessarily true. Strictly speaking, it is extremely unlikely for any two objects in the universe to contain an identical molecular structure; but, there are, nontheless, no known scientific laws which prevent it.

In a more pragmatic sense, it more likely, albeit not much more, that a pair of snowflakes are visually identical if their environments were similar enough, either because they grew very near one another, or simply by chance.

The American Meteorological Society has reported that matching snow crystals were discovered by Nancy Knight of the National Center for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

Everyday ice and snow is hexagonal ice (ice Ih). Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases. Only a little less stable (metastable) than Ih is cubic structure ice (Ic). But cooling Ih causes a different arrangement to form in which the protons move, XI.

With both cooling and pressure more types exist, each being created depending on the phase diagram of ice. These are II, III, V, VI, VII, VIII, IX, and X. With care all these types can be recovered at ambient pressure. The types are differentiated by their crystalline structure, ordering and density.

There are also two metastable phases of ice under pressure, both fully hydrogen disordered, these are IV and XII. Ice XII was discovered in 1996. As well as crystalline forms solid water can exist in amorphous states as amorphous solid water (ASW), low density amorphous ice (LDA), high density amorphous ice (HDA), very high density amorphous ice (VHDA) and hyperquenched glassy water (HGW).

Kurt Vonnegut's novel Cat's Cradle features Ice IX as a central element of the plot, although real Ice IX does not have the properties of Vonnegut's fictional ice-nine (i.e. the ability to freeze all water on Earth with the introduction of one granule).

Rime is a type of ice formed by fog freezing on cold objects. It contains a high proportion of trapped air, making it appear white rather than transparent, and giving it a density about one quarter of that of pure ice.

Ice can also form icicles, similar to stalactites in appearance, as water drips and re-freezes.

Clathrate hydrates are forms of ice that contain gas molecules trapped within its crystal lattice. Pancake ice is a formation of ice generally created in areas with less calm conditions.

Some other substances (particularly solid forms of those usually found as fluids) are also called "ice": dry ice, for instance, is a popular term for solid carbon dioxide.

Fog is a cloud in contact with the ground. It occurs when moisture from the surface of the Earth evaporates; as this evaporated moisture moves upward, it cools and condenses into the familiar phenomenon of fog.

Fog differs from cloud only in that fog touches the surface of the Earth, while clouds do not. It can form in a number of ways, depending on how the cooling that caused the condensation occurred:

Radiation fog is formed by the cooling of land after sunset

by thermal (infrared) radiation in calm conditions with clear sky. The cool ground then produces condensation in the nearby air by heat conduction. In perfect calm the fog layer can be less than a metre deep but turbulence can promote a thicker layer. Radiation fog is common in autumn and usually does not last long past sunrise.

Advection fog occurs when moist air passes over cool ground by advection (wind) and is cooled. This form is most common at sea when tropical air encounters cooler higher-latitude waters. It is also extremely common as a warm front passes over an area with significant snowpack.

Mist is a phenomenon of a liquid in small droplets floating through air. It can occur naturally as part of natural weather or volcanic activity, and is common in cold air above hot water, in exhaled air in the cold, and in a steam room of a sauna. It can also be created artificially with aerosol canisters.

The only difference between mist and fog is visibility. This phenomenon is called fog if the visibility is one kilometer or less. Otherwise it is known as mist. Seen from a distance, mist is blueish, while haze is more brownish.

Wind is the roughly horizontal movement of air (as opposed to an air current) caused by uneven heating of the Earth's surface.

It occurs at all scales, from local breezes generated by heating of land surfaces and lasting tens of minutes to global winds resulting from solar heating of the Earth.

The two major influences on the atmospheric circulation are the differential heating between the equator and the

poles, and the rotation of the planet (Coriolis effect).

Given a difference in barometric pressure between two air masses, a wind will arise between the two which tends to flow from the area of high pressure to the area of low pressure until the two air masses are at the same pressure, although these flows will be modified by the Coriolis effect in the extratropics.

Winds can be classified either by their scale, the kinds of forces which cause them (according to the atmospheric equations of motion), or the geographic regions in which they exist. There are global winds, such as the wind belts which exist between the atmospheric circulation cells.

There are upper-level winds, such as the jet streams. There are synoptic-scale winds that result from pressure differences in surface air masses in the middle latitudes, and there are winds that come about as a consequence of geographic features such as the sea breeze.

Mesoscale winds are those which act on a local scale, such as gust fronts. At the smallest scale are the microscale winds which blow on a scale of only tens to hundreds of metres and are essentially unpredictable, such as dust devils and microbursts.

Winds can also shape landforms, via a variety of eolian processes.

Prevailing winds are winds which come about as a consequence of global circulation patterns. These include the Trade Winds, the Westerlies, the Polar Easterlies, and the jet streams.

Because of differential heating and the fact that warm air rises and cool air falls, there arise circulations that (on a non-rotating planet) would lead to an equator-to-pole flow in the upper atmosphere and a pole-to-equator flow at lower levels.

Because of the Earth's rotation, this simple situation is vastly modified in the real atmosphere. In almost all circumstances the horizontal component of the wind is much larger than the vertical — the exception being violent convection.

The Trade Winds are the most familiar consistent and reliable winds on the planet, exceeded in constancy only by the katabatic winds of the major ice sheets of Antarctica and Greenland.

It was these winds that early mariners relied upon to propel their ships from Europe to North and South America. Their name derives from the Old English 'trade', meaning "path" or "track", and thus the phrase "the wind blows trade", that is to say, on track.

The Trades form under the Hadley circulation cell, and are part of the return flow for this cell. The Hadley carries air aloft at the equator and transports it poleward north and south. At about 30°N/S latitude, the air cools and descends. It then begins its journey back to the equator, but with a noticeably westward shift as a result of the action of the Coriolis force.

Along the east coast of North America, friction twists the flow of the Trades even further clockwise. The result is that the Trades feed into the Westerlies, and thus provide a continuous zone of wind for ships travelling between Europe and the Americas.

The Westerlies, which can be found at the mid-latitudes beneath the Ferrel circulation cell, likewise arise from the tendency of winds to move in a curved path on a rotating planet.

Together with the airflow in the Ferrel cell, poleward at ground level and tending to equatorward aloft (though not clearly defined, particularly in the winter), this predisposes the formation of eddy currents which maintain a more-or-less continuous flow of westerly air. The upper-level polar jet stream assists by providing a path of least resistance under which low pressure areas may travel.

