Climatology: Atmosphere and Energy

Composition and Structure of the Atmosphere

The atmosphere is the gaseous envelope surrounding the earth, held in place by the earth's gravitational field. While it extends to a height of roughly 16,000 kilometres, the effective atmosphere that supports weather phenomena and life is confined to the lower 80 kilometres. In terms of composition, the atmosphere consists of three components: gases, water vapour, and aerosols or particulate matter. Among the gases, nitrogen constitutes approximately 78 percent, oxygen about 21 percent, argon 0.93 percent, and carbon dioxide 0.03 percent, with trace amounts of krypton, xenon, neon, and helium making up the rest.

Nitrogen is an inert gas that controls the rate of combustion and forms an essential constituent of amino acids and proteins. Oxygen is required for respiration by most life forms, though the early atmosphere lacked free oxygen. Carbon dioxide, despite constituting only 0.03 percent of the atmosphere, is arguably the most consequential gas for climate. It is transparent to incoming short-wave solar radiation but opaque to outgoing long-wave terrestrial radiation, making it a greenhouse gas. Any gas that exhibits this property - transparency to insolation but opacity to terrestrial radiation - is termed a greenhouse gas; all such gases are efficient absorbers of infrared radiation. This property gives rise to the greenhouse effect, which maintains surface temperatures conducive to life. Since the Industrial Revolution, anthropogenic burning of fossil fuels has raised atmospheric CO₂ from approximately 300 ppm to 410 ppm by 2018, the highest level in at least 800,000 years and possibly 20 million years. The highest historical CO₂ levels were in the Cambrian era, while levels fell to around 180 ppm during the Pleistocene ice ages. This anthropogenic increase drives global warming and climate change.

Water vapour is the most important variable atmospheric constituent, ranging from 0.2 percent to 5 percent by volume. Around 90 percent of atmospheric water vapour is found below 5 kilometres altitude, and about 50 percent below 2 kilometres. Water vapour is itself a powerful greenhouse gas and the primary determinant of weather through its role in condensation and precipitation. Its distribution varies latitudinally, being highest in equatorial regions and lowest in hot desert belts, while it is also greatest during summer afternoons and lowest during cold pre-dawn hours.

Aerosols, or particulate matter, consist of natural particles such as pollen grains, mineral dust, sea salt, and volcanic ash, and anthropogenic particles such as industrial dust, soot, and residues from crop burning. Aerosols serve as cloud condensation nuclei or hygroscopic nuclei, around which water vapour condenses to form cloud droplets that subsequently grow through collision-coalescence into precipitation-bearing clouds. They also absorb, scatter, and reflect both incoming and outgoing radiation, thereby influencing the heat budget. Increasing anthropogenic aerosols have caused global dimming by reducing the intensity of insolation reaching the earth's surface, which has negatively affected photosynthesis and food production. A counterintuitive but important effect is that increased aerosols are associated with reduced rainfall, because the same water is split among a larger number of tiny hygroscopic nuclei, each too small to coalesce into precipitation-sized droplets, and because reduced insolation lowers evaporation and thus the supply of water vapour. This is particularly significant for monsoon-type climates like India's, which depend on differential heating and moisture supply. Aerosols also contribute to acid rain through both wet and dry deposition, and their mixing with fog generates photochemical smog and other secondary pollutants.

Layers of the Atmosphere

The atmosphere is divided into five principal layers based on temperature characteristics. The troposphere is the lowermost and most important layer because all weather and climatic phenomena occur exclusively here. Temperature decreases with altitude in the troposphere at the normal or environmental lapse rate of approximately 6.5°C per kilometre. This decrease occurs because the primary source of atmospheric heating is terrestrial long-wave radiation rather than direct solar absorption, meaning the atmosphere is heated from below; because water vapour, a potent greenhouse gas, is concentrated in the lower troposphere and diminishes upward; and because the density and heat-trapping capacity of the atmosphere decrease with altitude. The troposphere extends to the tropopause, which lies at 16 to 18 kilometres over the equator and 6 to 8 kilometres over the poles, making the tropopause temperature colder over the equator (approximately -65°C) than over the poles (approximately -45°C). High-velocity jet streams flow within the troposphere at heights of 6 to 12 kilometres.

