The Atmosphere

Geography Atmosphere

The Atmosphere

Blankets of volatile gases and liquids near and above the surface of the earth are of prime importance, along with the solar energy, for the sustenance of life on earth. These are distributed and recycled throughout the atmosphere and hydrosphere of the planet.

The Earth is surrounded by a relatively thin atmosphere consisting of a mixture of gases, primarily molecular nitrogen (77 percent) and molecular oxygen (21 percent). This gaseous envelope, commonly called the air, also contains much smaller amounts of gases such as argon, carbon dioxide, methane, and water vapour, along with minute solid and liquid particles in suspension.

It is not surprising that the Earth, as a small planet (with a rather weak gravitational field) at fairly warm temperatures (due to its proximity to the Sun), should lack the most common gases in the universe, hydrogen and helium. Whereas both the Sun and Jupiter are dominantly composed of these two elements, they could not be retained long on the Earth and would rapidly evaporate into interplanetary space. It is surprising, however, that more than 20 percent of the Earth’s atmosphere is composed of oxygen, a highly reactive gas that, under most planetary conditions, would have combined with other chemicals. The two parts per million of methane in the atmosphere, which is far out of chemical equilibrium, is actually of biogenic origin (produced in the digestive tracts of cows, for example).

The atmosphere extends from the surface of the Earth to heights of thousands of kilometres, where it gradually merges with the solar wind—a stream of charged atomic particles that flows outward from the outermost regions of the Sun. The composition of the atmosphere is more or less constant with height to an altitude of about 100 kilometres.

The atmosphere is commonly described in terms of distinct layers, or regions. Most of the atmosphere is concentrated in the TROPOSPHERE, which extends from the surface to an altitude of about 15 kilometres. The behaviour of the gases in this layer is controlled by convection. This process involves the turbulent, overturning motions resulting from buoyancy of near-surface air that is warmed by the Sun. Convection maintains a vertical temperature gradient (i.e., temperatures decline with altitude) of roughly 6º C per kilometre (1o C per 165m) through-out the troposphere. At the top of the troposphere, which is called the tropopause (zone of transition between troposphere and stratosphere), temperatures fall to about -60º C (-76º F). The troposphere is the region where virtually all water vapour exists and where all weather occurs.

The dry, tenuous stratosphere lies above the troposphere and extends to an altitude of about 50 kilometres. Convective motions are weak or absent in the stratosphere; motions instead tend to be horizontally oriented. The temperature in this layer increases with altitude.

In the upper stratospheric regions, absorption of ultraviolet light from the Sun breaks down oxygen molecules; recombination of oxygen atoms with O2 molecules into ozone (O3) creates the ozone layer, which shields the lower ecosphere from harmful radiations of short-wavelength ( Ultra violet radiations).

Above the relatively warm stratopause (zone of transition between stratosphere and mesosphere) is the even more tenuous mesosphere, in which temperatures again decline with altitude, reaching roughly -85º C at the mesopause. Temperatures then rise with increasing height through the overlying layer known as the thermosphere.

Above about 100 kilometres, in the ionosphere, there is an increasing fraction of charged, or ionized, particles. Spectacular visible auroras are generated in this region, particularly along circular zones around the poles, by episodic precipitation of energetic particles.

The general circulation of the Earth’s atmosphere is driven by solar energy, which falls preferentially in equatorial latitudes. Atmospheric redistribution of heat pole ward is strongly affected by the Earth’s rapid rotation and the associated Coriolis force at non-equatorial latitudes (which adds an east-west component to the direction of the winds), resulting in about three latitudinal cells of circulation in each hemisphere. Instabilities produce the characteristic high-pressure areas and low-pressure storms of the mid-latitudes as well as the fast, eastward-moving jet streams of the upper troposphere that guide the paths of storms.

The oceans are massive reservoirs of heat, and their slowly changing currents and temperatures also influence weather and climate, as in the so-called El Nino episodes

The Earth’s atmosphere is not a static feature of the environment. Rather its composition has evolved over time in concert with life and continues to change as human activities alter the ecosphere. Roughly halfway through the history of the Earth, the atmosphere’s unusual complement of free oxygen began to develop owing to photosynthesis by blue-green algae and subsequently evolving plant life. Accumulation of oxygen eventually made it possible for respirating animals to move out onto the land.

The Earth’s climate at any location varies with the seasons, but also there are longer-term variations in global climate. Volcanic explosions, such as the 1991 eruption of Mount Pinatubo in the Philippines, can inject great quantities of particulates into the atmosphere, which remain suspended for years, decreasing atmospheric transparency and resulting in measurable cooling worldwide. Rare, giant impacts of asteroids and comets can have even more profound effects. The dominant climate variations observed in the recent geologic record are the ice ages, which are linked to small variations in the Earth’s geometry with respect to the Sun.

