Climatology: Atmosphere and Winds

Core concept and process

Composition and layers of the atmosphere

By volume dry air is about 78 percent nitrogen, 21 percent oxygen, and small amounts of argon and carbon dioxide; water vapour, dust and ozone vary. Almost all weather, water vapour and dust sit in the lowest layer. The atmosphere is layered by how temperature behaves with height:

• Troposphere: surface to about 12 km (about 8 km at the poles, about 18 km at the equator). All weather, clouds and water vapour. Temperature falls with height. The boundary above is the tropopause.
• Stratosphere: about 12 to 50 km. Holds the ozone layer (about 15 to 35 km), which absorbs ultraviolet radiation, so temperature rises with height. It is stable and cloud-free, which is why jet aircraft cruise in the lower stratosphere. Boundary above is the stratopause.
• Mesosphere: about 50 to 80 km. The coldest layer (down to about minus 90° Celsius near the top); meteors burn up here. Boundary above is the mesopause.
• Thermosphere: about 80 to 400 km, containing the ionosphere (electrically charged layers that reflect radio waves back to earth, making long-distance radio possible). Aurorae form here. Temperature rises sharply.
• Exosphere: above about 400 km, extremely rarefied, merging into outer space.

The homosphere (uniform composition, up to about 80 km) and heterosphere (layered gases above) is an alternative compositional split.

Insolation, temperature and lapse rate

Insolation is incoming solar radiation. It is intense at the equator (rays near-vertical, short path through the atmosphere) and weak at the poles (oblique rays, long path), which drives all the heating gradients. The atmosphere is heated chiefly from below, by terrestrial (longwave) radiation off the warmed surface, not directly by the sun, so the troposphere cools upward at the normal lapse rate of about 6.5° Celsius per kilometre. Where this reverses (warm air over cold), it is a temperature inversion, common on calm clear nights and in valleys.

The heat budget balances incoming shortwave solar radiation against outgoing longwave terrestrial radiation, keeping the earth's average temperature stable. The atmosphere is largely transparent to incoming sunlight but absorbs and re-radiates outgoing heat, the greenhouse effect; the tropics receive a surplus and the poles a deficit, and winds and currents redistribute the heat. Temperature distribution is controlled by latitude, altitude, distance from the sea (continentality), ocean currents, prevailing winds, slope and aspect, and cloud cover. Lines joining equal temperature on a map are isotherms.

Pressure and the global pressure belts

Atmospheric pressure falls with altitude. Pressure on a map is shown by isobars. Four pressure belts repeat in each hemisphere, giving seven belts in all:

• Equatorial low (the doldrums) at the equator: thermally induced; intense heating lifts air, calm surface, heavy convectional rain. This is the rising limb of the Hadley cell and the Inter-Tropical Convergence Zone (ITCZ).
• Subtropical high (the horse latitudes) at about 30° N and S: dynamically induced; descending dry air gives clear skies and the world's hot deserts.
• Subpolar low at about 60° N and S: dynamically induced; warm and cold air converge along the polar front, giving rising air and temperate cyclones.
• Polar high at the poles: thermally induced; intensely cold dense air descends.

The belts shift north in the northern summer and south in the northern winter, following the overhead sun, which drives seasonal wind reversal.

The Coriolis force and the planetary winds

Because the earth rotates, moving air (and water) is deflected: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is Ferrel's law. The deflection is zero at the equator and maximum at the poles. Air flows from high-pressure belts to low-pressure belts as the three permanent (planetary) winds:

• Trade winds: from the subtropical highs toward the equatorial low; they blow from the north-east in the Northern Hemisphere and the south-east in the Southern (steady, hence "trade").
• Westerlies: from the subtropical highs toward the subpolar lows; south-westerly in the Northern Hemisphere, north-westerly in the Southern. Over the open Southern Ocean they are fierce, giving the Roaring Forties, Furious Fifties and Shrieking Sixties.
• Polar easterlies: from the polar highs toward the subpolar lows; cold and dry.

High in the troposphere, narrow fast ribbons of air called jet streams (the subtropical and polar-front jets) steer weather systems; the subtropical westerly jet and its seasonal shift are important for the onset and retreat of the Indian monsoon.

Local and regional winds

Seasonal winds (monsoons) reverse with the season as the pressure belts and land-sea heating shift. Local winds are short-lived and regional, often named, and are a favourite matching question (see table). Sea and land breezes are diurnal: a sea breeze blows from sea to land by day, a land breeze from land to sea by night. Mountain and valley breezes (anabatic up-slope by day, katabatic down-slope by night) work similarly.

Humidity, condensation and precipitation

Evaporation puts water vapour into the air; relative humidity is the percentage of vapour the air holds against its capacity at that temperature. On cooling to the dew point, vapour condenses. Condensation forms include dew, frost, fog, mist and clouds. Clouds are classified by height and form: cirrus (high, wispy), cumulus (heaped, fair-weather), stratus (layered, low), and cumulonimbus (towering thunderstorm cloud, brings heavy rain and hail). Precipitation comes in three types by uplift mechanism: convectional (heating lifts moist air, equatorial afternoons), orographic or relief (air forced up a mountain, wet windward and dry leeward rain-shadow), and cyclonic or frontal (warm air rises over cold along a front).

Air masses and fronts

An air mass is a large body of air uniform in temperature and humidity, named for its source region: continental polar (cP, cold dry), maritime polar (mP, cold moist), continental tropical (cT, hot dry) and maritime tropical (mT, warm moist). Where two contrasting air masses meet, the boundary is a front. A warm front (warm air rising gently over cold) brings widespread, prolonged rain; a cold front (cold air undercutting warm) brings short, heavy, often stormy rain; an occluded front forms when a cold front overtakes a warm one; a stationary front barely moves. The temperate cyclone is the weather system of the polar front, born where polar and tropical air masses clash.

A word on climate classification

Weather is the day-to-day state of the atmosphere; climate is the long-term average (the World Meteorological Organization uses a 30-year normal). Wladimir Koeppen's classification, the standard CAPF reference, groups world climates by temperature and rainfall into five main letters: A (tropical), B (dry), C (warm temperate), D (cold or continental), and E (polar), with sub-letters for the rainfall and temperature pattern. India spans several Koeppen types from Aw (tropical savanna) and BWh (hot desert, the Thar) to Cwg (monsoon with dry winter) and ET (tundra, the high Himalaya). The world climatic zones are treated fully in world physical geography.

Cyclones

A cyclone is a low-pressure system with inward-spiralling winds (anticlockwise in the Northern Hemisphere, clockwise in the Southern); an anticyclone is the opposite, a high-pressure outward-spiralling system.

• Tropical cyclones form over warm tropical oceans (sea-surface temperature above about 27° Celsius), need the Coriolis force so they form near but not on the equator (Coriolis is zero at the equator), and have a calm low-pressure eye ringed by the violent eyewall. They bring torrential rain, destructive winds and storm surge. They are called cyclones in the Indian Ocean and the South Pacific, hurricanes in the Atlantic and the eastern Pacific, typhoons in the western Pacific, and willy-willies in the older Australian usage.
• Temperate (extratropical or frontal) cyclones form along the polar front at the subpolar lows where warm and cold air masses meet, are larger and milder, bring widespread steady rain, and travel eastward with the westerlies. They dominate winter weather in the mid-latitudes, and their western-disturbance variant brings winter rain to north-west India.