The Polar Easterlies result from the outflow of the Polar high, a permanent body of descending cold air which makes up the poleward end of the Polar circulation cell. These winds, though persistent, are not deep.

However, they are cool and strong, and can combine with warm, moist Gulf Stream air transported northward by weather systems to produce violent thunderstorms and tornadoes as far as 60°N on the North American continent.

Records of tornadoes in northerly latitudes are spotty and incomplete because of the vast amount of uninhabited terrain and lack of monitoring, and it is certain that tornadoes have gone unseen and unreported. The deadly Edmonton tornado of 1987, which ranked as an F4 on the Fujita scale and killed 27 people, is evidence that powerful tornadoes can occur north of the 50th parallel.

The jet streams are rapidly moving upper-level currents. Travelling generally eastward in the tropopause, the polar jets reside at the juncture of the Ferrel cell and the Polar cell and mark the location of the polar cold front.

During winter, a second jet stream forms at about the 30th parallel, at the interface of the Hadley and Ferrel cells, as a result of the contrast in temperature between tropical air and continental polar air.

The jet streams are not continuous, and fade in and out along their paths as they speed up and slow down. Though they move generally eastward, they may range significantly north and south. The polar jet stream also marks the presence of Rossby waves, long-scale (4000 - 6000 km in wavelength) harmonic waves which perpetuate around the globe.

Seasonal winds

Seasonal winds are winds that only exist during specific seasons, such as the Indian monsoon.

Synoptic winds

Synoptic winds are winds associated with large-scale events such as warm and cold fronts, and are part of what makes up everyday weather. These include the geostrophic wind, the gradient wind, and the cyclostrophic wind.

As a result of the Coriolis force, winds in the northern hemisphere always flow clockwise around a high pressure area and counterclockwise around a low pressure area (the reverse occurs in the southern hemisphere). At the same time, winds always flow from areas of high pressure to areas of low pressure.

These two forces are opposite but not equal, and the path that results when the two forces cancel each other runs parallel to the isobars. Wind following this path is known as geostrophic wind. Winds are said to be truly geostrophic only when other forces (e.g. friction) acting on the air are negligible, a situation which is often a good approximation to the large-scale flow away from the tropics.

In certain circumstances, the Coriolis force acting on moving air may be almost or entirely overwhelmed by the centripetal force. Such a wind is said to be cyclostrophic, and is characterized by rapid rotation over a relatively small area. Hurricanes, tornadoes, and typhoons are examples of this type of wind.

Mesoscale winds

Synoptic winds occupy the lower boundary of what is considered to be "forecastable" wind. Winds at the next lowest level of magnitude typically arise and fade over time periods too short and over geographic regions too narrow to predict with any long-range accuracy.

These mesoscale winds include such phenomena as the cold outflow from thunderstorms. This wind frequently advances ahead of more intense thunderstorms and may be sufficiently energetic to generate local weather of its own. Many of the "special" winds, addressed in the last section of this article, are mesoscale winds.

Microscale winds

Microscale winds take place over very short durations of time - seconds to minutes - and spatially over only tens to hundreds of metres. The turbulence following the passage of an active front is composed of microscale winds, and it is microscale wind which produces convective events such as dust devils.

Though small in scope, microscale winds can play a major role in human affairs. It was the crash of a fully loaded Lockheed L-1011 at Dallas-Fort Worth International Airport in the summer of 1985, and the subsequent loss of 133 lives, that introduced the term "microburst" to many people, and that was a factor in the installation of doppler radar in airports and weather installations worldwide.

Winds by effect

In classical terminology, Aeolian winds, or winds producing Aeolian action, are winds which produce geologic changes. Modern tornadoes and hurricanes might at times be considered to produce such changes.

Largescale erosion, dune formation, and other geologic and topographic effects influenced by wind are still referred to as aeolian activity.

Local winds that are tied to specific temperature distributions

Some local winds blow only under certain circumstances, i.e. they require a certain temperature distribution.

Differential heating is the motive force behind land breezes and sea breezes (or, in the case of larger lakes, lake breezes), also known as on- or off-shore winds. Land is a rapid absorber/radiator of heat, whereas water absorbs heat more slowly but also releases it over a greater period of time.

The result is that, in locations where sea and land meet, heat absorbed over the day will be radiated more quickly by the land at night, cooling the air. Over the sea, heat is still being released into the air at night, which rises.

This convective motion draws the cool land air in to replace the rising air, resulting in a land breeze in the late night and early morning. During the day, the roles are reversed. Warm air over the land rises, pulling cool air in from the sea to replace it, giving a sea breeze during the afternoon and evening.

Mountain breezes and valley breezes are due to a combination of differential heating and geometry. When the sun rises, it is the tops of the mountain peaks which receive first light, and as the day progresses, the mountain slopes take on a greater heat load than the valleys.

This results in a temperature inequity between the two, and as warm air rises off the slopes, cool air moves up out of the valleys to replace it. This upslope wind is called a valley breeze. The opposite effect takes place in the afternoon, as the valley radiates heat. The peaks, long since cooled, transport air into the valley in a process that is partly gravitational and partly convective and is called a mountain breeze.

Mountain breezes are one example of what is known more generally as a katabatic wind. These are winds driven by cold air flowing down a slope, and occur on the largest scale in Greenland and Antarctica. Most often, this term refers to winds which form when air which has cooled over a high, cold plateau is set in motion and descends under the influence of gravity. Winds of this type are common in regions of Mongolia and in glaciated locations.

Because katabatic refers specifically to the vertical motion of the wind, this group also includes winds which form on the lee side of mountains, and heat as a consequence of compression. Such winds may undergo a temperature increase of 20 °C (36 °F) or more, and many of the world's "named" winds (see list below) belong to this group.

Among the most well-known of these winds are the chinook of Western Canada and the American Northwest, the Swiss föhn, California's infamous Santa Ana wind, and the French mistral.

The opposite of a katabatic wind is an anabatic wind, or an upward-moving wind. The above-described valley breeze is an anabatic wind.

A widely-used term, though one not formally recognised by meteorologists, is orographic wind. This refers to air which undergoes orographic lifting. Most often, this is in the context of winds such as the chinook or the föhn, which undergo lifting by mountain ranges before descending and warming on the lee side.