The Sun is believed to have been less luminous during the early history of the Earth, so if other planetary conditions were identical with those of today, the oceans would have been frozen. But it is expected that there was much more carbon dioxide in the Earth’s atmosphere during earlier periods, which would have enhanced greenhouse warming. In this phenomenon, heat radiated by the surface is trapped by gases such as carbon dioxide in the atmosphere and reradiated back to the surface, thereby warming it. There is presently 10 times more carbon dioxide buried in carbonate rocks in the Earth’s crust than in the atmosphere, in sharp contrast with Venus, whose atmospheric evolution followed a different course.

The amount of carbon dioxide in the atmosphere is rising steadily, and has evidently increased by more than 10 percent in the last 30 years owing to the burning of fossil fuels (e.g., coal, oil, and natural gas) and the destruction of tropical rain forests, such as that of the Amazon River basin. A further doubling by the middle of the 21st century could lead to a global warming of a few degrees, which would have profound effects on the sea level and on agriculture.

Of more immediate concern is the impact of human activities on the stratospheric ozone layer. Complex chemical reactions involving traces of man-made chlorofluorocarbons have recently created temporary holes in the ozone layer, particularly over Antarctica, during polar spring. More disturbing, however, is the discovery of a growing depletion of ozone over temperate latitudes, where a large percentage of the world’s population resides, since the ozone layer serves as a shield against ultraviolet radiation, which has been found to cause skin cancer.

Composition & Structure of Atmosphere

Atmosphere is a layer of mixture of gases enveloping the earth, held to it by gravitational force.

From the earth’s surface upward to an altitude of about 80 km the chemical composition of atmosphere is uniform in terms of the proportions of its component gases. The atmosphere can be divided into layers on the basis of temperatures and zones of temperature change. Above 80 km. atmospheric composition tends to be independent of height.

Troposphere

It is the lowermost atmospheric layer extending for about 8 km at the poles and 16 km at equator[1}. It is heated to an average temperature of 15ºC/59ºF by the outgoing radiations from Earth, which in turn is warmed by infrared and visible radiation from the Sun. Warm air cools as it rises in the troposphere and this rising of warm air causes rain and most other weather phenomena. The top of the troposphere is at approximately -60ºC. All phenomena of weather and climate, which physically affect man, take place within this layer.

Stratosphere

The second layer of atmosphere is called the stratosphere. The end boundary of troposphere that gives way to stratosphere is called Tropopause. (At this level the fall in temperature stops). Within the stratosphere, the increase in temperature with altitude is slow and constant at lower section, but becomes rapid at higher altitudes. Temperature increases with altitude from 10 km/6 mi to 50 km/31 miles. The upper limit of this layer is called stratopause.

Within the stratosphere, temperature increases from about -60oC at Tropopause to about OoC at stratopause. Little weather is generated here as there is very little water vapour and virtually no dust is present. The stratosphere provides ideal conditions for flying large and fast airplanes. Ozone is produced in tropical and mid-latitudes of stratosphere.

Mesosphere

Mesosphere is the atmospheric layer extending between the stratopause (at an altitude of about 50 km) and mesopause, the upper limit of mesosphere (at about 80-90 km). Within mesosphere, the temperature decreases with altitude from about 0^oC at stratopause to about 100^oC at mesopause. Vertical air currents are not strongly inhibited here and formation of ice crystal clouds called the noctilucent clouds takes place occasionally in the upper regions of the layer.

Thermosphere and Exospheres

Thermosphere is the uppermost layer of the atmosphere, extending from the mesopause, at an altitude of about 85 km. to 700 km. of the atmosphere. Temperature rises with altitude to extreme values of thousands of degrees. The meaning of these extreme temperatures can be misleading. High thermosphere temperatures represent little heat because they are defined by motions among so few atoms and molecules spaced widely apart from one another.

Exosphere is the boundary between the earth’s atmosphere and the inter planetary space. It extends from about 400 km. above the earth’s surface.

ATMOSPHERE (On the basis of composition)
	HETEROSPHERE	HOMOSPHERE
1.	Gases not evenly mixed	Gases evenly mixed.
2.	Height Begins at 80 km & extends up to 10,000 km	Extends from the earth’s surface up to an altitude of 80 km.
3.	Solar constant is measured at 480 km	N₂ & O₂ are the Main Gases
4.	Above 48⁰ Exposure (outer space). It contains individual atoms of light gases, weakly bound by gravity	CO₂ helps in maintaining global temperatures.

Temperature

The Heat Budget

Incoming heat being absorbed by the Earth, and outgoing heat escaping the Earth in the form of radiation are both perfectly balanced. If they were not balanced, then Earth would be getting either progressively warmer, or progressively cooler with each passing year. This balance between incoming and outgoing heat is known as Earth’s heat budget.

While on average, Earth’s heat budget is balanced, the interactions that take place as heat and electromagnetic radiation interacting with Earth, and its many objects, oceans, and atmosphere are complex. Over all they balance out, however, some places are hotter, or cooler day in and day out.