The stratosphere lies above the tropopause and is characterised by temperature increasing with altitude due to the presence of the ozone layer. Stratospheric ozone, concentrated between 20 and 35 kilometres, absorbs harmful ultraviolet radiation, protecting terrestrial life from UV damage and skin cancer. It is termed good ozone to distinguish it from tropospheric ozone, which is a secondary pollutant formed from vehicle emissions. Ozone-depleting substances - principally chlorofluorocarbons, hydrochlorofluorocarbons, and bromofluorocarbons - destroy stratospheric ozone by releasing chlorine and bromine atoms that catalytically break ozone molecules. The greatest depletion has occurred over the polar regions, particularly Antarctica. The Montreal Protocol of 1987 banned CFCs, and the 2016 Kigali Amendment extended the ban to HFCs, which are themselves super greenhouse gases, with complete phase-out targeted by 2048.

The mesosphere lies above the stratosphere and is the coldest layer, with temperatures falling to between -80°C and -100°C. Meteors burn up in this layer due to friction with the increasing density of the atmosphere as they descend. It is separated from the thermosphere by the mesopause, sometimes called the Karman line, which marks the boundary between the inner and outer atmosphere. The thermosphere is the outermost layer, where temperature rises continuously with altitude due to direct absorption of solar radiation by atomic oxygen and nitrogen, though so few molecules exist that the concept of temperature is different from that at the surface. Within the thermosphere, the ionosphere (from about 80 to 400 kilometres) contains electrically charged particles called ions that reflect radio waves back to the earth, enabling long-distance radio communication. The auroras - aurora borealis in the north and aurora australis in the south - occur in the ionosphere as a result of the interaction between the earth's magnetic field and charged particles from solar winds.

Insolation and the Heat Budget

Insolation refers to incoming solar radiation. The amount of insolation received at a location is determined by several factors. The rotation of the earth on its axis, inclined at 23.5 degrees to the perpendicular of its orbital plane (or 66.5 degrees to the plane itself), causes variation in the length of day and night and the occurrence of solstices and equinoxes. On 21 June, the northern summer solstice, the sun's rays fall vertically on the Tropic of Cancer; on 22 December, the southern summer solstice, they fall on the Tropic of Capricorn; and on 21 March and 23 September, the spring and autumnal equinoxes, they fall on the equator, causing equal day and night everywhere except the poles.

The angle of the sun's rays is the most critical factor governing insolation intensity. Vertical or near-vertical rays are concentrated over a smaller surface area and travel through a shorter atmospheric column, experiencing less scattering, absorption, and reflection. Oblique rays spread over a larger area and traverse more atmosphere, losing more energy before reaching the surface. This explains why equatorial regions receive more intense insolation than polar regions. The duration of the day also matters: longer days in summer allow more total insolation to be received even if instantaneous intensity is lower, while short winter days cause greater heat loss than heat gain.

Albedo is the proportion of incoming solar radiation that is reflected back from the earth's surface without contributing to surface heating. It is expressed as a percentage. Light-coloured surfaces have high albedo and reflect more radiation, while dark surfaces have low albedo and absorb more. Albedo increases from the equator toward the poles, because polar ice and snow have much higher reflectivity than tropical forests and soils. Among cloud types, cirrus clouds have the highest albedo because they are composed of ice crystals at high altitude. The order of decreasing albedo broadly follows: cirrus clouds, fresh snow, old snow, deserts, grasslands, deciduous forests, evergreen coniferous forests, tropical evergreen forests, black soils. Higher albedo means less heating of the surface and atmosphere, which is why increasing snow and ice cover through a positive feedback loop (cooling causing more ice, which increases albedo, which causes more cooling) is a major concern in climate science. Artificial albedo enhancement through reflective white-coloured surfaces or stratospheric aerosol injection has been proposed as a geoengineering measure against global warming.