Winds that are defined by an equilibrium of physical forces

These winds are used in the decomposition and analysis of wind profiles. They are useful for simplifying the atmospheric equations of motion and for making qualitative arguments about the horizontal and vertical distribution of winds. Examples are:

Geostrophic wind (wind that is a result of the balance between Coriolis force and pressure gradient force; flows parallel to isobars and approximates the flow above the atmospheric boundary layer in the midlatitudes if frictional effects are low)

Thermal wind (not actually a wind but a wind difference between two levels; only exists in an atmosphere with horizontal temperature gradients, i.e. baroclinicity)

Ageostropic wind (difference between actual and geostrophic wind; the wind component which is responsible for air "filling up" cyclones over time)

Gradient wind (like geostrophic wind but also including centrifugal force)

Names for specific winds in certain regions

In ancient Greek mythology, the four winds were personified as gods, called the Anemoi. These included Boreas, Notos, Euros, and Zephyros. The Greeks also observed the seasonal change of the winds, as evidenced by the Tower of the Winds in Athens.

In modern usage, many local wind systems have their own names. For example:

Alizé (northeasterly across central Africa and the Caribbean)

Alizé Maritime (a wet, fresh northerly wind across west central Africa)

Bora (northeasterly from eastern Europe to Italy)

Chinook (warm dry westerly off the Rocky Mountains)

Etesian (Greek name) or Meltemi (Turkish name) (northerly across Greece and Turkey)

Föhn (warm dry southerly off the northern side of the Alps)

Fremantle Doctor (afternoon breeze from the Indian Ocean which cools Perth, Western Australia during Summer)

Gregale (northeasterly from Greece)

Harmattan (dry northerly wind across central Africa)

Halny (in northern Carpathians)

Khamsin (southeasterly from north Africa to the eastern Mediterranean)

Levanter (easterly through Strait of Gibraltar)

Libeccio (southwesterly towards Italy)

Marin (south-easterly from Mediterranean to France)

Mistral (cold northerly from central France and the Alps to Mediterranean)

Nor'easter (eastern United States)

Santa Ana winds (southern California)

Sirocco (southerly from north Africa to southern Europe)

Southerly Buster (rapidly arriving low pressure cell that dramatically cools Sydney, Australia during Summer)

Tramontane (cold northwesterly from the Pyrenees or northeasterly from the Alps to the Mediterranean, similar to Mistral)

Vendavel (westerly through Strait of Gibraltar)

Zonda wind (on the eastern slope of the Andes in Argentina)

In meteorology, an anticyclone (i.e. opposite to a cyclone) is a weather phenomenon in which there is a descending movement of the air and a relative increase in barometric pressure over the part of the earth's surface affected by it.

At the surface the air tends to flow outwards in all directions from the central area of high pressure, and is deflected on account of the earth's rotation (see Ferrel's law) so as to give a spiral movement.

In the northern hemisphere an anticyclone rotates in the clockwise direction, while it rotates counterclockwise in the

southern hemisphere. The rotation is caused by the movement of colder higher pressure air that is moving away from the poles towards the equator being affected by the rotation of the earth.

Since the air in an anticyclone is descending, it becomes warmed and dried, and therefore transmits radiation freely whether from the sun to the earth or from the earth into space. Hence in winter anticyclonic weather is characterized by clear air with periods of frost, causing fogs in towns and low-lying damp areas, and in summer by still cloudless days with gentle variable airs and fine weather.

Anticyclones generally bring fair weather and clear skies as the dynamics of an anticyclone lead to downward vertical movement which suppresses convective activity and generally lowers the mean relative humidity, in contrast to the upward vertical movement in a cyclone.

However as the anticyclone moves over the earth surface it may heat up locally, acquire water from the land or oceans or encounter warmer wet air. Local geography may cause a range of localised weather phenomena specific to anticyclones, while the interaction of the different air masses, which occurs at weather fronts, may cause a range of weather events.

In meteorology, a cyclone is the rotation of a volume of air about an area of low atmospheric pressure. Cyclones are responsible for a wide variety of different meteorological phenomena such as tropical cyclones and tornadoes. Because of this, most weather forecasters avoid using the term cyclone without a qualifying term.

The center of a cyclone is a low-pressure region. Pressure gradient force, from high- to low-pressure regions, causes high winds around these regions. Wind flow around a large cyclone is almost invariantly anticlockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the Coriolis effect (viewed from above). Large anticyclonic storms are extremely rare on Earth, though Jupiter's Great Red Spot storm is anticyclonic.

A tornado is a violent spinning storm typically shaped like a funnel with the narrow end on the ground. Tornados are known for being extremely destructive and are almost always visible due to water vapor from clouds and debris from the ground.

Tornadoes form in storms all over the world, and though they have been recorded in all 50 U.S. states, they form most famously in a broad area of the American Midwest and South known as Tornado Alley.

Although, in pure number of incidences, the United States experiences more tornadoes than any other country, the

United Kingdom is the most tornado-prone country relative to

land area. The word "tornado" comes from the Spanish word for "turned", which in turn comes from the Latin word torqueo, meaning "to twist." Some common, related slang terms include: twister, whirlwind, wedge, funnel, willy-willy, or rope. However, willy-willy usually refers to a dust devil in Australia.

Cyclone is also another term for a tornado, although it must be noted that in parts of the world (notably Australia) a cyclone refers to what is more correctly known as a tropical cyclone (also known as a hurricane, or a typhoon), and meteorologists use the term cyclone to refer to a wide range of circular weather systems (using adjectives to disambiguate).

In general tornadoes are associated with a thunderstorm; however, National Weather Service in the United States considers all waterspouts—including "fair weather" waterspouts—to be tornadoes. Larger vortexes not associated with a thunderstorm are sometimes called landspouts.

Dust devils are small vortexes that form near the ground, which may or may not be considered tornadoes.

Tornadoes are developed from thunderstorms, most frequently supercell thunderstorms, though they also occur within squall lines and hurricanes. They are believed to be produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly.

Tornadoes, lightning, and sometimes hail are associated with thunderstorms. Many tornadoes appear at the tail end of mesocyclones. On weather radar screens, a characteristic "hook echo" marks the area where tornadoes are likely to exist.

Exactly how tornadoes form is complex and not fully understood. When thunderstorms develop, an increase in wind speed and/or a large change in direction with height ("wind shear") produces a horizontal, spinning area of air. The strong updrafts within the thunderstorm can draw this area of rotation up from horizontal to vertical. Towards the end of this area of rotation (the mesocyclone) is often a lower area of rain-free cloud and can be seen as a rotating "wall cloud".

