Climatology — Humidity, precipitation, air masses, cyclones & climate classification

Climatology questions on this topic are rarely isolated definitions. UPSC normally joins humidity, uplift, pressure belts, air masses and storm tracks into one causal chain.

• Moisture as stored energy: Water vapour is a greenhouse gas and also the source of latent heat. When moist air rises, cools and condenses, latent heat is released; this extra heating can deepen convection and help a low-pressure system intensify.
• Three humidity measures: Absolute humidity is actual water vapour per unit volume; specific humidity is water vapour mass per unit air mass; relative humidity is the percentage of water vapour present compared with the maximum possible at that temperature.
• Temperature trap: Relative humidity can rise at night without adding new moisture, because cooler air has lower moisture-holding capacity. A hot afternoon can show lower relative humidity even when the actual vapour content remains high.
• Dew point: The dew point is the temperature to which air must cool for saturation. A high dew point means the air is genuinely moist; it is usually a better discomfort and rainfall clue than relative humidity alone.
• Adiabatic cooling: Rising air expands under lower pressure and cools. Unsaturated air cools at a faster dry adiabatic rate; saturated air cools more slowly because condensation releases latent heat.
• Cloud-to-rain link: Saturation produces cloud droplets, but precipitation needs droplets or ice crystals to grow large enough to overcome updrafts. This is why all clouds do not produce rainfall.
• UPSC trap: Humidity, rainfall and cyclones are not synonyms. Humidity is vapour in the air, precipitation is water falling from the atmosphere, and a cyclone is a rotating low-pressure system.
• Indian relevance: Monsoon rainfall, western disturbances, tropical cyclones, fog over the Indo-Gangetic Plain and orographic rain on the Western Ghats all depend on moisture supply plus a lifting mechanism.
• World relevance: Deserts near subtropical highs, Mediterranean winter rain, mid-latitude frontal rain and equatorial convection are different results of the same moisture-uplift logic.
• Stability clue: Stable air resists vertical motion and favours fog, haze or layered cloud; unstable air supports rising parcels, cumulonimbus clouds and showers. A rainfall question often hides this stability test behind words like clear night, heated surface or cold air over warm sea.
• Latent-heat memory: Evaporation stores energy in invisible form and condensation releases it. This is why humid tropical air can produce much stronger weather than equally warm but dry continental air.