Atmosphere, Insolation, Temperature, Pressure Belts and Winds: OAS Climatology Part 1

These climatology notes for OPSC OAS Part 1 cover the atmosphere's composition and vertical structure, insolation and the heat budget, temperature distribution and lapse rates, the global pressure belts, planetary and local winds, and jet streams. Humidity, clouds, precipitation, air masses, cyclones, El Nino and Koppen classification are covered in Part 2.

Composition of the Atmosphere

The atmosphere is a thin envelope of gases held to the Earth by gravity. Nitrogen and oxygen together make up about 99% of dry air by volume.

Water vapour averages about 3% by volume at the surface. It varies from near zero over deserts and polar regions to about 4% in the humid tropics. It is the source of all clouds and precipitation and the atmosphere's most important absorber of long-wave radiation.

SSC 2011 asked what holds atmospheric air to the Earth — gravity. WBCS 2018 asked the average share of water vapour — 3%.

Vertical Structure of the Atmosphere

The boundary between the troposphere and the stratosphere is the tropopause, a zone of mixing. The troposphere is thickest over the equator because of strong convection, and thinnest over the poles. The stratosphere's temperature inversion acts as a lid that suppresses vertical convection, which is why there is no weather there. The mesosphere cools with height because solar heating falls and radiative emission to space increases.

CDS 2009 and CDS 2017 asked where ozone is concentrated — the stratosphere. Astt. Comm. 2018 asked the layer with the lowest temperature — the mesosphere.

Insolation, Solar Constant and the Heat Budget

Insolation at the top of the atmosphere depends chiefly on latitude, because latitude fixes the angle of incidence and the length of the day. Oblique rays spread the same energy over a larger area and pass through a longer atmospheric path, so the equator receives far more energy per unit area than the poles.

Of the incoming solar radiation, roughly 30% is reflected back to space. This reflected fraction is the Earth's albedo. The rest is absorbed by the atmosphere, clouds and the surface, and is eventually re-radiated as long-wave terrestrial radiation.

The atmosphere is largely transparent to short-wave solar radiation but absorbs long-wave terrestrial radiation through water vapour, carbon dioxide, methane and ozone. This is the greenhouse effect. The radiation window, roughly between 8 and 12 micrometres, lets part of the terrestrial radiation escape directly to space.

CDS 2013 asked what the 30% of solar radiation returned to space is called — the Earth's albedo.

Temperature: Lapse Rates and Distribution

The normal lapse rate in the troposphere is about 6.5°C per km of ascent, roughly 1°C per 165 metres. Adiabatic cooling and warming result from the expansion and compression of air, not from heat exchange with the surroundings.

Temperature distribution is shaped by latitude, altitude, distance from the sea, ocean currents, prevailing winds and vegetation. Five regularities worth memorising:

• Temperature decreases from the equator to the poles.
• The annual range of temperature is least at the equator and greatest in continental interiors at high latitudes.
• The equatorial belt shows the least variation in incoming solar radiation through the year, because the noon Sun stays high.
• Coastal areas have a smaller seasonal range than interiors because water heats and cools more slowly than land — the maritime versus continental effect.
• The thermal equator lies north of the geographical equator because the Northern Hemisphere has more land.

CDS 2018 asked what causes adiabatic temperature change — expansion and compression of air. IAS 2010 asked where the annual range of temperature is least — the equator.

Atmospheric Pressure and the Global Pressure Belts

Pressure is the weight of the overlying air column. It falls with altitude, so at high altitude water boils below 100°C, which is why a pressure cooker is needed there.

The Intertropical Convergence Zone (ITCZ) is the belt where the north-east and south-east trade winds converge near the equator. It shifts with the apparent movement of the Sun, northward in July and southward in January, and its shift is greater over land than over sea because land heats faster.

In the Hadley cell, air rises at the ITCZ, moves poleward aloft, and descends over the subtropical highs, feeding the trade winds at the surface. The Ferrel cell lies between 30° and 60°, and the polar cell between 60° and the pole.

MPPSC 2018 asked where the ITCZ is located — at the equator. CDS 2020 asked where Hadley cell air finally descends — the subtropical high-pressure cells.

Planetary Winds and the Three-Cell Model

Wind is air moving from high to low pressure. Three forces shape surface winds: the pressure gradient force, which drives them; the Coriolis force, which deflects them right in the Northern Hemisphere and left in the Southern; and friction, which slows them near the ground. Above the friction layer, geostrophic wind blows roughly parallel to the isobars.

The energy that drives the whole atmospheric circulation comes from the difference in energy content between the high-insolation tropics and the low-insolation poles.

The Southern Hemisphere westerlies are stronger and more persistent than the Northern Hemisphere ones chiefly because the Southern Hemisphere has far less land to break the flow. The Roaring Forties, Furious Fifties and Shrieking Sixties are the great wind belts of the Southern Ocean.

The south-west monsoon of the Northern Hemisphere summer owes its direction to the Coriolis force acting on air drawn from the Indian Ocean towards low pressure over north-west India.

IAS 2009 asked which is a planetary wind — trade winds. NDA 2018 asked about the Roaring Forties — strong Southern Hemisphere westerlies.

Local Winds and Their Regions

Windward slopes are wetter than leeward slopes because moist air is forced to rise, cool and precipitate; the leeward side lies in the rain shadow. A descending wind is warm and dry, which is why Chinook, Fohn and Santa Ana raise temperatures sharply.

NDA 2019 asked where the Shamal blows — Mesopotamia. UPPSC 2019 asked which wind is wrongly matched — Mistral with Australia, when Mistral belongs to France.

Jet Streams

Jet streams are narrow, fast bands of upper-tropospheric wind, found near the tropopause at about 8–15 km altitude, and are continuous over long distances. The polar front jet attains its maximum strength in winter, when the pole-to-equator temperature contrast is greatest, not in summer.

Three jet streams matter for India: the subtropical westerly jet, which lies south of the Himalaya in winter and helps steer western disturbances; the tropical easterly jet, which dominates the upper air over India in summer and supports monsoon rainfall; and the polar front jet, which influences temperate weather. The jet stream's northward shift in summer is linked to the onset of the monsoon.

Odisha Angle: Heat, Local Winds and Early Warning

Odisha's climate shows a sharp contrast between the moderated coastal belt and the extreme interior western districts. Titlagarh in Balangir district is a byword for summer heat, with maxima regularly crossing 45°C, while coastal Bhubaneswar and Puri have a much smaller annual range because of the maritime effect.

Western Odisha experiences the hot, dry Loo in May and early June. The coastal belt receives pre-monsoon thunderstorms called Kalabaisakhi, the Nor'westers of April and May, which bring sharp showers, lightning and gusty winds, causing casualties and crop damage. The India Meteorological Department's Regional Meteorological Centre at Bhubaneswar issues heat wave, thunderstorm and cyclone warnings for the state, and Odisha has one of India's earlier state heat wave management plans.

Practice Questions