The solar energy travels towards the Earth at light speed, in the form of ultraviolet radiation, visible light, and infrared radiation. When this energy reaches the Earth, immediately 30% of it bounces off, being reflected back into space. The ability to reflect the light and radiation of the Sun is known as an object’s albedo[2]. Because the Earth reflects 30% of the light that hits it, it is said that the Earth has an albedo of 30. In contrast, our moon has an albedo of [3] to 11. Because 30% of the electromagnetic energy from the Sun has been reflected away, only 70% remains to interact with the Earth, and warm it up. 20% of the energy from the Sun is absorbed by the atmosphere as a whole heating it up. 2% is reflected by the snow. This leaves around 50% of the Sun’s energy to heat both the surface of the Earth, as well as the oceans, lakes and streams. This heat is absorbed by the earth and later released as long wavelength radiations.

Latent Heat of Condensation

A proportion of the solar energy that reaches the earth’s surface is used to change liquid water from rivers, lakes, and oceans to water-vapour in the process known as ‘EVAPORATION’. The solar energy used to do this is then stored in the water-vapour as ‘latent’ or ‘potential energy’. Later the water-vapour in the atmosphere may change to form liquid water again through a process called ‘CONDENSATION’. The energy released through this process is known as the ‘LATENT HEAT OF CONDENSATION’. Like other means of heat transfer in the earth system, ‘latent heat of condensation’ takes part in the heating of the atmosphere. In fact, it plays a major role in this process, and in addition, is a source of energy for STORMS and CYCLONES.

Convection

Heat energy transfer that involves the movement of a fluid (gas or liquid) is termed as convection. Fluid in contact with the source of heat expands and tends to rise within the bulk of the fluid. Cooler fluid sinks to take its place, setting up convection currents. This is the principle of natural convection in many domestic hot-water systems and space heaters.

Temperature inversion

Under special circumstances the normal trend of decrease in temperature with increasing height is reversed under special circumstances. Some of the situations leading to temperature inversion:

1) Frontal situation of middle latitudes represents case of temperature inversion as in a front, warm air rises over cold air.

2) On a clear night when heat is radiated from the earth surface, the air near to the surface is cooled by conduction of heat to cold ground. The lower layer of the atmosphere is cooler than air aloft.

3) At night the colder denser air on the upper slopes of a valley side descends into the valley bottom displacing warmer air aloft thereby making the valleys coler than the hill tops.

4) When air is subsiding in an anticyclone the air is warmed adiabatically & this air is warmer than the air at the ground level creating a case of temperature inversion.

Role of constituents

Nitrogen: is a very stable gas due to its strong bond and is normally not reactive to elements forming earth’s surface & is also stable in the presence of solar radiation. Nitrogen is removed from the atmosphere and deposited at the Earth's surface mainly by specialized nitrogen fixing bacteria, and by way of lightning during precipitation. Nitrogen returns to the atmosphere primarily through biomass combustion and denitrification.

Oxygen: The atmosphere’s free oxygen began to develop owing to photosynthesis by blue-green algae and subsequently evolving plant life. Accumulation of oxygen eventually made it possible for respirating animals to move out onto the land.

Water vapour: Water vapor varies in concentration in the atmosphere (average 1.27%) both spatially and temporally. The highest concentrations of water vapor are found near the equator over the oceans and tropical rain forests. In polar areas and subtropical continental deserts the volume of water vapor can approach zero percent. Water vapor has several very important functional roles on our planet:

It redistributes heat energy on the Earth through latent heat energy exchange.

The condensation of water vapor creates precipitation that falls to the Earth's surface providing needed fresh water for plants and animals.

It helps warm the Earth's atmosphere through the greenhouse effect.

Carbon dioxide: does not absorb visible light, but it absorbs Infrared radiations from both insolation as well from terrestrial radiation and is largely responsible for green house effect. Volume of CO₂ is increasing because of burning of fossil fuels, deforestation and land use change.

Aerosols: Atmosphere also has a capacity to keep small solid particles; sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors. Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds. They also scatter and diffuse insolation. These are concentrated in the lower layers of the atmosphere mainly in subtropical and temperate regions due to dry winds but sometimes convectional air currents may transport them to great heights.

Methane: is a very strong greenhouse gas. The primary sources for the methane are: rice cultivation; domestic grazing animals; termites; landfills; coal mining; and, oil and gas extraction. Anaerobic conditions associated with rice paddy flooding results in the formation of methane gas.

Nitrous Oxide: is also a green house gas. Natural emissions of N₂O result from bacterial breakdown of nitrogen in soils and in the earth's oceans while human-related sources of N₂O are chemical fertilizers, animal manure, sewage treatment, combustion of fossil fuel, and production of nitric acid.

Ozone :is created naturally in the stratosphere by the combining of atomic oxygen (O) with molecular oxygen (O₂) activated by sunlight. Ozone is destroyed naturally by the absorption of ultraviolet radiation and collision with other molecules. Ozone absorbs ultraviolet radiations. Without ozone, this high energy light would penetrate further into the earth's atmosphere and destroy animal (and plant) cells. The majority of the ozone (about 97%) found in the atmosphere is concentrated in the stratosphere at an altitude of 15 to 55 km. Ozone levels have been decreasing due to the buildup of human created CFC[4] in the atmosphere.