If the rotation intensifies, a funnel cloud can develop where the cloud water vapor is draw down towards the ground. Usually the funnel cloud follows the intensity of the vortex towards the ground and this indicates the formation of a tornado, often referred to as "touching down", however this is not a reliable indicator as tornados can have a partial funnel cloud or be invisible.

It is not uncommon for a tornado to suddenly become visible when it fills with debris from the ground. Why the rotation can intensify and form tornadoes is not understood.

Recent Doppler radar studies, such as the Doppler on Wheels project, have shown that at least some tornadoes have "eyes" or "eyewalls" with central downdrafts like hurricanes; this parallel had been modeled as well as reported anecdotally for some time.[

Tornadoes normally rotate in a cyclonic (counterclockwise) direction in the northern hemisphere, as the warm air in which thunderstorms usually form sweeps north and jet streams come from the west, creating a situation in which the storms rotate. In the northern hemisphere, this rotation is counterclockwise, and in the southern hemisphere, clockwise.

The tornadoes usually rotate the same way. Sometimes opposite direction swirls develop under a thunderstorm. About 1 in 100 tornadoes in the northern hemisphere rotate in an anticyclonic direction.

No two tornadoes look exactly alike, nor have any two tornadoes behaved in exactly the same way. There are true incidents of tornadoes repeatedly hitting the same town several years in a row; however, forecasting the exact position a tornado will strike at a certain time is presently impossible.

Tornadoes can be nearly invisible, marked only by swirling debris at the base of the funnel. While tornadoes are invisible at night, some nocturnal tornadoes have been observed glowing diffusely due to lightning activity. Verified observations by Hall and others suggest a cellular structure inside tornadoes.

Some tornadoes are composed of several mini-funnels. A tornado must by definition have both ground and cloud contact. Thus, the oft-mentioned exclamation "Tornado on the ground!" is indeed redundant.

Not every thunderstorm, supercell, squall line, or hurricane will produce a tornado. Luckily, it takes exactly the right combination of atmospheric variables (wind, temperature, pressure, humidity, etc.) to spawn even a weak tornado. On the other hand, roughly 1,000 tornadoes a year are reported in the contiguous United States.

Even though no two tornadoes are exactly alike, they always have the same general characteristics that classify them as tornadoes. First, a tornado is a microscale rotating area of wind, from a few feet to a few miles wide. A thunderstorm can rotate, but that does not mean it is a tornado.

Second, the vortex, rotating wind, must be attached to a convective cloud base (such as thunderstorms embedded in squall lines, supercell thunderstorms, or the outer fringes of landfalling hurricanes), and must also be in contact with the ground. Third, a spinning vortex of air must have caused enough damage to be classified by the Fujita scale as a tornado.

In the United States (and sometimes in other countries, as well), the intensity of a tornado is measured on the Fujita-Pearson Tornado Scale (also known simply as Fujita scale). The intensity can be derived directly with high resolution Doppler radar wind speed data, or empirically derived from structural damage compared to engineering data.

Note that intensity does not refer in any way to the size, or width, of a tornado. The scale ranges from F0 for the weakest to F5 for the most powerful known tornadoes. No F6 tornado has yet been detected.

The TORRO scale, developed in the United Kingdom and used primarily in Europe, covers a broader range in finer detail, and is based solely on wind speed. It ranges in a similar way from a T0 to T11 for the most powerful known tornado in the United States.

Of all tornadoes formed in the U.S., F0 and F1 tornadoes account for a large percentage of occurrences. On the other end of the scale, the massively destructive F5s account for fewer than 1% of all tornadoes in the U.S.

While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.

Terms used in weather reports for tropical cyclones that

have surface winds over 64 knots (73.6 mph) or 32 m/s vary by region:

Hurricane: North Atlantic Basin and North Pacific Ocean east of the dateline

Typhoon: Northwest Pacific west of the dateline

Severe tropical cyclone: Southwest Pacific west of 160°E and the southeast Indian Ocean east of 90°E

Severe cyclonic storm: North Indian Ocean

Tropical cyclone: Southwest Indian Ocean and the South Pacific east of 160°E.

Cyclone (unofficially): South Atlantic Ocean

There are many regional names for tropical cyclones, including Bagyo in the Philippines and Taino in Haiti.

The formation of tropical cyclones is the topic of extensive ongoing research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible:

Sea surface temperatures above 26.5 degrees Celsius (79.7 degrees Fahrenheit) to at least a depth of 50 meters (164 feet). The moisture in the air above the warm water is the energy source for tropical cyclones.

Upper-atmosphere conditions conducive to thunderstorm formation. Temperature in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.

A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through tropical oceans.

A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone's rotation. (2004's Hurricane Ivan was the strongest storm to form closer than 10 degrees from the equator; it started forming at 9.7 degrees north.)

Low vertical wind shear (change in wind speed or direction over height). High wind shear can break apart the vertical structure of a tropical cyclone.

Tropical cyclones occasionally form despite not meeting these conditions.

Only specific weather disturbances can result in tropical cyclones. These include:

Tropical waves, or easterly waves, which, as mentioned above, are westward moving areas of convergent winds. This often assists in the development of thunderstorms, which can develop into tropical cyclones. Most tropical cyclones form from these. A similar phenomenon to tropical waves are West African disturbance lines, which are squally lines of convection that form over Africa and move into the Atlantic.

Tropical upper tropospheric troughs, which are cold-core upper level lows. A warm-core tropical cyclone may result when one of these (on occasion) works down to the lower levels and produces deep convection.

Decaying frontal boundaries may occasionally stall over warm waters and produce lines of active convection. If a low level circulation forms under this convection, it may develop into a tropical cyclone.

Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. However, each particular basin has its own seasonal patterns.

In the North Atlantic, a distinct hurricane season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the North Atlantic hurricane season is September 10. The Northeast Pacific has a broader period of activity, but in a similar timeframe to the Atlantic.

The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November.

In the Southern Hemisphere, tropical cyclone activity begins in late October and ends in May. Southern Hemisphere activity peaks in mid-February to early March.

Worldwide, an average of 80 tropical cyclones form each year.

Locations of formation

Most tropical cyclones form in a worldwide band of thunderstorm activity called the Intertropical convergence zone (ITCZ).

