Controls of Evaporation

Evaporation is the conversion of liquid water into vapour from a surface at temperatures below boiling. From open water, its rate is governed mainly by the available energy and the atmosphere's capacity to remove vapour, expressed through the vapour-pressure deficit between the water surface and overlying air.

Energy and temperature controls

Evaporation requires latent heat, so the energy available at the surface sets a fundamental limit.

• Greater solar radiation and other heat inputs generally raise water-surface temperature and accelerate evaporation.
• Warmer water has a higher saturation vapour pressure, allowing more energetic molecules to escape from the surface.
• Cloud cover, shading and heat storage within the water body modify the energy reaching the surface and therefore its temperature.

Moisture and air movement

The vapour-pressure gradient determines how readily water vapour moves away from the surface.

• Low relative humidity produces a larger vapour-pressure deficit and favours faster evaporation; humid air suppresses it.
• Wind removes the moist boundary layer immediately above the water and replaces it with drier air, increasing evaporation.
• Atmospheric turbulence strengthens vertical mixing and prevents vapour from accumulating near the surface.

Surface and water characteristics

• A larger exposed surface area increases total water loss, although it does not by itself raise evaporation per unit area under identical conditions.
• Dissolved salts lower water's vapour pressure, so saline water generally evaporates more slowly than fresh water under comparable conditions.
• Lower atmospheric pressure facilitates molecular escape, but pressure is usually a weaker control than temperature, humidity and wind.