Ozone is also highly concentrated at the Earth's surface in and around cities. Most of this ozone is created as a byproduct of human created photochemical smog. This build-up of ozone is toxic to organisms living at the Earth's surface.

PRESSURE & WINDS

WINDS

Horizontal flow of air is known as winds and this flow is caused by combination of pressure gradient, Coriolis force and frictional forces caused by uneven heating, rotation of earth and distribution of land and oceans.
Pressure-gradient force is the rate of change of pressure with respect to distance is the pressure gradient. The larger the pressure gradient force the stronger will be the resulting wind. (i.e. closer Isobars)
Coriolis Force- As winds move north or south they are deflected due to the rotation of the Earth. As the Earth spins on its axis, a person standing on the equator moves from west to east at around 1600km/hour. At the poles, on the other hand, that person would not move at all, just spin around in place. So, the equator and anything on it moves west to east faster than any other place on Earth. The west to east motion decreases from the equator to the pole.

As winds move away from the equator, their west to east momentum carries them to the east of a true poleward trajectory. In the northern hemisphere they are deflected to the right. In the southern hemisphere they are deflected to the left.

The strength of the coriolis force is zero at the equator, half its maximum strength at 30° latitude, and maximum at the poles. Fast winds and winds covering the greatest distances are deflected the most. In the absence of friction (approximated in the upper atmosphere), the coriolis force would cause the winds to blow parallel to isobars, in circles, clockwise around high pressure and counterclockwise around lows in the northern hemisphere. These isobar parallel circular winds, geostrophic winds, only occur in the upper atmosphere, away from friction with the Earth's surface.

Friction Force: Friction with the Earth's surface only affects wind speed up to altitudes around 500 m (1640 ft). Friction prevents geostrophic winds at low altitude. Rather, low level winds move at an angle across isobars.

The net effect of these forces is that near surface winds spiral outward away from high pressure centers and inward toward low pressure centers.

In the northern hemisphere, where winds are deflected to the right, winds spiral clockwise around high pressure and counterclockwise around low pressure
In the southern hemisphere, where winds are deflected to the left, winds spiral counterclockwise around high pressure and clockwise around low pressure

Low pressure centers are zones of convergence, with winds spiralling inward. These are called cyclones. Tornadoes and hurricanes are strong cyclonic storms. High pressure centers are zones of diveregence, with winds spiralling outward. These are called anticyclones.
Friction with Earth's surface slows the movement of air and alters wind direction. It is greatest at the surface and its influence generally extends to an elevation of 1 - 3 km. Over the sea surface the friction is minimal.
Upper-air winds, called geostrophic winds, blow parallel to the isobars and reflect a balance between the pressure-gradient force and the Coriolis effect. Upper-air winds are faster than surface winds because friction is greatly reduced. Friction slows surface winds, which in turn reduces the Coriolis effect. The result is air movement at an angle across the isobars toward the area of lower pressure.

Pressure Belts

ATMOSPHERIC PRESSURE

The pressure exerted by the atmosphere as a result of its weight on the surface of the earth, expressed in millibars (1000 mb = 1 bar = 1 million dynes per square centimeter).

The average pressure over the earth’s surface at sea-level is 1013.25 mb, equivalent to the weight of a column of 29.92 inches (76.0 centimeters) of mercury at 0°C or to a weight of air 14.66 pounds per square inch or 1033.3 gram per square centimeter.

Two factors leading to the formation of high and low pressure are thermal and dynamic.

A study of the distribution of air pressure reveals that the air pressure is not uniformly distributed over the Earth’s surface. In order to destroy this uneven distribution of pressure, winds are caused. In other words, winds are caused due to uneven distribution of pressure. Winds move from high to low pressure areas.

i. Thermal Control: The sunrays make different angles at different latitudes. Where the rays are more vertical, the amount of insolation received is more, i.e., the place with more vertical rays is heated more than that where the sunrays are less vertical. Such a place, which is hot, heats the air with the result that the air pressure decreases. In this way many belts of high and low pressures are produced.

ii. Dynamic Control: The Earth is rotating about its axis. Due to this rotation the pressure belts shift from their ideal positions. For example, the winds at poles shift towards equator.

Direction of Winds

Had the Earth not been rotating about its axis the winds would have blown in the direction of pressure gradient. The earth on account of its rotation produces a force known as Coriolis force after the name of the Mathematician Coriole. The force displaces the winds from the direction of wind (or pressure gradient). Ferrel formulated a law to deduce the direction of winds in the northern and southern hemispheres. It is known as Ferrel’s law.

According to Ferrel’s law, if we stand with our faces in the direction in which the wind is blowing the wind will turn towards our right hand in the northern hemisphere and towards our left hand in the southern hemisphere.

Buys Ballot’s Law: It relates the wind direction to the position of air pressure areas. According to this law, if we stand with our backs in the direction in which the wind is blowing we will find the area of low pressure towards our left hand in the northern hemisphere and towards our right hand in the southern hemisphere.