Nearly all of them form between 10 and 30 degrees of the equator and 87% form within 20 degrees of it. Because the Coriolis effect initiates and maintains tropical cyclone rotation, such cyclones almost never form or move within about 10 degrees of the equator, where the Coriolis effect is weakest.

However, it is possible for tropical cyclones to form within this boundary if there is another source of initial rotation. These conditions are extremely rare, and such storms are believed to form at most once per century. Hurricane Ivan of 2004 developed within 10 degrees of the equator.

A combination of a pre-existing disturbance, upper level divergence and a monsoon-related cold spell led to Typhoon Vamei at only 1.5 degrees north of the equator in 2001. It is estimated that such conditions occur only once every 400 years.

Lightning is a powerful natural electrostatic discharge produced during a thunderstorm. Lightning's abrupt electric discharge is accompanied by the emission of visible light and other forms of electromagnetic radiation.

The electric current passing through the discharge channels rapidly heats and expands the air into plasma, producing acoustic shock waves (thunder) in the atmosphere.

The first process in the generation of lightning is the forcible separation of positive and negative charge carriers within a cloud or air. The mechanism by which this happens is still the subject of research, but one widely accepted theory is the polarisation mechanism.

This mechanism has two components: the first is that falling droplets of ice and rain become electrically polarised as they fall through the atmosphere's natural electric field, and the second is that colliding ice particles become charged by electrostatic induction.

Once charged, by whatever mechanism, work is performed as the opposite charges are driven apart and energy is stored in the electric fields between them. The positively charged crystals tend to rise to the top, causing the cloud top to build up a positive charge, and the negatively charged crystals and hailstones drop to the middle and bottom layers of the cloud, building up a negative charge.

Cloud-to-cloud lightning can appear at this point. Cloud-to-ground lightning is less common. Cumulonimbus clouds that do not produce enough ice crystals usually fail to produce enough charge separation to cause lightning.

When sufficient negatives and positives gather in this way, and when the electric field becomes sufficiently strong, an electrical discharge occurs within the clouds or between the clouds and the ground, producing the bolt.

It has been suggested by experimental evidence that these discharges are triggered by cosmic ray strikes which ionise atoms, releasing electrons that are accelerated by the electric fields, ionising other air molecules and making the air conductive by a runaway breakdown, then starting a lightning strike.

During the strike, successive portions of air become conductive as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions (stepped channels called step leaders). The conductive filament grows in length. At the same time, electrical energy stored in the electric field flows radially inward into the conductive filament.

When a charged step leader is near the ground, opposite charges appear on the ground and enhance the electric field. The electric field is higher on trees and tall buildings. If the electric field is strong enough, a discharge can initiate from the ground.

This discharge starts as positive streamer and, if it develops as a positive leader, can eventually connect to the descending discharge from the cloud. Lightning can also occur within the ash clouds from volcanic eruptions[1],[2], or can be caused by violent forest fires which generate sufficient dust to create a static charge.

A bolt of lightning usually begins when an invisible negatively charged stepped leader stroke is sent out from the cloud. As it does so, a positively charged streamer is usually sent out from the positively charged ground or cloud. When the two leaders meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud.

This "return stroke" is the most luminous part of the strike, and is the part that is really visible. Most lightning strikes usually last about a quarter of a second. Sometimes several strokes will travel up and down the same leader strike, causing a flickering effect. This discharge rapidly superheats the leader channel, causing the air to expand rapidly and produce a shock wave heard as thunder.

It is possible for streamers to be sent out from several different objects simultaneously, with only one connecting with the leader and forming the discharge path. Photographs have been taken on which non-connected streamers are visible such as that shown on the right.

This type of lightning is known as negative lightning because of the discharge of negative charge from the cloud, and accounts for over 95% of all lightning.

An average bolt of negative lightning carries a current of 30 kiloamperes, transfers a charge of 5 coulombs, has a potential difference of about 100 megavolts and dissipates 500 megajoules (enough to light a 100 watt lightbulb for 2 months).

Positive lightning makes up less than 5 per cent of all lightning. It occurs when the stepped leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground.

Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike several kilometers or miles distant from the clouds. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.

As a result of their power, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999 [3]. Here is a page showing some pictures of positive lightning.

Positive lightning has also been shown to trigger the occurrence of upper atmospheric lightning. It tends to occur more frequently in winter storms and at the end of a thunderstorm.

An average bolt of positive lightning carries a current of 300 kiloamperes, transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (a thousand million volts), dissipates enough energy to light a 100 watt lightbulb for up to 95 years, and lasts for tens or hundreds of milliseconds.

Some lightning strikes take on particular characteristics, and scientists and the public have given names to these various types of lightning.

Intracloud lightning, sheet lightning, anvil crawlers

Intracloud lightning is the most common type of lightning which occurs completely inside one cumulonimbus cloud, and is commonly called an anvil crawler. Discharges of electricity in anvil crawlers travel up the sides of the cumulonimbus cloud branching out at the anvil top.

Cloud-to-ground lightning is a great lightning discharge between a cumulonimbus cloud and the ground initiated by the downward-moving leader stroke. This is the second most common type of lightning. One special type of cloud-to-ground lightning is anvil lightning, a form of positive lightning, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged.

In anvil lightning, the leader stroke issues forth in a nearly horizontal direction till it veers toward the ground. These usually occur miles ahead of the main storm and will strike without warning on a sunny day. They are signs of an approaching storm.

Bead lightning, ribbon lightning, staccato lightning

Another special type of cloud-to-ground lightning is bead lightning. This is a regular cloud-to-ground stroke that contains a higher intensity of luminosity. When the discharge fades it leaves behind a string of beads effect for a brief moment in the leader channel. A third special type of cloud-to-ground lightning is ribbon lightning.

These occur in thunderstorms where there are high cross winds and multiple return strokes. The winds will blow each successive return stroke slightly to one side of the previous return stoke, causing a ribbon effect. The last special type of cloud-to-ground lightning is staccato lightning, which is nothing more than a leader stroke with only one return stroke.

Cloud-to-cloud lightning

Cloud-to-cloud lightning is a somewhat rare type of discharge lightning between two or more completely separate cumulonimbus clouds.

Ground-to-cloud lightning

Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward-moving leader stroke. Most ground-to-cloud lightning occurs from tall buildings, mountains and towers.

Heat lightning or summer lightning

Heat lightning (or, in the UK, "summer lightning") is nothing more than the faint flashes of lightning on the horizon from distant thunderstorms. Heat lightning was named because it often occurs on hot summer nights. Heat lightning can be an early warning sign that thunderstorms are approaching.