The effects of the atmosphere on incoming solar radiation are fourfold: scattering, in which gas molecules redirect radiation in all directions (the preferential scattering of blue light by nitrogen and oxygen molecules explains why the sky appears blue); absorption, principally by water vapour, dust, and ozone; reflection, both from cloud tops and from the earth's surface; and transmission, where radiation passes through the atmosphere to reach the surface. Collectively, the atmosphere absorbs about 14 percent of incoming solar radiation directly, clouds reflect another 20 percent, and the earth's surface reflects a further 4 percent, leaving about 48 percent of incoming solar radiation to be absorbed at the earth's surface and converted into long-wave terrestrial radiation, which then heats the lower atmosphere.

Temperature: Factors, Distribution, and Inversions

Temperature is the measure of sensible heat at a location, standardised to sea level. The primary mechanism of atmospheric heating is terrestrial long-wave radiation emanating from the earth's surface, supplemented by conduction (direct contact between the ground and the overlying air layer), convection (the dominant mechanism of vertical heat transfer through rising air currents), and advection (the horizontal transport of heat by winds). Among these, convection is the most efficient mechanism for distributing heat vertically through the troposphere.

Six major factors govern the spatial distribution of temperature. Latitude is the most fundamental: in general, temperature decreases from the equator toward the poles as insolation becomes more oblique. However, the highest temperatures on earth are not found at the equator but rather in the subtropical high pressure belts between 25 and 35 degrees North and South, because the equatorial belt has cloud cover and rainfall throughout the year that reduce surface heating. Altitude causes temperature to decrease at the normal lapse rate of 6.5°C per kilometre, explaining why hill stations are cooler than surrounding plains, why Kilimanjaro maintains an ice cap despite its equatorial location, and why the Himalayas are the most heavily glaciated mountain range outside the polar regions. Distance from the sea creates the contrast between continental climates, which experience extreme annual and diurnal ranges of temperature, and maritime climates, which are moderated by land and sea breezes and the thermal buffering of large water bodies. The land-water temperature contrast arises because water is transparent and distributes heat through a deep column, is mobile through currents and waves that redistribute heat, and has a higher specific heat capacity, all of which make it heat and cool more slowly than land. Atmospheric stability and cloud cover affect temperature by reducing insolation through cloud reflection in unstable conditions. Ocean currents carry warm or cold water into adjacent coastal regions, raising or lowering temperatures relative to what latitude alone would predict; the British or West European type of climate owes its anomalously mild winters to the warming influence of the North Atlantic Drift.

Isotherms are imaginary lines connecting places of equal temperature reduced to sea level. They are more regular and widely spaced in the Southern Hemisphere, which is dominated by water and thus has more uniform temperatures, while they are irregular and closely spaced in the Northern Hemisphere due to the greater land-sea contrast. In summer, isotherms bend equatorward over the sea (since the sea is cooler than the land at the same latitude) and poleward over the land, while the pattern reverses in winter. The annual range of temperature is therefore higher in the Northern Hemisphere.

Temperature inversion refers to an anomalous condition in which temperature increases rather than decreases with altitude, producing a negative lapse rate. It occurs exclusively in the troposphere and most commonly very close to the ground. Non-advectional or radiation inversion, also called ground inversion, requires a cold ground surface, long cold winter nights with clear and cloudless skies, dry air near the ground (since water vapour is a greenhouse gas that would trap heat), and the absence of wind movement. These conditions allow the ground to radiate heat rapidly, chilling the overlying air more than the air above. Ground inversion is most persistent in permafrost regions such as the tundra and Arctic year-round, and occurs in winter in middle and subtropical latitudes. It is responsible for fog, dew, and frost during calm winter mornings. Advectional temperature inversion involves air movement and includes valley inversion, where cold dense air drains down mountain slopes at night under gravity and pools in valley floors, making valleys susceptible to frost and undesirable for agriculture and settlement; sea fog, caused by warm moist air moving over a cold ocean surface; and sea smoke, where cold air moves over warm water causing condensation.