Planetary Winds

The general permanent circulation of surface winds throughout the world is denoted by the term planetary winds. The wind belts are basically controlled by the latitudinal pressure belts and by the forces produced by rotation of the earth.

Winds
Planetary Winds	Extent	Nature
Trade winds	5o and 30o N & S	At equator they are humid and warm and cause rainfall
Westerlies	40o and 65o N & S	At 40o N & S-Roaring Forties; At 50o S-Furious fifties At 60oS-Shrieking Sixties cause winter rains on the western margins of temperate landmasses
Polar easterlies	70o N&S to poles	Very cold and directed by local weather disturbances

Doldrums

It is the equatorial belt of variable winds and calmness over the equatorial belt of low pressure lying between 5^osouth and 5^o North latitude. This zone has no pressure gradients to induce a persistent flow of wind.

Trade Winds

The word ‘trade’ comes from th

e Saxon word tredon, which means to tread or follow a regular path. Moving north and south of the equator, the main wind belts are trade winds, covering roughly the zone between 5^oand 30^o north and south. They blow from the subtropical high-pressure areas (horse latitude) towards equatorial low-pressure areas (doldrums). Under the influence of the Coriolis force they blow from the northeast in the northern hemisphere (northeast trades) and from the southeast in the southern hemisphere (southeast trades). They are also called tropical easterlies.

Horse Latitudes [5]

Are the subtropical belts of variable winds and columns that lie between the latitudes 25^o and 35^o south and north. They coincide with the subtropical high-pressure belts. The high pressure is probably caused by the rising air of equatorial latitudes, which after cooling descends here.

Westerlies

Blow from subtropical high-pressure areas (Horse latitudes) to sub-polar low-pressure areas between 35^o and 60^o N and S latitudes. Variable in direction and strength westerlies contain depressions. In the northern hemisphere they blow from southwesterly direction and in the southern hemisphere from north westerly direction. In the northern hemisphere, land masses cause considerable disruption to the Westerlies wind belt. But between 40^o and 60^o S, lies the almost unbroken ocean belt. Westerlies are strong and persistent here, giving rise to the mariner’s expressions, “the roaring forties”, “the furious fifties” and “the screaming sixties”.

Polar Easterlies

Constitute the wind system, characteristic of the arctic and polar zones. They blow from polar high-pressure areas to subpolar low-pressure areas and are north easterly in the northern hemisphere and south easterly in southern hemisphere. .

Monsoon Winds

Derived from the Arabic word ‘mausim’, meaning season. ‘Monsoon’ is applied to winds whose direction is reversed completely from one season to the next. Land masses of Asia and North America powerfully control the temperature and pressure conditions in the northern hemisphere. As pressure conditions control winds, these areas also develop wind system, quite independent of the belted wind system in the southern hemisphere.

Summer Monsoon: During summer, a thermal’ or ‘heat’ low is developed over southern Asia in the lower levels of the atmosphere. It is a cyclone with a considerable airflow. This low pressure attracts rain bearing winds from the Indian ocean towards the continental landmass. These winds after crossing the equator turn to the right to enter the landmass as southwest monsoonal winds bringing most of the yearly rainfall to the Indian subcontinent.

Winter Monsoon: Reverse airflow from that of summers takes place in winter in Asia. The land area is dominated by a strong centre of high pressure, from which there is an outward flow of air. Blowing southward and southeastward towards the equatorial oceans, the winter monsoon brings dry, clear weather for several months.

Jet Streams

The term was introduced in 1947, by Swedish born U.S. meteorologist Car Gustaf Rossby, and stands for a very strong steady westerly wind blowing at high altitudes (6,000 to about 1400 meters above the earth’s surface) just below the tropopause. It is usually confined to a narrow band and its speed reaches up to 350-450kph. The highest speed occurs during winter. There are two main jet streams: (a) polar front jet stream, irregular in its location and commonly discontinuous, (b) subtropical jet stream (between 20o and 30o latitudes, north and south), fairly consistent for a given season. These jet streams play an important role in the formation of monsoons over the Indian subcontinent.

Local Winds

These winds affect only limited area and blow for short periods of time, and are generated by immediate influences of the surrounding area. Most local winds are developed by depressions. Some types of local winds are discussed below.

Land and Sea Breeze: The local wind that blows from sea to land during the day is called the sea breeze. This is due the differential heating between the land and the sea. During the night the land cools more quickly than the sea and a reverse process sets in. Land breeze is a cold wind that blows from the land to the sea (or large lake etc.).

Mountain and Valley Winds: These are local winds, responding to local pressure gradients set up by heating or cooling of the lower air.

