Water in the Atmosphere

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 10.

Where does the water on a cold glass come from?

Take a glass of chilled water out of the fridge on a humid day. In a few minutes its outside is wet. Nobody poured anything on it, and the glass has no leak. The water was already in the room, as an invisible gas. This chapter explains how air holds water, what makes it let go, and why it lets go in so many different ways: dew on grass, fog on a winter road, thunder-clouds in the afternoon, snow on a mountain.

The big idea: water travels in a loop

Water vapour makes up between 0 and 4 per cent of the atmosphere by volume, yet it drives almost all weather. Water exists in the air in three forms: gas (vapour), liquid (droplets) and solid (ice crystals). The air gets its moisture from water bodies through evaporation and from plants through transpiration. It gives moisture back through condensation and precipitation.

The loop in one line: oceans, rivers and plants → evaporation and transpiration → vapour in the air → condensation (cooling) → clouds → precipitation → land and ocean.

Every question in this chapter is one stage of this loop, so keep asking: is the air taking up water here, or giving it up?

Humidity: how much water the air holds

Water vapour present in the air is called humidity. It is measured in more than one way, and CUET tests the differences between them.

Why relative humidity changes even when no water is added. How much vapour air can hold depends entirely on its temperature: warm air has a bigger "capacity", cold air a smaller one. Relative humidity compares the water actually present with that capacity. So the same parcel of air shows a lower relative humidity when it warms up (capacity rises, content stays) and a higher one when it cools.

Pause and predict: a parcel of air is at 50 per cent relative humidity. It cools until its capacity is exactly halved. What is its relative humidity now, and what happens if it cools a little more?

Check your answer: it is now at 100 per cent, so it has reached its dew point. Cooling any further means the excess vapour must condense into liquid.

Why does water vapour thin out with height? Vapour enters the air from the surface (oceans, lakes, plants), so the lowest layers are the most moist. Air also gets colder with height, and cold air can hold very little vapour. Both reasons push the same way, which is why the amount of water vapour decreases rapidly with altitude.

Evaporation: how water gets in

Evaporation is the change of water from liquid to gas. Heat is its main cause. The energy needed to turn a unit mass of liquid into vapour without any change in temperature is called the latent heat of vaporisation. The energy is not lost; it is stored in the vapour and released again when the vapour condenses.

Three conditions speed up evaporation:

Think of wet clothes. They dry fastest on a hot, dry, breezy day, which has all three conditions at once.

Condensation: how water comes out

Condensation is the change of water vapour into liquid water, and it is caused by loss of heat. As moist air cools, a point arrives where it can hold no more vapour, and the excess condenses.

In free air, vapour does not condense on nothing. It needs tiny particles to collect on, called hygroscopic condensation nuclei. Dust, smoke and salt from the ocean make particularly good nuclei because they absorb water. Condensation can also happen when moist air touches a colder object, or when the air temperature is close to the dew point.

Condensation takes place under three conditions, but one is the most favourable.

The most favourable condition for condensation is a decrease in air temperature. If an option says "increase in temperature", it is wrong.

Beyond the textbook. For the exam: condensation in free air needs cooling to the dew point and hygroscopic nuclei such as dust, smoke and sea salt. In reality: in perfectly clean air, droplets would not form until the air was supersaturated, meaning well above 100 per cent relative humidity. Particles give vapour a surface to start on, which is why clouds form so readily over oceans (salt) and why smoky industrial cities get dense fog. Why it matters: it explains why fog and mist are described as "mini clouds" that depend on nuclei.

Pause and predict: your spectacles fog up when you walk from an air-conditioned room into a humid street. Which of the three condensation conditions is at work?

Check your answer: moist air touching a colder object. The warm outside air next to the cold lens is cooled to its dew point, so vapour condenses on the lens.

Forms of condensation

After condensation, the moisture appears as dew, frost, fog or cloud. These forms are classified by temperature and location. The key split is the dew point: condensation happens when it is above freezing and also when it is below freezing, and the product differs.

The ideal conditions for dew are a clear sky, calm air, high relative humidity and cold, long nights. Frost needs the same conditions, except that the air temperature must be at or below freezing. There is a reason behind each condition. A clear sky lets the ground lose heat freely at night, calm air keeps the chilled layer sitting on the surface, high humidity means little cooling is needed to reach the dew point, and a long night gives the surface more time to cool.