Temperature inversions are significant because fog formed under inversions reduces visibility and causes transport disruptions; the trapping of pollutants and aerosols under the inversion layer intensifies smog formation; and the stability associated with inversions suppresses convection and precipitation. London smog and the notorious winter smog of Delhi are both products of temperature inversions trapping pollutants. Photochemical smog differs from classical industrial smog: it is a secondary pollutant formed during the day when vehicle-emitted nitrogen oxides and volatile organic compounds react under sunlight to produce tropospheric ozone, peroxyacetyl nitrates, and aldehydes. These cause respiratory damage, destroy chlorophyll and reduce photosynthesis, corrode plastics, and act as carcinogens.

Pressure Belts and Atmospheric Circulation

Atmospheric pressure is the weight of a column of air per unit area and is measured in millibars, with the average sea-level pressure being 1013.25 mb. Pressure decreases with altitude and is maximum at sea level. The global distribution of pressure is organised into latitudinal belts of alternating high and low pressure that drive the world's wind systems. There are seven principal pressure belts: one equatorial low pressure belt, two subtropical high pressure belts, two subpolar low pressure belts, and two polar high pressure belts. These belts fall into two types based on their origin.

Thermally induced pressure belts arise from differential heating. The equatorial low pressure belt, lying between about 10°N and 10°S, is produced by intense surface heating throughout the year, which causes air to rise through convection. This belt is also known as the Intertropical Convergence Zone because the trade winds from both hemispheres converge here, and as the Doldrums because the rising air leaves the surface relatively calm. The ITCZ shifts northward and southward with the apparent movement of the sun, and during the Southwest monsoon season it occupies the plains of India, where it is also called the monsoon trough. The polar high pressure belts at the poles are thermally induced by intense cooling, which causes air to subside and create surface high pressure.

Dynamically induced pressure belts arise from the mechanics of atmospheric circulation rather than local thermal conditions. The subtropical high pressure belts, located between 25 and 35 degrees North and South, form because air that rose at the equator travels poleward at high altitude, gradually cools, becomes denser, and is forced to subside at these latitudes. The descending air inhibits cloud formation and produces the hot deserts on the western sides of continents, which lie in the belt of offshore trade winds that carry no moisture. This belt is also known as the horse latitudes because the calms made sailing ships carrying horses becalmed for so long that the animals had to be jettisoned. The subpolar low pressure belts, located between 60 and 65 degrees North and South, are dynamically induced by the convergence and forced ascent of the warm westerlies and cold polar easterlies. This belt is called the belt of fronts and is the birthplace of temperate cyclones. All pressure belts except the polar high undergo seasonal shifting northward during the northern summer and southward during the northern winter, following the apparent movement of the sun. This seasonal shift is directly responsible for the Mediterranean type of climate, which transitions between the westerly rainfall regime in winter and the subtropical trade wind dryness in summer.

Global Wind Systems and Jet Streams

Winds are generated and directed by four forces acting simultaneously: the pressure gradient force (PGF), which drives air from high to low pressure perpendicular to isobars; the Coriolis force (CF), which deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; friction, which reduces wind speed and causes surface winds to cross isobars at an oblique angle rather than blow strictly parallel to them; and the centripetal or gravitational force, which draws air toward low pressure centres. The Coriolis force is proportional to the speed of movement and the rotation of the earth; it is zero at the equator and maximum at the poles, which is why tropical cyclones cannot form at the equator despite high sea surface temperatures.

