Water vapour makes up only a tiny slice of the air. Yet it drives every cloud, every raindrop, and every hailstone that ever falls.
This article covers how that water vapour turns into dew, fog, clouds, and rain — and why some rain falls gently while other rain crashes down as hail.
MCQ Questions🏛️ Must Know
Humidity and Saturation
- Absolute Humidity This is the actual weight of water vapour in a given volume of air, measured in grams per cubic metre.(NCERT 11)
- Relative Humidity This compares how much moisture air holds against its full capacity at that temperature. It is higher over oceans and lower over continents.(NCERT 11)
- Saturation and Dew Point Air is saturated once it holds all the moisture it can at a given temperature. The exact temperature where this happens is called the dew point.(NCERT 11)
- Evaporation and Condensation Evaporation turns liquid water into vapour, adding moisture to the air. Condensation is the reverse, turning vapour back into water and releasing that moisture.(NCERT 11)
Dew, Frost, Fog, and Cloud Types
- Dew This forms when moisture condenses onto cool surfaces like grass, but only when the dew point stays above freezing.(NCERT 11)
- Frost This is dew’s cold-weather cousin. It forms the same way, except the dew point sits at or below freezing, so ice crystals form instead of liquid droplets.(NCERT 11)
- Fog, Mist, and Smog Fog is condensation near the ground that cuts visibility sharply. Mist is simply a wetter version of fog. Smog is fog mixed with urban smoke.(NCERT 11)
- Four Cloud Types Cirrus clouds are thin, feathery, and highest up (8,000-12,000 m). Cumulus clouds look like cotton wool. Stratus clouds are flat layers covering wide areas. Nimbus clouds are dark and rain-bearing.(NCERT 11)
📘 Good to Know
Precipitation: Rain, Snow, Sleet, and Hail
- Rain and Snow Precipitation falls as rain above freezing, or as snowflakes, delicate hexagonal ice crystals, once the temperature drops below 0°C.(NCERT 11)
- Sleet This is frozen raindrops or refrozen melted snow. It forms when a warmer air layer above overlies a colder, subfreezing layer near the ground.(NCERT 11)
- Hailstones These form when raindrops pass repeatedly through very cold layers, building up several concentric rings of ice, one over the other.(NCERT 11)
Test Yourself
🌟 Great to Know
Three Types of Rainfall
- Convectional Rain Heated air rises, cools, and condenses into cumulus clouds, often with thunder and lightning. It is common in the equatorial belt and hot summer afternoons.(NCERT 11)
- Orographic Rain Moist air is forced upward by a mountain, cools, and drops its rain on the windward slope. The leeward slope stays dry, in what’s called a rain-shadow area.(NCERT 11)
- Cyclonic Rain This comes from the rising air along weather fronts, tied to the extra-tropical cyclones covered in the previous chapter.(NCERT 11)
World Distribution of Rainfall
- Equator to Poles Rainfall generally decreases steadily from the equator toward the poles. Coasts also get more rain than continental interiors.(NCERT 11)
- Heaviest Rainfall Zones The equatorial belt, windward mountain slopes in the cool temperate zone, and monsoon coasts all receive over 200 cm a year.(NCERT 11)
- Driest Zones Continental interiors sitting in a rain shadow, along with high-latitude regions, receive under 50 cm a year, the lowest rainfall band on Earth.(NCERT 11)
📝 Exam Point of View
Exam Point of View: Cloud and Sleet Traps
- Real PYQ CDS-II 2023, General Knowledge asked which cloud statements were correct. The trap: it claimed stratus, nimbostratus and stratocumulus were “high clouds,” when they’re actually low clouds.
- Real PYQ The same paper tested cloud height order: nimbostratus, altostratus, cirrocumulus, then cirrus, lowest to highest. Get the tiers right (low, middle, high) and the order follows.
- Real PYQ NDA & NA (I) 2025, General Ability tested sleet formation. The trap: it swapped which air layer sits on top, claiming cold air overlies warm, when it’s actually the reverse.
- The Pattern These questions almost always swap two paired facts (which cloud is high vs low, which layer is warm vs cold). Learn each pair together, not as two separate facts.
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