Maarg Manthan · Topic 7.6

Water Vapour, Evaporation and Condensation

Geography › Pressure, Winds and Atmospheric Moisture · Topic 7.6

Water vapour is the invisible gaseous form of water in the atmosphere, entering mainly by evaporation from the oceans and leaving by condensation. Humidity measures the amount of vapour in the air, as absolute, specific or relative humidity. Warm air can hold more vapour, so air reaches saturation and condenses at the dew point when it cools, releasing latent heat and forming dew, fog and clouds.

6 min read · UPSC Prelims · Geography · Pressure, Winds and Atmospheric Moisture

At a Glance

Water vapour share 0 to 4 per cent of the air
Humidity types Absolute, specific, relative
Evaporation Liquid to vapour; cools the surface
Condensation Vapour to liquid; releases latent heat
Dew point Temperature at which air is saturated
Exam link UPSC Prelims, Geography: Pressure, Winds and Atmospheric Moisture

Where Does It Fit?

This is the sixth topic of the category on pressure, winds and atmospheric moisture. The previous topic classified winds, and this one begins the study of the moisture that winds carry. The next topic covers clouds, fog and forms of condensation. Previous topic: Planetary, Seasonal and Local Winds. Next topic: Clouds, Fog and Forms of Condensation.

What Is Water Vapour and Why Does It Matter?

In short: water vapour is the invisible gas form of water, and it is the fuel of weather.

Water vapour is water in the gaseous state, mixed in the air. It forms a small part of the atmosphere, varying from almost zero to about 4 per cent by volume, but it plays a large role. It is the source of all clouds and precipitation, it is the most abundant greenhouse gas, and it stores and transports heat as latent heat. Most of it is in the lowest 5 km of the atmosphere. Only about 0.001 per cent of the Earth’s water is in the atmosphere at a time, as vapour, and the rest is in oceans (about 97 per cent), ice and groundwater. The movement of water between the ocean, land and atmosphere is the hydrological cycle, in which evaporation and condensation are the key steps. The composition of the air is in Evolution and Composition of the Atmosphere.

What Are the Types of Humidity?

Term	Meaning
Absolute humidity	The actual mass of water vapour per unit volume of air, in grams per cubic metre
Specific humidity	The mass of water vapour per unit mass of air, in grams per kilogram
Relative humidity	The ratio, as a percentage, of the amount of water vapour in the air to the amount it could hold at the same temperature when saturated
Saturation	The state in which the air holds as much water vapour as it can at that temperature, so the relative humidity is 100 per cent
Dew point	The temperature at which air becomes saturated when cooled, and condensation begins

The capacity of air to hold vapour rises sharply with temperature: for every 1 degree Celsius rise, it increases by about 7 per cent. Therefore warm air can carry far more moisture than cold air. Relative humidity is high in the early morning when the air is cool, and falls in the afternoon as the temperature rises, even if the vapour content is unchanged. It is measured by a hygrometer or psychrometer (wet and dry bulb thermometers). The high capacity of warm air means that a warmer climate can produce heavier downpours, and scientists link the rise in extreme rainfall events to global warming, as in Heat Transfer and Heat Budget of the Earth.

How Does Evaporation Work?

Evaporation is the process by which liquid water changes into water vapour. It takes place at the surface of oceans, lakes, rivers, soils and from plants by transpiration, together called evapotranspiration. The oceans supply about 86 per cent of all evaporation. It needs heat, called latent heat of vaporisation, which is taken from the surroundings, so evaporation cools the surface, as when sweat dries on the skin. The rate of evaporation increases with:

Higher temperature: more energy for molecules to escape.
Lower humidity of the air: dry air takes up more vapour.
Wind: removes the saturated air layer above the surface and brings dry air.
Larger water surface and clear skies: more area exposed and more insolation.

How Does Condensation Occur?