In Florida, heat lightning is often seen out over the water at night, the remnants of storms that formed during the day along a seabreeze front coming in from the opposite coast.

Some cases of "heat lightning" can be explained by the refraction of sound by bodies of air with different densities. An observer may see nearby lightning, but the sound from the discharge is refracted over his head by a change in the temperature, and therefore the density, of the air around him. As a result, the lightning discharge appears to be silent. [4]

Ball lightning

Ball lightning is described as a floating, illuminated ball that occurs during thunderstorms. They can be fast moving, slow moving or nearly stationary. Some make hissing or crackling noises or no noise at all. Some have been known to pass through windows and even dissipate with a bang. Ball lightning has been described by eyewitnesses but rarely, if ever, recorded by meteorologists.

The engineer Nikola Tesla wrote, "I have succeeded in determining the mode of their formation and producing them artificially" (Electrical World and Engineer, 5 March 1904).

There is some speculation that electrical breakdown and arcing of cotton and gutta-percha wire insulation used by Tesla may have been a contributing factor, since some theories of ball lightning require the involvement of carbonaceous materials. Some later experimenters have been able to briefly produce small luminous balls by igniting carbon-containing materials atop sparking Tesla Coils.

Several theories have been advanced to describe ball lightning, with none being universally accepted. Any complete theory of ball lightning must be able to describe the wide range of reported properties, such as those described in Singer's book "The Nature of Ball Lightning" and also more contemporary research. Japanese research shows that ball lightning has been seen several times without any connection to stormy weather or lightning.

Ball lightning field properties are more extensive than realised by many scientists not working in this field. The typical fireball diameter is usually standardised as 20–30 cm, but ball lightning several meters in diameter has been reported (Singer). A recent photograph by a Queensland ranger, Brett Porter, showed a fireball that was estimated to be 100 meters in diameter.

The photograph has appeared in the scientific journal Transactions of the Royal Society. The object was a glowing globular zone (the breakdown zone?) with a long, twisting, rope-like projection (the funnel?).

Fireballs have been seen in tornadoes, and they have also split apart into two or more separate balls and recombined. Fireballs have carved trenches in the peat swamps in Ireland. Vertically linked fireballs have been reported.

One theory that may account for this wider spectrum of observational evidence is the idea of combustion inside the low-velocity region of axisymmetric (spherical) vortex breakdown of a natural vortex (e.g., the 'Hill's spherical vortex'). The scientist Coleman was the first to propose this theory in 1993 in Weather, a publication of the Royal Meteorological Society.

Ball lightning is hardly ever seen. In fact, there are only a few pictures of it.

St Elmo's fire was correctly identified by Franklin as electrical in nature. It is not the same as ball lightning.

Sprites, elves, jets and other upper atmospheric lightning

Reports by scientists of strange lightning phenomena above storms date back to at least 1886. However, it is only in recent years that fuller investigations have been made. This has sometimes been called megalightning.

Sprites are now well-documented electrical discharges that occur high above the cumulonimbus cloud of an active thunderstorm. They appear as luminous reddish-orange, neon-like flashes, last longer than normal lower stratospheric discharges (typically around 17 milliseconds), and are usually spawned by discharges of positive lightning between the cloud and the ground.

Sprites can occur up to 50 km from the location of the lightning strike, and with a time delay of up to 100 milliseconds. Sprites usually occur in clusters of two or more simultaneous vertical discharges, typically extending from 65 to 75 km (40 to 47 miles) above the earth, with or without less intense filaments reaching above and below.

Sprites are preceded by a sprite halo that forms because of heating and ionisation less than 1 millisecond before the sprite. Sprites were first photographed on July 6, 1989, by scientists from the University of Minnesota and named after the mischievous sprites in the plays of Shakespeare.

Recent research carried out at the University of Houston in 2002 indicates that some normal (negative) lightning discharges produce a sprite halo, the precursor of a sprite, and that every lightning bolt between cloud and ground attempts to produce a sprite or a sprite halo.

Research in 2004 by scientists from Tohoku University found that very low frequency emissions occur at the same time as the sprite, indicating that a discharge within the cloud may generate the sprites.

Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 40 to 50 km (25 to 30 miles) above the earth. They are also brighter than sprites and, as implied by their name, are blue in colour. They were first recorded on October 21, 1989, on a video taken from the space shuttle as it passed over Australia.

Elves often appear as a dim, flattened, expanding glow around 400 km (250 miles) in diameter that lasts for, typically, just one millisecond. They occur in the ionosphere 100 km (60 miles) above the ground over thunderstorms. Their colour was a puzzle for some time, but is now believed to be a red hue. Elves were first recorded on another shuttle mission, this time recorded off French Guiana on October 7, 1990.

Elves is a frivolous acronym for Emissions of Light and Very Low Frequency Perturbations From Electromagnetic Pulse Sources. This refers to the process by which the light is generated; the excitation of nitrogen molecules due to electron collisions (the electrons having been energised by the electromagnetic pulse caused by a positive lightning bolt).

On September 14, 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around 80 km (50 miles) into the atmosphere. The jet was located above a thunderstorm over the ocean, and lasted under a second.

Lightning was initially observed travelling up at around 50,000 m/s in a similar way to a typical blue jet, but then divided in two and sped at 250,000 m/s to the ionosphere, where they spread out in a bright burst of light.

On July 22, 2002, five gigantic jets between 60 and 70 km (35 to 45 miles) in length were observed over the South China Sea from Taiwan, reported in Nature [8]. The jets lasted under a second, with shapes likened by the researchers to giant trees and carrots.

Researchers have speculated that such forms of upper atmospheric lightning may play a role in the formation of the ozone layer.

One theory about the cause of the Space Shuttle Columbia disaster is that the craft was struck by atmospheric lightning [9].

Streak lightning

All lightning is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke. Because most of these strokes occur inside a cloud, we do not see many of the individual return strokes in a thunderstorm.

Triggered lightning

Lightning has been triggered directly by human activity in several instances. Lightning struck the Apollo 12 soon after takeoff, and has struck soon after thermonuclear explosions. It has also been triggered by launching rockets carrying spools of wire into thunderstorms. The wire unwinds as the rocket climbs, making a convenient path for the lightning to use. These bolts are typically very straight. For more information, see triggered lightning.

