Atmospheric Circulation and Weather

Focus closely on the vertical rate of pressure change, which decreases with height at a rate of 1 mb for every 10 meters vertically. Learn the distinction between warm air columns (which produce low surface pressure but high upper-level pressure) and cold air columns. Skip high-altitude mathematical derivations of hydrostatic balance. Watch out for traps regarding horizontal pressure distribution vs vertical pressure gradients; horizontal gradients are tiny compared to vertical ones, yet they drive all horizontal wind movement.

UPSC tests the three-way balance between Pressure Gradient Force (PGF), Frictional Force, and Coriolis Force. Memorize that the Coriolis force acts perpendicular to wind direction, is zero at the equator, and increases to a maximum at the poles. Understand how Geostrophic winds develop parallel to isobars when Coriolis force balances PGF in the friction-free upper troposphere (above 2-3 km). Traps: UPSC may state that the Coriolis force changes wind speed; it does not, it only deflects wind direction.

Extremely high yield. Study the three-cell model (Hadley, Ferrel, Polar cells) and their corresponding pressure belts (ITCZ, Subtropical Highs, Subpolar Lows). Note that the ITCZ shifts north and south with the apparent movement of the sun, directly modulating the Indian Monsoon. Jet streams are westerly, high-velocity wind bands in the upper troposphere. Trap: UPSC tested a statement claiming jet streams occur only in the Northern Hemisphere; remember they exist in both hemispheres.

Focus on diurnal winds like land/sea breezes and mountain/valley winds. Katabatic winds are cold, dense air masses sinking downslope at night due to terrestrial radiative cooling, while Anabatic winds are warm air moving upslope. Understand the concept of air masses (mT, cP, cT, mP) and the source regions required for their development. Skip extensive catalogs of global regional winds not mentioned in the text, but memorize key types like Chinook, Mistral, and Harmattan.

Understand Frontogenesis and the four front types: cold, warm, stationary, and occluded. Pay special attention to the Occluded front, which forms when a rapidly moving cold front overtakes a warm front, completely lifting warm air aloft. Contrast Extra-Tropical Cyclones (which form along polar fronts, move west-to-east under the influence of westerly winds, and affect large land areas) with Tropical Cyclones. Skip thermodynamic mathematical representations of frontal slopes.

UPSC frequently tests this section. Memorize the precise structural requirements for tropical cyclone genesis: Sea Surface Temperature (SST) above 27°C, presence of Coriolis force (hence, they cannot form directly on the equator), weak vertical wind shear, and pre-existing low-pressure disturbance. Focus on the 'Eye of the Storm': a zone of calm, descending dry air with higher temperatures than its surroundings. Trap: UPSC falsely claimed that the eye is cooler and has rising updrafts.