Evaporation, transpiration and latent heat

Detail

1. Evaporation: what it is and why it happens

• Evaporation is the change of water from liquid to vapour (gas).
• Heat is the main cause. Heat gives water molecules enough energy to break away from the water surface and enter the air.
• The first energy source is the sun. The atmosphere and the Earth's surface also supply energy [3].
• About 90% of the moisture in the atmosphere comes from water evaporating from oceans, seas, lakes and rivers. The remaining 10–15% comes from plants (transpiration) [2].

2. Factors that control the rate of evaporation

Factor	Effect on evaporation	Why
Warm air	Faster	Warm air has a higher capacity: it can hold more water vapour before it becomes saturated (full of vapour).
Dry air	Faster	Air with low moisture content has more room to take in vapour.
Moving air (wind)	Faster	Wind removes the saturated layer (air already full of vapour) sitting just above the water and brings in unsaturated (drier) air.
Large water surface	More total evaporation	More surface means more molecules can escape at the same time.

Where evaporation is highest: the subtropical oceans under the trade winds

• These lie at about 20°–30° N and S, under the subtropical high-pressure belts.
• Sinking air there gives clear skies, so strong sunshine reaches the sea → warm water.
• The air is dry, because sinking air warms and its relative humidity falls.
• The trade winds blow steadily over the sea → they keep carrying away the moist layer.
• Result: evaporation is high over these oceans, even higher than at the equator. Near the equator the sky is cloudy, the air is very humid and winds are calm (the doldrums).
• The moisture picked up over these oceans is carried by the trade winds to the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet). The Indian Ocean is the source of the moisture in India's south-west monsoon.

3. Evaporation cools the surface it leaves

• Cause: water needs heat to evaporate. It takes this heat from the surface it is on.
• Process: the surface loses heat, but the vapour carries that heat away.
• Result: the surface becomes cooler.
• Examples:
• Sweating: sweat evaporates from the skin and takes body heat with it.
• Earthen pots (matkas, surahis): clay has tiny pores, so water slowly seeps to the outer surface and evaporates. This cools the water inside. Matkas cool best in the dry heat of north-west India (Rajasthan, Delhi) in May–June. They work poorly in humid coastal places like Kolkata or Chennai, because humid air slows evaporation.
• The same idea is behind desert coolers, which cool well in dry air and poorly during the humid monsoon.

4. Transpiration and evapotranspiration

• Transpiration is the release of water vapour from plants through stomata, which are tiny pores, mostly on the underside of leaves.
• Roots take up water from the soil → it moves up through the stem → it evaporates out of the leaves.
• Almost all of this water goes into the air. About 99% of the water a plant takes up is transpired, and only about 1% is used for growth [3].

• Transpiration does two jobs. It cools the plant, and it pulls water and minerals up to the leaves for photosynthesis [2].

• Evapotranspiration = evaporation (from soil, water bodies and wet leaves) + transpiration (from plants). It is the total water that moves from the land surface into the air [2].

5. Forests recycle rain

• Moisture recycling means that rain falls on a forest, the trees return it to the air through evapotranspiration, and that vapour falls again as rain in the same area or further downwind (in the direction the wind blows).
• Amazon (South America; Brazil, Peru, Colombia and others):
• In the southern and western Amazon Basin, 25–35% of the rain is recycled from inside the basin. In some places up to 50% of the forest's rain is produced by the forest itself [4].
• Historical deforestation is the main reason (52–72% of the cause) for falling rainfall over the southern Amazon. Annual rainfall there has dropped by 8–11% (study published 2026) [4].
• How cutting trees lowers rain:
  • fewer trees → less evapotranspiration → less vapour in the air;
  • the air becomes more stable (it resists rising), so fewer rain clouds form;
  • more moisture flows out of the region → less rain [4].

• Feedback loop: less rain → forest dies back → even less rain. This raises the risk of a tipping point, where parts of the Amazon could turn into savanna [4].

• India: cutting forests in the Western Ghats (Maharashtra to Kerala) and north-east India (Assam, Meghalaya, Arunachal Pradesh and others) can reduce rain locally and downwind in the same way. These are among India's wettest, most forested regions.

6. Latent heat: the "hidden heat"

• Latent heat is heat that is taken in or given out when a substance changes state (solid, liquid or gas) without any change in its temperature. "Latent" means hidden, because a thermometer does not show it.
• Latent heat of vaporisation is the heat needed to turn a unit mass of liquid water into vapour at the same temperature.
• It is about 2,500 joules per gram near 0 °C.
• This heat is stored in the vapour.
• It is released when the vapour condenses back into water droplets, for example in clouds.