Lightning throughout the Solar System

Lightning requires the electrical breakdown of gas, so lightning cannot exist in the vacuum of space. However, lightning has been observed within the atmospheres of other planets, such as Venus and Jupiter, and electrical discharges between Jupiter and Io often occur within the gas cloud sent out by Io's volcanos. Lightning on Jupiter is estimated to be 100 times as powerful, but fifteen times less frequent, than that which occurs on Earth.

Lightning on Venus is still a controversial subject after decades of study. During the Soviet Venera and U.S. Pioneer missions of the '70s and '80s, signals suggesting lightning may be present in the upper atmosphere were detected [10]. However, recently the Cassini-Huygens mission fly-by of Venus detected no signs of lightning at all.

In optics, refraction occurs when light waves travel from a medium with a given refractive index to a medium with another.

At the boundary between the media the wave's phase velocity is altered, it changes direction, and its wavelength increases or decreases but frequency remains constant. For example, a light ray will refract as it enters and leaves

glass; understanding of this concept led to the invention of lenses and the refracting telescope.

An example of this is looking into a bowl of water. Air has a refractive index of about 1.0003, and water has a refractive index of about 1.33. If a person looks at a straight object, such as a pencil, which is placed at a slant, partially in the water, the object appears to bend at the water's surface.

This is due to the light rays from the object being bent as they move from the water to the air. This causes water to appear shallower than it really is. The depth that the water appears to be when viewed from above is known as the apparent depth. The diagram on the right shows an example of refraction in

water waves. Ripples travel from the left and pass over a shallower region inclined at an angle to the wavefront. The waves travel more slowly in the shallower water, so the wavelength decreases

and the wave bends at the boundary. The dotted line represents the normal to the boundary. The dashed line represents the original direction of the waves. The phenomenon explains why waves on a shoreline never hit the shoreline at an angle. Whichever direction the waves travel in deep water, they always refract towards the normal as they enter the shallower water near the beach.

Refraction is also responsible for rainbows and for splitting up of white light into a rainbow-spectrum as it passes through a glass prism. Glass has a higher refractive index than air and the different frequencies of light travel at different speeds (dispersion), causing them to be refracted at different angles, so that you can see them. The different frequencies correspond to different colours observed.

Snell's law is used to calculate the degree to which light is refracted when traveling from one medium to another.

Recently some metamaterials have been created which have a negative refractive index. With metamaterials, we can also obtain the total refraction phenomena when the wave impedances of the two media are matched. There is no reflected wave.

Ocean currents can flow for thousands of kilometers. They are very important in determining the climates of the continents, especially those regions bordering on the ocean.

Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than

any other region at the same latitude. Another example is the Hawaiian Islands, where the climate is somewhat cooler (sub-tropical) than the tropical latitudes in which they are located because of the California Current.

Surface ocean currents are generally wind driven and develop their typical clockwise spirals in the northern hemisphere and counter-clockwise rotation in the southern hemisphere due to the imposed wind stresses. In wind driven currents the Ekman spiral effect results in the currents flowing at an angle to the driving winds. The areas of surface ocean currents move somewhat with the seasons, this is most notable in equatorial currents.

Deep ocean currents are driven by density and temperature gradients. Thermohaline circulation, also known as the ocean's conveyor belt, refers to the deep ocean density-driven ocean basin currents. These currents that flow under the surface of the ocean, and are thus hidden from immediate detection, are called submarine rivers.

These are currently being researched by floating devices, which maintain their depth according to slightly differing densities of waters. Upwelling and downwelling areas in the oceans are areas, where significant vertical movement of ocean water is observed.

Arctic Ocean

East Greenland Current

Norwegian Current

Atlantic Ocean

Angola Current

Antilles Current

Benguela current

Brazil Current

Canary Current

Cape Horn Current

Caribbean Current

East Greenland Current

Falkland Current

Gulf Stream

Guinea Current

Labrador Current

North Atlantic Current

North Brazil Current

North Equatorial Current

Norwegian Current

Portugal Current

South Atlantic Current

South Equatorial Current

Spitzbergen Current

West Greenland Current

West Wind Drift

Pacific Ocean

Alaska Current

Aleutian Current

California Current

Cromwell current (a deep current)

East Australian Current

Equatorial Counter Current

Humboldt Current (or Peru Current)

Kamchatka Current

Kuroshio Current (or Japan Current, Kuro Siwo)

Mindanao Current

North Equatorial Current

North Pacific Current (or North Pacific Drift)

Oyashio Current (or Oya Siwo)

South Equatorial Current

West Wind Drift

Indian Ocean

Agulhas Current

East Madagascar Current

Equatorial Counter Current

Indonesian Through-flow

Leeuwin Current

Madagascar Current

Mozambique Current

Somali Current

South Australian Counter Current

South Equatorial Current

Southwest and Northeast Monsoon Drift (or Indian Monsoon Current)

West Australian Current

West Wind Drift

Southern Ocean

Antarctic Circumpolar Current

Weddell Gyre

The ozone layer, or ozonosphere, is that part of the Earth's stratosphere which contains relatively high concentrations of ozone (O3). "Relatively high" means a few parts per million, much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere.

The ozone layer was discovered in 1913 by the french physicists Charles Fabry and Henri Buisson. Its properties

were explored in detail by the British meteorologist G.M.B. Dobson, who developed a simple spectrophotometer that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations which continues to operate today. The "Dobson unit", a convenient measure of the total amount of ozone in a column overhead, is named in his honor.

The photochemical mechanisms that give rise to the ozone layer were worked out by the British physicist Sidney Chapman in 1930. Ozone in the earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create ozone, O3.

The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere.

Tropospheric ozone has two sources: about 10 % is transported down from the stratosphere while the remainder is created in situ in smaller amounts through different mechanisms.

About 90% of the ozone in our atmosphere is contained in the stratosphere, the region from about 10 to 50km (32,000 to 164,000 feet) above Earth's surface. Ten percent of the ozone is contained in the troposphere, the lowest part of our atmosphere where all of our weather takes place.

Ozone concentrations are greatest between about 15 and 40 km, where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only a few millimeters thick.

Although the concentration of ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation from the Sun. UV radiation is divided into three categories, based on its wavelength; these are referred to as UV-A, UV-B, and UV-C. UV-C, which would be very harmful to humans, is entirely screened out by ozone at around 35 km altitude.