Local and Regional winds
	Winds	Region/country	Nature
1.	Fohn	Alps/Europe (Germany)	Dry/Warm
2.	Chinook	Rockies USA & Canada	Dry/Warm
3.	Mistral	Alps/France to Mediterranean Sea(Rhine Valley)	Dry/Cold
4.	Sirocco	N. Africa/Sicily/Italy	Dry/Hot
5.	Khamsin	Egypt/N Africa	Dry/Hot
6.	Harmattan	West Africa/Ghana/Nigeria	Dry/Hot
7.	Norwesters	Bengal/Assam/India	Moist/Host
8.	Berg	South Africa	Dry/Cold
9.	Pampero	Argentina	Dry/Cold
10.	Zonda	Chile/Peru/Brazil	Dry/Warm
11.	Brick filder	Australia	Dry/Hot
12.	Bruan	Siberia/Russia	Dry/Cold
13.	Bora	Italy/Yugoslavia	Dry/cold
14.	Southerly Buster	Australia	Dry/Cold
15.	Samun	Persia/Iran	Dry/Hot
16.	Nevados	Ecuador	Dry/Hot
17.	Nor Wester	New Zealand (South Island)	Dry/Hot
18.	Leveche	Algeria/Morocco	Dry/Hot

Katabatic Winds: A cold downslope wind caused by the gravitational movement of cold dense air near the earth’s surface is a katabatic or drainage wind. Such cold dense air may accumulate in winter over a high plateau or high interior valley. Favorable conditions cause some of this cold air to spill over low divides and flow down as a strong cold wind.

Fohn and Chinook: These result when strong regional winds passing over a mountain range are forced to descend on the lower side with the result that under the adiabatic compression, the air is heated and dried. These winds thaw the winter snow thereby clearing the ground for cultivation.

Wind Terminology

Winds are named after their source. This is, a wind that comes out of the north-east is called a ‘NORTH-EAST WIND’. One coming from the south, even though going toward the north, is called a ‘SOUTH WIND’.

WINDWARD refers to the direction from which the wind blows. The side of something that faces the direction from which the wind is coming is called the ‘WINDWARD SIDE’.

LEEWARD, on the other hand, means the direction toward which the wind is blowing. Thus a ‘leeward shore’ would have offshore winds since it faces the way-winds are blowing.

PREVAILING WINDS; winds can blow from any direction. Yet in some places winds may tend to blow more from one direction than any other. These are called as the ‘PREVAILING WINDS’

FRONTS

It is the zone along which two contrasting air masses meet, which originated in different source areas, therefore have differing temperatures and humidity characteristics. Best known fronts are Polar and the Intertropical Fronts.

Depression is a mass of air whose isobars form an oval or circular shape, with low pressure at the centre. The air converges at the centre and rises to be disposed off. In a depression, the winds rotate anticlockwise in the northern hemisphere. While in the Southern hemisphere, the circular movement of winds is in a clockwise direction. Depressions are rarely stationary and tend to follow definite tracks. They are most influential over the ocean spreads and they weaken as they move over land areas. They are of two types:

i. Temperate Cyclones

ii. Tropical Cyclones

Temperate Cyclones

1. They are found both on land and sea.

2. Their isobars are usually V. shaped.

3. They have a low pressure-gradient.

4. The wind speed is low and never very strong.

5. They occupy areas measuring thousands of square km.

6. They travel from west to east (mainly under Rossby waves).

7. Rainfall is slow. Sometimes heavy showers take place.

8. Rainfall continues for many days.

9. All the sectors (cold front, warm front, warm sector, cold sector) of the cyclone have different temperatures, pressure, humidity etc. characteristics.

10. There are two fronts (cold and warm) in temperate cyclone.

11. More cyclones are produced in winter than in summer.

12. The direction of winds are rapidly changed at the front. Veering and backing of winds take place.

13. There is not a single place where winds are inactive and rains are absent.

14. The energy of the cyclone depends upon the difference of the densities of air masses.

Tropical Cyclones

Tropical cyclone are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges. This is one of the most devastating natural calamities. This system incorporates 6 different geographical locations of its regular development:

Gulf of California (Hurricane)

Gulf of Mexico (Hurricane)

Bay of Bengal & Arabian Sea (cyclone)

Mozambique Channel (Cyclone)

East China Sea (Typhoon)

Coral sea (willy willy)

Tropical cyclones are formed mainly over the warm tropical seas where water temperatures are above 27⁰C and there is presence of coriolis effect. Over the tropical seas these areas of low pressure attract winds which under the influence of coriolis spiral and rise upwards. These rising winds are laden with moisture captured from the ocean surface and due to the upward movement the winds get cooled and there is condensation leading to the formation of rain bearing cumulonimbus clouds. It is during this process of condensation that the latent heat is released and this is the source of energy for the cyclone. Following the low pressure depressions the cyclone moves landwards and on the way intensifies due to constant supply of moisture from the ocean. It strikes land and starts weakening as the moisture supply is cut off and the latent heat is no more available to power the cyclone. The centre of the cyclone is known as ‘eye’. The winds at the eye are calm and there is no rainfall.

Some facts about tropical cyclones

Their isobars are usually complete circles.

The pressure gradient is steep.

The wind speed is about 100 km. per hour or even more.

They have a small area.