The three global or permanent wind belts are the trade winds, the westerlies, and the polar easterlies. Trade winds originate from the subtropical high pressure belts and blow toward the equatorial low pressure belt. Due to the Coriolis effect they become the northeast trades in the Northern Hemisphere and the southeast trades in the Southern Hemisphere. They blow as onshore winds on the eastern coasts of continents, delivering heavy rainfall and supporting tropical deciduous and monsoon climates, while they are offshore on the western coasts, contributing to aridity and desert conditions. The western coast deserts of Africa (Namibia, Sahara), South America (Atacama, Patagonia), and Australia are all partly produced by the offshore nature of the trade winds on those coasts. The southwest monsoon of India originates as southeast trade winds from the Southern Hemisphere that cross the equator, are deflected by the Coriolis force, and enter India from the southwest.

The westerlies blow from the subtropical high pressure belt toward the subpolar low pressure belt. Because they blow from higher to lower latitudes, they are warm and onshore on the western coasts of continents in middle latitudes, accounting for the mild, wet British or West European type of climate. In the Southern Hemisphere, where these winds encounter no continental barriers between the latitudes of 40 and 65 degrees South, they travel with exceptional speed and ferocity, earning the names Roaring Forties, Furious Fifties, and Shrieking Sixties. The polar easterlies are extremely cold winds originating from polar high pressure belts, flowing toward the subpolar lows. They are responsible for blizzards in North America, where there are no east-west mountain barriers to check their penetration southward; the equivalent winds are called Norther in the United States and Canada, Burran in Siberia, and were colloquially called a Bomb Cyclone in the 2017 North American event.

The tricellular meridional circulation divides atmospheric circulation into three cells per hemisphere. The Hadley cell encompasses the tropical circulation: surface heating at the equator drives air upward, it diverges poleward at high altitude, subsides at the subtropical high pressure belt, and returns to the equator as trade winds. The Ferrel cell covers the middle latitudes: the westerlies carry air poleward from the subtropical high, converge with polar air at the subpolar low, ascend, and return equatorward at altitude. This cell was described by William Ferrel and operates in a thermally indirect sense, driven mechanically rather than thermally. The Polar cell covers the high latitudes: very cold air subsides at the poles, flows equatorward as polar easterlies, and rises at the subpolar low.

Regional and local winds occupy a secondary level of importance below global circulation. The monsoon is the most climatically significant regional wind system, defined by a seasonal reversal of roughly 180 degrees in wind direction. Monsoon climates receive the bulk of their rainfall in a period of four months. Local winds affect only a small area but exert disproportionate influence on local weather. Warm local winds include the Chinook or snow eater on the eastern leeward slopes of the Rocky Mountains, where descending air is warmed adiabatically and melts winter snow, making agriculture possible; the Fohn or Foehn on the northern slopes of the Alps, responsible for Switzerland's reputation as the climatic oasis of Europe; and the Harmattan, a dry warm wind from the Sahara blowing toward the equatorial western coast of Africa, relieving the oppressive humidity of the coastal climate and earning the name doctor wind. The Sirocco originates in the northern Sahara, crosses the Mediterranean picking up moisture, and delivers reddish or blood rain to southern Europe; it is locally known as Laventer in Spain, Khamsin in Egypt, Ghibli in Libya, and Simoom in West Asia. Cold local winds include the Mistral, a cold dry wind funnelled through the Rhone valley in France at up to 120 km/h; and the Bora, a cold dry wind off the eastern coast of the Adriatic that picks up moisture and delivers heavy snowfall over Italy.

Jet streams are narrow, meandering bands of high-speed upper air circulation flowing generally from west to east in a circumpolar manner at altitudes of 6 to 14 kilometres from the earth's surface, as defined by the World Meteorological Organization. They form where the horizontal temperature gradient in the upper atmosphere is greatest, which corresponds to the subtropical and subpolar latitudes. In the upper atmosphere where friction is absent, wind speed increases until the Coriolis force fully balances the pressure gradient force, causing winds to blow parallel to isobars rather than crossing them at an angle - such winds are called geostrophic winds, and jet streams are an example. A fully developed jet stream develops a meandering sinusoidal path known as Rossby waves, which arise from the tendency of cold polar air to penetrate equatorward and warm subtropical air to push poleward. Jet streams also exhibit vertical eddies: a downward eddy creates surface high pressure and stable conditions below, while an upward eddy creates surface low pressure and encourages precipitation and cyclogenesis.