The three latent heats

Type	Change of state	Heat taken in	Heat given out
Latent heat of vaporisation / condensation	liquid ↔ vapour	evaporation	condensation
Latent heat of fusion	solid ↔ liquid	melting	freezing
Latent heat of sublimation	solid ↔ vapour directly	ice → vapour	vapour → ice (e.g. frost, ice crystals in clouds)

• Sublimation needs the most heat. It equals fusion plus vaporisation, because it skips the liquid stage.
• Trap: in physics, vapour turning straight into ice is called deposition. NCERT calls this direct change "sublimation".

7. Latent heat moves energy from the surface into the air

Heat budget link (out of 100 units of insolation, the sun's energy reaching the top of the atmosphere):

• The atmosphere absorbs 34 units of the heat given off by the Earth:
• 6 units are absorbed directly from terrestrial radiation (heat given off by the Earth's surface);
• 9 units come through convection and turbulence;

• 19 units come as latent heat of condensation, the single biggest share.

• Chain: the sun heats the ocean → water evaporates and stores latent heat → the vapour rises → it condenses at height → the heat is released into the air, not at the surface.

• This is how heat moves upwards (from the surface into the air) and poleward (from the tropics towards the poles).

8. Latent heat powers storms

• Thunderstorms:
• Warm, moist air rises and condenses → latent heat is released → the rising air stays warmer than the air around it → it keeps rising.

• This builds tall cumulonimbus clouds.

• Tropical cyclones (cyclones in the Bay of Bengal and Arabian Sea, hurricanes in the Atlantic, typhoons in the north-west Pacific):

• The main energy source is the latent heat released when water vapour condenses. Solar heating supplies the energy at the start, by driving evaporation from the warm sea [6].
• Condensation in the eyewall (the ring of tall clouds around the eye) releases latent heat. The storm turns this heat into the energy of its winds, working like a steam engine (Carnot heat engine) [5].
• Released heat makes the air buoyant (lighter, so it keeps rising) → surface pressure falls → more moist air flows in [5][6].
• If latent heat release is more than the energy lost to surface friction (drag between the wind and the surface), the cyclone intensifies. If it is less, the cyclone weakens [5].
• Result on landfall (when the cyclone moves from sea onto land): the supply of moisture is cut and friction increases → the cyclone weakens. Examples are Odisha and Andhra Pradesh coasts after Bay of Bengal cyclones.

Prelims Hooks

• Evaporation is fastest when the air is warm, dry and windy. Calm, humid air slows it.
• Evaporation is highest over subtropical oceans under the trade winds (about 20°–30° N/S), not at the equator.
• About 90% of atmospheric moisture comes from oceans, seas, lakes and rivers. Only about 10–15% comes from transpiration [2].
• Transpiration happens through stomata. Evapotranspiration = evaporation + transpiration.
• Latent heat of vaporisation is about 2,500 J/g near 0 °C. It is absorbed during evaporation and released during condensation, with no change in temperature.
• Heat budget: latent heat = 19 units, the largest share of the 34 units the atmosphere absorbs from the Earth (convection and turbulence = 9, direct absorption = 6).
• Order of latent heat, from highest to lowest: sublimation > vaporisation > fusion.
• Trap: freezing and condensation release heat. Melting, evaporation and sublimation absorb heat.
• The main energy source of a tropical cyclone is the latent heat of condensation, not the sun directly [6].

Mains Points

• Forests as rain-makers (GS-I/GS-III): about 25–50% of Amazon rainfall is recycled by the forest, and deforestation has cut southern Amazon rainfall by 8–11% [4]. The same logic applies to the Western Ghats and north-east India, which feed peninsular rivers and support plantation agriculture. Protecting forests is therefore also protecting water and rainfall.
• Latent heat as the engine of weather: it carries surface heat into the atmosphere (19 of 100 units), moves heat from the tropics towards the poles, and drives thunderstorms and cyclones. Warmer seas mean more evaporation and more latent heat, which can feed stronger cyclones over the Bay of Bengal and Arabian Sea (disaster management, GS-III).
• Evaporative cooling and water use: matkas and desert coolers work best in dry north-west India and poorly in humid coastal areas. High evaporation from reservoirs in semi-arid Rajasthan, Gujarat and the Deccan means lost water, which is a case for water-saving storage and covered canals.
• Evaporation and the monsoon: strong evaporation over the warm Indian Ocean and Arabian Sea supplies the moisture for the south-west monsoon. Changes in sea temperature affect how much rain India gets.