Climatology I: The Atmosphere, Insolation and Heat Budget, Pressure and Winds, and Atmospheric Circulation

Insolation is incoming solar radiation, mostly short-wave. At the top of the atmosphere it averages about 1,361 W/m² on a surface perpendicular to the Sun (the solar constant). What reaches a place depends on the angle of incidence, the length of day, the transparency of the atmosphere and, slightly, the Earth–Sun distance — the Earth is closest to the Sun (perihelion, about 147 million km) around 3 January and farthest (aphelion, about 152 million km) around 4 July, which shows that distance does not cause the seasons. Because the Earth's average temperature stays constant, it must lose exactly as much energy as it receives: this is the heat budget.

Satellite measurements put the planetary albedo nearer 30 per cent, but the NCERT figures are the ones Indian papers set. The budget balances for the globe, not for each latitude: roughly between 40°N and 40°S there is a surplus of radiation and poleward of it a deficit, and the imbalance is removed by the poleward transfer of heat by winds (including latent heat) and ocean currents. Temperature at a place further depends on latitude, altitude, distance from the sea, ocean currents, prevailing winds and relief. Isotherms bend equatorward over continents and poleward over oceans in the Northern Hemisphere winter, and the reverse in summer; the thermal equator lies north of the geographical one because the Northern Hemisphere holds more land. A temperature inversion — warmer air above colder — forms on long, clear, calm winter nights as the ground radiates heat (a surface inversion), and cold air draining into valleys creates frost pockets.

Wind is set in motion by the pressure gradient force and shaped by two others. The Coriolis force, an effect of the Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern; its magnitude is 2Ωv sin φ, so it is zero at the equator, greatest at the poles and proportional to wind speed. Friction slows wind near the surface. Above the friction layer (about 1 km), pressure gradient and Coriolis force balance and the geostrophic wind blows parallel to the isobars. Near the surface friction turns wind across the isobars towards low pressure, so air spirals anticlockwise into lows and clockwise out of highs in the Northern Hemisphere. Buys Ballot's law follows: in the Northern Hemisphere, stand with your back to the wind and low pressure is on your left.

The belts are tied together by the three-cell model. The Hadley cell is thermally direct: air rises at the equator, flows poleward aloft, sinks at about 30° and returns as the trade winds. The polar cell is also direct. Between them the Ferrel cell is thermally indirect — air sinks at 30° and rises at 60°, driven by the two cells beside it rather than by heating — and its surface limb is the westerlies. Local and periodic winds ride on this pattern: land and sea breezes, anabatic (valley, upslope, by day) and katabatic (mountain, downslope, by night) winds, warm dry descending winds such as the foehn of the Alps and the chinook of the Rockies, cold winds such as the mistral of the Rhône valley and the bora of the Adriatic, and hot winds such as the loo of the north Indian plains and the sirocco from the Sahara.

An air mass is a large body of air with fairly uniform temperature and humidity at each level, acquired over a source region — a large, uniform surface under stagnant air, such as a warm ocean or a snow-covered continent. Bergeron's classification labels air masses by the nature of the surface (c continental, dry; m maritime, moist) and by latitude (E equatorial, T tropical, P polar, A arctic or antarctic): so cT is hot and dry (the Sahara), mT warm and moist (the Gulf of Mexico), cP cold and dry (Siberia, northern Canada in winter) and mP cool and moist (the North Atlantic and North Pacific). A front is the boundary between two air masses. At a cold front cold air undercuts warm air along a steep surface, giving towering cumulonimbus, short heavy showers and a sharp drop in temperature. At a warm front warm air rises gently over cold air along a slope of about 1 in 100 to 1 in 200, giving a long sequence from cirrus and cirrostratus through altostratus to nimbostratus and steady rain. When the faster cold front overtakes the warm front, the warm air is lifted off the ground in an occlusion; a front that does not move is stationary.

Aloft, the westerlies flow in great meanders, the Rossby waves (usually three to six around the hemisphere), and are concentrated into jet streams — narrow, fast bands near the tropopause. The polar front jet follows the polar front between roughly 40° and 60°; the subtropical westerly jet lies near 25°–35° at about 12 km, and over India it sits south of the Himalaya in winter; the tropical easterly jet appears over peninsular India near 15°N in summer. Jet streams steer surface depressions and their meanders build ridges and troughs that decide where cyclones deepen. Temperate (extratropical) cyclones are frontal: the Norwegian polar-front model of the Bjerknes school traces a wave on a stationary front that grows into an open wave with a warm sector between warm and cold fronts, occludes and dies. Tropical cyclones are non-frontal heat engines fuelled by latent heat from warm seas. Anticyclones are highs of subsiding air with clockwise outflow in the Northern Hemisphere and fair, stable weather; a persistent blocking high can hold a heat wave or drought in place for weeks.