Condensation is the change of water vapour into liquid water or ice. It occurs when air is cooled to its dew point, or when moisture is added until the air is saturated, and when there are tiny particles, called condensation nuclei, such as dust, salt and smoke, on which vapour can condense. The heat absorbed in evaporation is released as latent heat of condensation, which warms the surrounding air, and this is a major source of energy for storms such as cyclones, as in Tropical Cyclones – Formation and Conditions. Air is usually cooled in one of four ways: by contact with a cold surface, by radiation at night, by mixing with colder air, or most importantly by rising and expanding, which is adiabatic cooling, as in Vertical Distribution of Temperature and Temperature Inversion.

Form	Where it forms	Condition
Dew	On cool surfaces near the ground	Clear, calm night; surface cools below the dew point; above freezing
Frost	On surfaces	Dew point below freezing, so vapour turns directly into ice crystals
Fog and mist	Near the ground	Cooling of a layer of air to the dew point
Clouds	Higher in the atmosphere	Rising air cools below the dew point

As an analytical point, condensation requires both cooling and nuclei, so the polluted air in cities may form more fog than clean air, as the number of particles is larger. The forms of condensation are in Clouds, Fog and Forms of Condensation.

Why Is This Topic Important?

Concepts Humidity types asked
Weather Foundation of cloud and rain
Climate change Warm air holds more vapour

What Came Next?

The next topic describes clouds, fog and the forms of condensation. Next topic: Clouds, Fog and Forms of Condensation.

Key People and Terms

Humidity Water vapour content of air
Relative humidity Percentage of vapour relative to capacity
Dew point Temperature of saturation
Latent heat Heat absorbed or released in phase change
Condensation nuclei Particles on which vapour condenses
Evapotranspiration Evaporation plus plant transpiration

Exam Corner

Points to Remember

Water vapour is 0 to 4 per cent of air.
Relative humidity is 100 per cent at saturation.
Capacity rises about 7 per cent per degree.
Oceans supply about 86 per cent of evaporation.
Condensation releases latent heat.

Do Not Confuse With

Absolute and relative humidity Actual amount versus percentage of capacity
Evaporation and condensation Absorbs versus releases latent heat
Dew and frost Above versus below freezing

Memory Hook

Warm air holds more; cool it to the dew point, and the vapour must show.

Mains Angle

Questions ask about humidity and condensation. These points add depth.

Capacity Rises with temperature
Energy Latent heat drives storms
Climate Heavier rain in a warmer world

A Question You May Face

An original practice question, not a past paper question.

“Explain the processes of evaporation and condensation and their role in weather.”

How to Answer

Vapour Types of humidity.
Evaporation Factors.
Condensation Dew point and nuclei.
Role Latent heat and rain.

GS Relevance

Prelims: types of humidity, dew point, latent heat, hydrological cycle. GS Paper 1: climatology.

Frequently Asked Questions

What is relative humidity?

Relative humidity is the ratio, expressed as a percentage, of the amount of water vapour present in the air to the maximum amount it could hold at that temperature. Saturated air has a relative humidity of 100 per cent, and it varies with temperature even when the vapour content is constant.

What is the dew point?

The dew point is the temperature to which air must be cooled for it to become saturated, so that condensation begins. If the air is cooled below it, the excess water vapour condenses into dew, fog or clouds, depending on where it happens.

What factors affect evaporation?

Evaporation increases with higher temperature, lower humidity of the air, wind that removes saturated air, a large exposed water surface and abundant sunshine. It takes up latent heat from the surroundings, so evaporation cools the surface from which it occurs.

What is latent heat?

Latent heat is the heat absorbed or released when water changes its state without a change of temperature. Evaporation absorbs latent heat, and condensation releases it, warming the air and supplying energy to thunderstorms and cyclones.

Why do warm regions have heavier rainfall?

Warm air can hold much more water vapour, about 7 per cent more for every degree Celsius, so there is more moisture available for condensation. When it rises and cools, a warm moist air mass releases large amounts of water and latent heat, producing heavy rainfall.

PYQ Practice — Statement Analysis

1 Warm air can hold more water vapour than cold air.

True

Capacity rises with temperature.

2 Condensation absorbs latent heat from the atmosphere.

False

It releases latent heat.

3 The dew point is the temperature at which air becomes saturated.