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Evaporation

The conversion of liquid water to water vapor. Cools surfaces and powers the atmosphere with moisture. The reason humid heat feels so much worse than dry heat.

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What is Evaporation?

Evaporation is the conversion of liquid water into water vapor (gas) — happening continuously at any temperature when water molecules at the surface gain enough kinetic energy to escape into the air. It is the opposite of condensation (vapor → liquid).

Evaporation has enormous consequences:

For South Asia, evaporation rates are among the world’s highest:

How evaporation works

The physical process:

  1. Water molecules in liquid have varying kinetic energies (thermal motion)
  2. Hottest molecules at the surface have enough energy to escape into the air
  3. They leave the liquid as vapor — taking their heat energy with them
  4. The remaining liquid cools (loses average kinetic energy)
  5. Vapor diffuses away in the air

The rate of evaporation depends on:

Evaporation and South Asian weather

Sea-surface evaporation:

Land surface evaporation:

Reservoir and lake evaporation:

Irrigation system efficiency:

Evaporation and human comfort

Sweating is the human body’s primary cooling mechanism — and it works through evaporation:

  1. Sweat glands secrete water onto skin
  2. Sweat evaporates — absorbs 580 cal/g of heat from skin
  3. Skin cools — heat dissipates from body

In dry conditions:

In humid conditions:

This is why South Asian coastal cities (Mumbai, Karachi, Chennai, Dhaka) during monsoon feel far worse than inland deserts (Jaisalmer, Bikaner) even at higher temperatures.

The wet-bulb temperature quantifies this — at 35°C wet-bulb, evaporative cooling cannot dissipate body heat, and death follows within hours.

Evapotranspiration

For land surfaces with vegetation, evaporation combines with plant transpiration (water released from leaves) into a combined process called evapotranspiration (ET):

Crop water requirements are calculated using ET:

This drives irrigation scheduling and reservoir management across South Asia.

Climate change and evaporation

Climate change is accelerating evaporation:

Implications:

For South Asia, this means a water-stress double whammy — same total rainfall delivered in fewer events, with more evaporation losses in between.

Practical implications

For households:

For agriculture:

For sports/exercise:

Frequently asked questions

Why does sweating cool me down? Because sweat evaporates — and each gram of evaporated water absorbs 580 calories of heat from your skin. This is by far the most efficient way the body sheds heat. Without sweating, you’d overheat in minutes during exertion.

Why does humid heat feel worse than dry heat? Because humid air slows or prevents sweat evaporation. Your body cannot cool itself effectively. Even moderate humid heat (32°C / 80% RH) can cause heat exhaustion, while dry heat (40°C / 20% RH) feels uncomfortable but is rarely dangerous.

Does ice evaporate? Yes — directly from solid to vapor via sublimation. This is why ice cubes in a freezer slowly disappear, and why dry ice (frozen CO₂) goes straight from solid to gas. Sublimation also occurs from snow surfaces in mountains.

Why is South Asian monsoon so wet? Because the Indian Ocean evaporates enormous amounts of water year-round (warm SSTs + tropical sun). When monsoon winds reverse and blow over the subcontinent, they carry this moisture inland. The Indian Ocean’s evaporation feeds 75% of annual rainfall in 4 months.

Where can I see humidity and evaporation data? Mausam Online displays relative humidity on every city page. Specific evaporation data is published by IMD via the Pan Evaporation network. For “feels-like” temperature (which combines humidity + temperature for human comfort), see your city’s hourly forecast on Mausam Online.

Related Terms

humidity condensation water-cycle wet-bulb-temperature feels-like-temperature heat-stroke
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