Humidity, evaporation and condensation

Water vapour makes up 0 to 4 per cent of the air by volume and plays a key role in weather. Water exists in the air as gas, liquid and solid. It enters the air by evaporation from water bodies and transpiration from plants, and returns by condensation and precipitation: a continuous exchange between air, oceans and continents.

Humidity is the water vapour in the air:

• Absolute humidity: the actual amount of water vapour, as weight per unit volume of air, in grams per cubic metre. It varies from place to place.
• Relative humidity: the moisture present as a percentage of the air's full capacity at that temperature. Since warm air can hold more, relative humidity changes as temperature changes. It is greater over the oceans and least over the continents.
• Air holding its full capacity is saturated: it cannot take any more moisture at that temperature.
• The temperature at which a sample of air becomes saturated is its dew point.

Evaporation turns liquid water into vapour. Heat is its main cause. The latent heat of vaporisation is the energy needed to turn a unit mass of liquid into vapour without changing its temperature.

• Warmer air can absorb and hold more moisture; drier air also has more room to take moisture in.
• Moving air replaces the saturated layer with unsaturated air, so more wind means more evaporation.

Condensation turns water vapour back into water. It is caused by loss of heat: when moist air cools until it can no longer hold all its vapour, the extra condenses.

• In free air it happens around tiny hygroscopic condensation nuclei: dust, smoke and sea salt are especially good because they absorb water.
• It also happens when moist air touches a colder object, or when temperature is near the dew point.
• It depends on the amount of cooling and the relative humidity; it is affected by volume, temperature, pressure and humidity. It occurs when air cools to the dew point at constant volume; when both volume and temperature fall; or when moisture is added by evaporation. The most favourable condition is a fall in air temperature.

Example (new): a glass of iced water on a humid afternoon soon has droplets on the outside: the air touching the cold glass is cooled below its dew point, and its extra vapour condenses on the glass.