They travel from east to west.

Rainfall is heavy.

Rainfall does not last beyond a few fours. If cyclone stays at a place, the rainfall may continue for many days.

The temperature at the centre is almost equally distributed.

The fronts are usually absent in tropical cyclones.

These cyclones are produced in summer than in winter.

The difference of the densities of air masses does not contribute to the energy of the cyclone. Its energy is derived from the latent heat of condensation.

These cyclones are not formed between 5⁰ N and 5⁰ S of the equator due to absence of coriolis force at the equator.

Anticyclones

This is a mass of air whose isobars also form an oval or circular shape, but in which pressure is high at the centre, decreasing towards the outside. Winds in an anticyclone form a clockwise out spiral in the northern hemisphere, whereas, they form an anticlockwise out spiral in the southern hemisphere.

Classification of Anticyclones: According to Hanzlik anticyclones can be divided into two classes:

1. Cold Anticyclones.

2. Warm Anticyclones.

Enhanced F Scale for Tornado Damage

Dr. T. Theodore Fujita first introduced The Fujita Scale in February 1971. He wanted something that categorized each tornado by intensity and area. The scale was divided into six categories.

The Enhanced F-scale still is a set of wind estimates (not measurements) based on damage. Its uses 28 damage indicators. Each damage indicators is further classified into 8 levels of damage. These estimates vary with height and exposure.

FUJITA SCALE			EF SCALE
F Number	Fastest 1/4-mile (mph)	3 Second Gust (mph)	EF Number	3 Second Gust (mph)
0	40-72	45-78	0	65-85
1	73-112	79-117	1	86-109
2	113-157	118-161	2	110-137
3	158-207	162-209	3	138-167
4	208-260	210-261	4	168-199
5	261-318	262-317	5	200-234

Humidity AND PRECIPITATION

It is the degree of water vapours present in the air. For any specified temperature, there is a definite limit to the maximum quantity of moisture that can be held by the air. This limit is known as the saturation point. Humidity can be measured by a hygrometer or sling psychrometer.

Relative Humidity: is the ratio between the amount of water vapour actually present in an air mass and the maximum amount that the air mass can hold at a particular temperature. It is expressed as a percentage. At the saturation point, relative humidity is 100%. It varies inversely with temperature, given a fixed amount of water vapour. Secondly, if an exposed water surface is present, the relative humidity can be increased by evaporation.

Absolute Humidity: is the actual amount of moisture present in air. Amount of water vapor per unit volume of air is usually expressed in grams per cubic meter. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation occurs. As the absolute humidity cannot remain a constant figure for the same body of air, modern meteorology makes us of another measure of moisture content-specific humidity.

Specific humidity: It is the ratio of the weight of water vapour to the weight of moist air. Expressed in units of grams of water vapour per kilogram of moist air, specific humidity is often used to describe the moisture characteristics of a large mass of air.

Precipitation

Formation of water-particles or ice within the cloud, that fall towards earth’s surface is precipitation. It occurs when condensation takes place rapidly within the cloud. Main types are rain, drizzle, sleet, snow and hail.

i. Convectional precipitation is caused by heating of moist air in the lower layers of atmosphere, which rises, expands, and is cooled adiabatically to its dew point. Towering cumulonimbus clouds may form. Convection rain is often accompanied by lightning and thunder. In tropical latitudes this type of rain is usually torrential.

ii. Orographic means, “Related to mountains”. This Precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises on the windward side it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results, when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rain shadow may exist on the leeward side. Several of the important dry deserts of earth are of this type like the Patagonia.

iii. Cyclonic Precipitation (Depression or frontal) occurs when large masses of air of different temperatures meet. The warm moist air of one air mass moves over the cold heavier air of another. Or, it is caused by air rousing through horizontal convergence in an area of low pressure. Cyclonic rain is common throughout the doldrums where the trade winds meet. It is the precipitation along the frontal surfaces of a depression in mid and high latitudes.

Thunderstorm

It is an intense local storm accompanied by lightning and thunder that develops in large cumulonimbus clouds, which result from rapid ascent of air under very unstable conditions. Such conditions occur either at the cold front of a depression or when the ground is intensely heated, and there must always be sufficient moisture in the air for cloud formation.

Measuring Precipitation

Rainfall is usually measured by an instrument called the rain gauge, which can be operated on a simple level merely by setting out a straight side, flat bottomed fan and measuring the depth to which water accumulates in a particular period.

Dew Point

Is the temperature at which the air is fully saturated and below which, condensation normally occurs and water vapour starts to condense to form water droplets. It may be equal to, less than or greater than 0^o C. Dew is the deposition of water droplets on the ground and objects, such as plants near the ground. It occurs when the temperature of the ground surface falls and the air in contact with it is cooled below its dew point. Water vapours from the air or diffused from the soil then gets condensed and are deposited as droplets. The favourable conditions are moist air, light winds and clear skies at night to ensure maximum cooling by radiation.