UV-B radiation is the main cause of sunburn; excessive exposure can also cause genetic damage, resulting in problems such as skin cancer. The ozone layer is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at Earth's surface is 350 million times weaker than at the top of the atmosphere.

Nevertheless, some UV-B reaches the surface. Most UV-A reaches the surface; this radiation is significantly less harmful, although it can potentially cause genetic damage.

Depletion of the ozone layer would allow more of the UV radiation, and particularly the more harmful wavelengths, to reach the surface, causing increased genetic damage to living things.

The "thickness" of the ozone layer - that is, the total amount of ozone in a column overhead - varies by a large factor worldwide, being in general smaller near the equator and larger as one moves towards the poles.

It also varies with season, being in general thicker during the spring and thinner during the autumn. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity.

The tropical climates appear in the equator and in the areas next to him, where the air is always warm and humid.

The deserts appear on both sides of the equator, in zones where the air goes down after warm air contacts the polar winds, by what the climate there is moderated and variable. Next to the poles themselves, the air is cold and dry.

Besides the basic stripes, four intermediate zones exist: subtropical, esteparia, of taiga and Mediterranean. The mountains represent a special case, since an elevation closely of 4500 m of altitude can have the same environmental impact as an increase of 10th in the latitude.

The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research.

The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen. Heat (from the still-molten

crust, and the sun) dissipated this atmosphere.

About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia.

This led to the "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen (though very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggested that it may have had up to 40% hydrogen ).

This second atmosphere had approximately 100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.

During the next few billion years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans.

One of the earliest types of bacteria were the cyanobacteria. Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the earth's atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen).

Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere.

Photosynthesizing plants would later evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen.

As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere, resulting in mass extinctions and further evolution.

With the appearance of an ozone layer (ozone is an allotrope of oxygen) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".

troposphere: From the Greek word tropos meaning to turn or mix. The troposphere is the lowest layer of the atmosphere starting at the surface going up to between 7 km at the poles and 17 km at the equator with some variation due to weather factors. The troposphere has a

great deal of vertical mixing due to solar heating at the surface. This heating warms air masses, which then rise to release latent heat as sensible heat that further buoys the air mass. This process continues until all water vapor is removed. In the troposphere, on average, temperature decreases with height due to expansive cooling.

stratosphere: from that 7–17 km range to about 50 km, temperature increasing with height.

mesosphere: from about 50 km to the range of 80 km to 85 km, temperature decreasing with height.

thermosphere: from 80–85 km to 640+ km, temperature increasing with height. The boundaries between these regions are named the tropopause, stratopause, and mesopause. The average temperature of the atmosphere at the surface of earth is 14 °C.

ionosphere – the region containing ions: approximately the mesosphere and thermosphere up to 550 km. exosphere – above the ionosphere, where the atmosphere thins out into space. magnetosphere – the region where the Earth's magnetic field interacts with the solar wind from the Sun.

It extends for tens of thousands of kilometers, with a long tail away from the Sun. ozone layer – or ozonosphere, approximately 10 - 50 km, where stratospheric ozone is found. Note that even within this region, ozone is a minor constituent by volume. upper atmosphere – the region of

the atmosphere above the mesopause. Van Allen radiation belts – regions where particles from the Sun become concentrated.

Between the physical properties on which the thermometers are based he emphasizes the dilation of the gases, the dilation of a column of mercury, the electrical resistance of some metal, the change of the electromotive force of contact between two metals, the distortion of a metallic plate or the change of the magnetic susceptibility of true you go out paramagnéticas.

The thermometer of dilation of liquids is the most well-known. It consists of a blister full of liquid joined a capillary dry sherry, all this shut up in a glass capsule or quartz in the shape of rod. The sensibility that is achieved depends on the dimensions of the deposit and on the diameter of the capillary, and in the most favorable cases it is of hundredth of grade.

The status of temperatures in which it is more trustworthy depends on the nature of the used liquid. For example, with alcohol good sensibility and reliability is achieved between-100 ºC and 100 ºC, whereas the thermometer of mercury is indicated between-30th and 600 ºC.

This concept of "decreasing pressure means bad weather" is the basis for a primitive weather prediction device called a weather glass or thunder glass. It can also be called a "storm glass" or a "Goethe thermometer" (the writer Goethe popularized it in Germany).

It consists of a glass container with a spout. The container is filled with water up to about the middle of the spout; some air is left in the main body of the container. The design is such that when the air pressure decreases, the pressure of the air pocket inside the device will push some

of the water up the spout. If the air pressure is low enough, some of the water may even drip out of the spout. These devices are essentially a water-based version of the mercury barometer. The "Thunder Glass" is extremely susceptible to the ambient temperature which will alter the height of the water column in the spout.

Mercury barometers

A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjusts until the weight of the mercury column

balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more downward force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the downward force placed on the reservoir. The first barometer of this type was devised by Evangelista Torricelli, a student of Galileo Galilei, in 1643.

Torricelli had set out to create a perfect vacuum, and an instrument to measure air pressure. He succeeded in creating a vacuum in the top of a tube of mercury. Torricelli also noticed that the level of the fluid in the tube changed slightly each day and concluded that this was due to the changing pressure in the atmosphere. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millibars: the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type can usually measure atmospheric pressures in the range between 28 and 31 inches of mercury.

Aneroid barometers

Another type of barometer, the aneroid barometer, uses a small, flexible metal box called an aneroid cell. The box is tightly sealed after some of the air is removed, so that small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives a series of mechanical levers and other devices which are displayed on the face of the aneroid barometer.

Applications

A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activity.

Compensations

Temperature

The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. No such compensation is required for an aneroid barometer.

Altitude

As the air pressure will be reduced at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presense of weather systems).

Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.

A precipitation of 5 mm indicates that if all the water of the rain will accumulate in a flat area without slipping to evaporate, the height of the water layer would be 5 mm.

The millimeters (mm) are equivalent to the liters per square meter. The pluviómetro gathers the atmospheric water in his diverse states. The whole is named a

precipitation. For the solid states, the measurements carry out once reached the liquid state. Two basic models of pluviómetros exist: of direct reading and registrars.

Those of direct reading have a receptacle and a funnel. Every 12 hours the receptacle empties in a burette adjusted with a section ten times minor that that of reception, with what it is possible to establish a relation between the height in burette and the precipitation in millimeters per square meter. The pluviómetros registrars can be of three types: of bore, of vat basculante or of float, according to the procedure that they use to register the measurement once reached certain level.