The four major jet streams are the Polar Front Jet Stream (the most powerful), the Subtropical Westerly Jet Stream, the Subtropical Easterly Jet Stream, and the Polar Night Jet Stream. Jet streams are stronger in winter when the temperature gradient between air masses is greater, reaching speeds of 500 to 600 km/h, while they are weaker and more variable in summer. Their significance for India is profound. The Subtropical Westerly Jet carries temperate cyclones from the Mediterranean and Black Sea regions into northwestern India during winter, delivering the western disturbances that provide essential moisture to the rabi crop, especially wheat, and snowfall to the Himalayas that sustains the perennial character of north Indian rivers. The Subtropical Easterly Jet, which flows exclusively over monsoon Asia and is generated by the intense heating of the Tibetan Plateau, helps sustain the southwest monsoon. The bifurcation of the westerly jet around the Tibetan Plateau determines the onset of the monsoon: when its southern branch disappears in early June, the monsoon advances onto the Indian subcontinent. The Polar Night Jet Stream carries tropospheric pollutants into the stratosphere during the polar vortex, contributing to ozone depletion. Modern aviation exploits jet streams for fuel efficiency: westbound flights lose time against the jet but gain it on eastward legs.

Humidity, Condensation, and Precipitation

Humidity describes the moisture content of air. Absolute humidity is the mass of water vapour per unit volume of air, expressed in grams per cubic centimetre. Humidity capacity is the maximum amount of water vapour a parcel of air can hold at a given temperature; it increases with temperature, meaning warmer air can hold more moisture. Relative humidity is the ratio of absolute humidity to humidity capacity, expressed as a percentage. When relative humidity reaches 100 percent, the air is saturated, and the temperature at which this occurs for a given parcel of air is the dew point. Relative humidity is inversely proportional to temperature for a constant amount of water vapour: as air warms, its humidity capacity rises and relative humidity falls; as it cools, humidity capacity falls and relative humidity rises. Condensation and precipitation depend on achieving saturation, so relative humidity is the critical parameter. Air with relative humidity below 20 percent or above 80 percent is injurious to human health, which partly explains why deserts and equatorial regions are sparsely populated despite their other features. Globally, relative humidity is highest in the equatorial belt and lowest in the subtropical high pressure belt, where high temperatures and subsiding dry air combine to suppress moisture.

When a parcel of air rises, it expands as pressure decreases, and in doing so cools without exchanging heat with surrounding air - a process called adiabatic cooling. The dry adiabatic lapse rate is the rate of cooling of an unsaturated parcel of rising air, which is higher (approximately 10°C per km) than the environmental lapse rate of 6.5°C per km. This difference is the basis for atmospheric stability analysis: if a parcel of air lifted from the surface cools faster than the surrounding environment, it becomes denser than its surroundings at any given altitude and returns to its original position - this is the stable condition. The saturated adiabatic lapse rate applies once condensation begins, and is lower (approximately 5 to 6°C per km) because the latent heat released by condensation partially offsets the adiabatic cooling. The comparison of these lapse rates determines whether clouds grow and whether convective rainfall occurs.

Forms of condensation include dew (liquid deposition on surfaces when the ground cools below the dew point on calm clear nights), frost (solid deposition when the dew point is below freezing), fog (a low-lying cloud formed by the cooling of warm moist air from below, associated with temperature inversion), mist (light fog with visibility greater than one kilometre), and clouds. Clouds are aggregations of microscopic water droplets and ice particles held in suspension by air currents. They form when moist air rises and cools adiabatically to the dew point, and water vapour condenses around hygroscopic nuclei to form cloud droplets that grow through collision-coalescence. Cloud seeding, or artificial rainfall, introduces additional hygroscopic nuclei such as silver iodide, potassium iodide, dry ice (solid CO₂), or common salt to accelerate this process.