Frost

Is a weather condition that occurs when the air temperature is at or below 0^oC. Moisture on the ground surface and objects freezes to form an icy deposit. Conditions favourable for its formation are similar to those in the case of dew formation.

Fog

Cloud that collects at the surface of the Earth composed of water vapours that have condensed on particles of dust in the atmosphere. Cloud and fog are both caused by the air temperature falling below dew point. The thickness of fog depends on the number of water particles it contains.

Officially, fog refers to a condition when visibility is reduced to 1 km/0.6 mi or less, and mist or haze to that

Giving a visibility of 1-2 km or about 1 mi.

Types of Fog

An advection fog is formed by the meeting of two currents of air, one cooler than the other, or by warm air flowing over a cold surface. Sea fogs commonly occur where warm and cold currents meet and the air above them mixes. A radiation fog forms on clear, calm nights when the land surface loses heat rapidly (by radiation); the air above is cooled to below its dew point and condensation takes place. A mist is produced by condensed water particles, and a haze by smoke or dust.

Smog

Also called smoke fog, is a form of fog that occurs in areas where the air contains a large amount of smoke. Smoke particles provide a high concentration of nuclei around which condensation occurs. Condensation can occur around these nuclei even when the air is not saturated and therefore it forms earlier, becomes denser and lasts longer than fog that develops in unpolluted air. The smoke, because of its chemical contents gives an acid taste to fog.

Mist

Is the term for a reduction of visibility between 1-2 km caused by condensation producing water droplets within the lower layers of atmosphere. It is intermediate between fog and haze.

Haze

Is normally formed by water particles that have condensed around nuclei in the atmosphere, but may also be a result of particles of smoke, dust or salt in the air. In meteorology, it is an obscurity of the lower atmosphere that limits visibility to under 2 km but over 1 km.

Clouds

Latin Root	Translation	Example
cumulus stratus cirrus nimbus	heap layer curl of hair rain	fair weather cumulus altostratus cirrus cumulonimbus

Clouds are masses of minute water droplets and/or ice crystals formed by the condensation of water vapour and held in suspension in atmosphere. Condensation which results from cooling, usually takes place around nuclei such as dust, smoke particles and salt. The cooling may be caused by convection, uplift over mountains, or ascent in depressions. Clouds may be present at heights ranging from ground level up to over 13,000 meters.

Types of Clouds

Clouds are classified on the basis of appearance, form and altitude. On the basis of form, there are two major groups: (i) stratiform, or layered types, and (ii) cumuliform or massive, globular types.

Other Classifications

High-Level Clouds: Cloud types include: cirrus and cirrostratus.

Mid-Level Clouds: Cloud types include: altocumulus, altostratus.

Low-Level Clouds: Cloud types include: nimbostratus and stratocumulus.

Clouds with Vertical Development: Cloud types include: fair weather cumulus and cumulonimbus.

Other Cloud Types :Cloud types include: contrails, billow clouds, mammatus, orographic and pileus clouds.

Beaufort Scale

The Beaufort Scale is defined by the Glossary of Meteorology (AMS) as a system of estimating and reporting wind speeds numerically from 0 (calm) to 12 (hurricane).

BEAUFORT WIND SCALE A scale of wind force, devised by Beaufort in 1805, modified in 1926; it is related to the descriptions of wind effects and estimated velocity at 10 m. above the ground.
Scale No.	Wind	Force(m.p.h)	Observed effects
0	calm	0	Smoke rises vertically
1.	light air	1–3	Wind direction shown by smoke drift, but not by vane
2.	light breeze	4–7	Wind felt on face; leaves rustle; wind vane moves
3.	gentle breeze	8–12	Leaves and small twigs in motion; a flag is extended
4.	moderate breeze	13–18	Raises dust; small branches move
5.	fresh breeze	19–24	Small trees sway; small crests on waves on lakes
6.	strong breeze	25–31	Large branches in motion; wind whistles in telephone wires
7.	moderate gale	32–38	Whole trees in motion
8.	fresh gale	39–46	Breaks twigs off trees
9.	strong gale	47–54	Slight structural damages to houses
10.	whole gale	55–63	Trees uprooted. Considerable structural damage
11.	storm	64–75	Widespread damage
12.	hurricane	above 75	Devastation

[1] The difference in height is due to lower gravitational pull at equator and more expansion by heating.

[2] Albedo is the reflection coefficient of a surface and is measured as the percentage of light reflected back by that surface. For snow it stands close to 90, for clouds it varies between 50 to 80, it is 15-20 for forests and meadows and is 30 for deserts.

[3] This means that if you were standing on the Moon looking up at the Earth, our planet would appear almost 3 times brighter than looking at the Moon from Earth.

[4] CFC or chloro fluoro carbons are free radicals and are very reactive and destroy the ozone by actively combining with ozone molecules.

[5] The name horse latitudes probably originated during a period when horses were being shipped from New England to the West Indies. The term was applied to a part of the North Atlantic Ocean where sailing ships were becalmed for long periods of time. Frequently, the horses died of heat and thirst and were thrown overboard.