Fog and mist compared. Both form near the ground and differ in visibility and moisture.

The pairing looks backwards, since the thicker-looking one (fog) is the drier one. Learn it as: fog = denser to see through, drier; mist = more moisture on each nucleus.

Beyond the textbook. For the exam: fog cuts visibility to under 1 km, mist to between 1 km and 2 km, and mist holds more moisture. In reality: aviation weather reports use only visibility to separate the two, with fog below 1 km and mist from 1 km up to 5 km. Why it matters: if a question asks you to tell fog from mist, use the visibility figures from the textbook, not a general weather app.

Clouds: condensation in free air

A cloud is a mass of minute water droplets or tiny ice crystals formed by the condensation of water vapour in free air at considerable elevations. Because clouds form at different heights, they take different shapes. They are grouped by height, expanse, density and transparency (or opaqueness) into four basic types.

The Latin roots make the names easy to hold: cirrus is a curl of hair, cumulus a heap, stratus a layer and nimbus rain. Nimbus is the rain-bearing cloud.

The four basic types combine to give the full set of clouds.

Pause and predict: on a hot afternoon a tall, towering cloud builds up and delivers a short, heavy burst of rain with thunder. Which category is it in?

Check your answer: clouds with extensive vertical development, usually cumulonimbus. Strong heating pushes air upward through a great depth, so the cloud grows tall rather than spreading in a layer.

Beyond the textbook. For the exam: cirrus is the highest cloud, at 8,000 to 12,000 m. In reality: the height bands are rough guides that shift with latitude, because the atmosphere is deeper in the tropics than near the poles, so high clouds sit higher in the tropics. Why it matters: it explains why a fixed figure in one source can differ slightly from another, but for CUET the textbook figures are the ones to use.

Precipitation: water returns to the surface

Condensation makes droplets grow. When the resistance of the air can no longer hold them against gravity, they fall. The release of moisture after condensation is precipitation, and it can be liquid or solid.

Sleet and hail are limited in occurrence and sporadic in both time and space.

Beyond the textbook. For the exam: hailstones form when raindrops pass through colder layers and have concentric layers of ice. In reality: hail grows inside tall thunderstorm clouds, where strong rising air carries ice pellets up again and again through freezing levels. Each trip adds a coat of ice, which is why a hailstone cut in half looks like an onion. Why it matters: it connects hail with the convectional rain described next.

Three types of rainfall

Rainfall is classified by origin into three types. The one thing they share is rising, cooling air.

The rain shadow, step by step. Orographic rain is the type that produces the dry "shadow".

The sequence in one line: moist air meets a mountain → forced up the windward slope → expands and cools → condenses → heavy rain on the windward side → air crosses the crest and descends the leeward slope → warms up → its capacity to hold moisture rises → leeward slope stays dry (the rain shadow area).

The air that reaches the leeward side has already lost its moisture, and descending air warms and can absorb moisture rather than release it. A good Indian example is the Western Ghats: the narrow western coastal plain on the windward side gets heavy rain, while the plateau interior to the east lies in a drier rain shadow.

Beyond the textbook. For the exam: each rainfall is labelled as convectional, orographic or cyclonic, according to what makes the air rise. In reality: real storms often mix lifting processes. Rain on a mountain slope may come from air that was already being lifted by a cyclone or by strong heating, and the mountain adds to it. Why it matters: when a CUET question gives one clear cause (a mountain barrier, a front, daytime heating), choose the matching type, and do not worry about the mixing.

World distribution of rainfall

Different places receive different amounts of rain in a year, and in different seasons. The broad rules are these.

The totals are grouped into precipitation regimes.

Seasonal distribution decides how useful rain is, not just the total. Rainfall is spread evenly through the year in the equatorial belt and in the western parts of cool temperate regions. In monsoon lands, by contrast, most rain falls in one season.

Exam traps

60-second revision

Where this chapter connects

• Previous: Atmospheric Circulation and Weather Systems explains the cyclones behind cyclonic rainfall and the westerlies behind the rainfall pattern at 45° to 65°.
• Next: World Climate and Climate Change uses temperature and rainfall together to classify the climates of the world.
• Related: Composition and Structure of Atmosphere covers water vapour as a constituent of the air, and Climate applies orographic rain and rain shadows to India.