Clouds are classified by height and form. High clouds (cirrus family, above 7 km) include cirrus, distinguished by their thin fibrous appearance resembling mares' tails and indicating fair weather; cirrocumulus, producing the dotted mackerel sky pattern; and cirrostratus, thin sheet clouds associated with halo phenomena around the sun and moon. Middle clouds (alto family, 2.5 to 7 km) include altostratus, which gives a watery sun and light precipitation, and altocumulus or woolpack clouds indicating fair weather. Low clouds (strato family, below 2.5 km) include stratus, the dense low grey layer; and nimbostratus, the dark rain-bearing version associated with prolonged gentle precipitation. Clouds of great vertical extent include cumulus, large white-domed fair-weather clouds; and cumulonimbus (Cu-Ni), the most energetic cloud type, extending from near the surface to the tropopause, characterised by a flat base and anvil-shaped or cauliflower top. Cumulonimbus is associated with heavy rainfall, cloud bursts, lightning, thunder, and hail. It forms through extreme convective heating of the ground, which is an everyday afternoon phenomenon in the equatorial belt (producing the four o'clock rain) and a seasonal summer phenomenon in middle and higher latitudes, including India, where it generates the thunderstorms known as Andhi in the Great Plains and Kal Baisakhi in Bengal and Assam. Hail forms inside cumulonimbus clouds through the alternate freezing and melting of ice particles as they are repeatedly lofted by updrafts and descend by downdrafts, accumulating layers of ice until they are heavy enough to fall.

The three types of rainfall reflect the three mechanisms by which air is forced to rise, cool, and achieve saturation. Convectional rainfall results from intense surface heating, which drives moist air upward through convection; it is characteristic of equatorial and tropical regions and summer afternoons in middle latitudes, and produces the characteristic heavy, short-duration showers with lightning and thunder. Orographic or relief rainfall occurs when moist air is forced to rise over a mountain barrier, cooling and precipitating on the windward side while the leeward side falls in a rain shadow. For orographic rainfall to occur, the mountain must stand broadly perpendicular to the wind direction; mountain ranges parallel to the wind, such as the Aravallis relative to the southwest monsoon, fail to intercept moisture and leave extensive rain shadows, explaining the aridity of western Rajasthan. Classic Indian rain shadow areas include the narrow drought-prone strip on the eastern leeward side of the Western Ghats, Leh-Ladakh and the Tibetan Plateau in the rain shadow of the Himalayas, and the mid-latitude deserts such as Patagonia behind the Andes. Frontal or cyclonic rainfall is associated with the convergence of contrasting air masses and the formation of fronts.

Air Masses, Fronts, Cyclones, and Anticyclones

An air mass is a large body of air that has acquired broadly uniform temperature, pressure, and humidity characteristics over a homogeneous source region. Source regions require either a continental or oceanic surface of great extent and stable atmospheric conditions that allow the air to remain in place long enough (three to four days) to take on the thermal and moisture properties of the underlying surface. Air masses therefore do not form over the equatorial low pressure belt where the atmosphere is unstable. The four principal source regions are the cold continental interiors of Canada and Russia; cold maritime regions of the Pacific and Atlantic; vast hot dry continental interiors of the Sahara and Arabian deserts; and tropical oceanic regions of the Atlantic, Pacific, and Indian Oceans. Air masses are classified by their temperature relative to their destination (warm air masses move into cooler areas; cold air masses move into warmer areas) and by their origin (continental or maritime). The southwest monsoon of India is a warm maritime air mass originating over the tropical Indian Ocean.

A front is a sloping boundary between two contrasting air masses. Fronts form where contrasting air masses converge, principally in the subpolar low pressure belt where warm westerlies meet cold polar easterlies. The formation of fronts is called frontogenesis, and their decay is frontolysis; the former leads to cyclogenesis and the latter to cyclolysis. A warm front develops when warm air actively advances into cold air territory; being lighter, the warm air rises along a gentle slope, producing stratus and nimbostratus clouds that deliver prolonged, gentle, widespread precipitation. A cold front develops when cold air actively undercuts warm air, vertically displacing it steeply; this generates cumulonimbus clouds and delivers intense but short-duration precipitation over a narrow area, accompanied by thunder and lightning. An occluded front forms when the faster-moving cold front overtakes the warm front, lifting the warm sector entirely off the ground and marking the mature stage and eventual decay of a temperate cyclone. A stationary front forms when two contrasting air masses flow parallel to each other rather than converging.

