Synoptic Meteorology, Pressure Patterns and the Indian Monsoon
Synoptic charts show isobars, fronts, pressure centres and troughs. Candidates must interpret pressure systems: a depression (low) has converging cyclonic flow with rising air, cloud and precipitation; an anticyclone has diverging anticyclonic flow with subsidence, clear skies and often haze or fog beneath an inversion. Troughs and ridges are elongated extensions of lows and highs. In the northern hemisphere flow is anticlockwise around a low and clockwise around a high.
Pressure tendency and spacing
Falling pressure indicates an approaching low or front; rapid falls (3 hPa or more in 3 hours) suggest a deepening system. Closely spaced isobars mean strong wind; the geostrophic wind is inversely proportional to latitude and directly proportional to pressure gradient.
The Indian monsoon
India's climate is dominated by the southwest monsoon from about June to September, driven by differential heating of land and sea and by the ITCZ's northward migration. It brings moist southwesterly flow, heavy orographic rain on the windward Western Ghats and a rain shadow to the lee. The northeast monsoon (October to December) brings rain to the southeast coast. Pre-monsoon heat produces dust storms and thunderstorms; the monsoon trough and low-pressure systems (monsoon depressions) bring widespread rain. The Himalayan barrier and the Tibetan Plateau shape the circulation.
Worked example: a chart with tight isobars around a low west of the coast and a falling tendency means strengthening onshore wind and heavy monsoon rain downwind.
Trap: anticyclones are not always cloud-free at the surface - subsidence inversions can trap low stratus and fog.