Cyclones are low pressure centres with closed isobars of increasing pressure outward. Wind flows into the centre in an anticlockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere, due to the Coriolis effect. Temperate or extratropical cyclones originate between 35 and 65 degrees latitude through frontogenesis. They move from west to east under the influence of the westerly jet stream, cover areas up to 3,000 kilometres in diameter, and have heterogeneous temperature sectors. Their wind speeds are relatively low (around 35 km/h) and they lack closed isobars, typically appearing as V-shaped pressure troughs. Precipitation occurs along the fronts: gentle and prolonged along the warm front, heavy and brief along the cold front. They can originate in any season but intensify in winter when temperature contrasts are greatest. Western disturbances affecting northwest India are temperate cyclones carried by the subtropical westerly jet from the Mediterranean and Black Sea regions. In North America, the absence of east-west mountain barriers allows cold polar air to penetrate far southward, where it generates tornadoes and twisters when it meets warm Gulf air; a tornado over water becomes a waterspout.

Tropical cyclones originate in the warm tropical and subtropical oceans between 10 and 30 degrees latitude, excluding the equatorial belt where the Coriolis force is insufficient. Their energy source is the latent heat released by the condensation of warm moist oceanic air; they therefore weaken rapidly on making landfall. They require sea surface temperatures above 27°C, sufficient Coriolis force, a pre-existing disturbance, low vertical wind shear, and anticyclonic conditions in the upper atmosphere. They are called cyclones in the Indian Ocean region, hurricanes in the Atlantic and Gulf of Mexico, typhoons in the South China Sea, Taifuu in Japan, and Willy-Willies in Australia. They are classified as depressions when wind speed is below 60 km/h, cyclonic storms between 60 and 120 km/h, and cyclones above 120 km/h. The central low pressure zone, the eye, is calm, clear, and rainless; surrounding it is the eyewall of dense cumulonimbus clouds producing the most violent weather. Storm surges - abnormal rises in sea level caused by extreme low pressure - are the most destructive element of tropical cyclones, causing coastal flooding, erosion, and agricultural inundation.

The eastern coast of India is more vulnerable to tropical cyclones than the western coast for several reasons. Cyclones originating in the Bay of Bengal and Andaman Sea curve northward and approach the eastern coast, while those in the Arabian Sea tend to curve away from the western coast toward Pakistan and Oman. The Bay of Bengal is warmer than the Arabian Sea because it is enclosed, receives more freshwater from major rivers, and is less agitated by strong upwelling and monsoon winds. Additionally, remnants of Pacific typhoons enter the Bay of Bengal through the South China Sea and intensify existing depressions. The ITCZ does not shift in regions with cold ocean currents, explaining why tropical cyclones are absent on the western coasts of South America, Africa, and Australia. The naming of Indian Ocean cyclones began in 1999, with names assigned by rotation among eight member countries: Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka, and Thailand, in alphabetical order.

Anticyclones are high pressure centres with decreasing pressure outward. In the Northern Hemisphere winds spiral clockwise out of anticyclones; in the Southern Hemisphere they spiral anticlockwise. Descending air in an anticyclone is warmed adiabatically, which inhibits cloud formation and precipitation; anticyclones are therefore associated with stable, dry, cloudless conditions, earning them the description of weatherless systems. They form primarily over the subtropical high pressure belt and the polar high pressure belt. India falls under anticyclonic conditions during winter, as cold temperatures over the Himalayas and north India generate high pressure at the surface; these stable anticyclonic conditions are periodically disrupted by westerly temperate cyclones carrying western disturbances from the Mediterranean region.