Humidity, evaporation, and condensation

A low-pressure area usually causes air near the surface to converge and rise. As the air rises, surrounding pressure decreases, so the air expands and cools adiabatically. Cooling lowers its capacity to hold water vapour; when saturation is reached, condensation forms cloud droplets or ice crystals. Further growth of these particles can produce precipitation, although rainfall is not guaranteed in every low-pressure area.

A warm sea supplies both heat and water vapour to the lower atmosphere. Moist air can rise through convection; as it cools, water vapour condenses and releases latent heat. This release helps maintain buoyancy and supports further ascent, lowering the mass of air over the surface and encouraging pressure to fall. The exact development of a low also depends on circulation, convergence, and wind shear.

Humidity is the amount of water vapour present in the atmosphere. Remember it as water vapour content in air.

In evaporation, liquid water changes into water vapour. Treat it as change from liquid to gas.

In condensation, water vapour cools and changes into tiny water droplets. This process is important in cloud formation.

Warm air can hold more water vapour. Remember the direct relation between temperature and vapour-holding capacity.

Relative humidity shows how close air is to saturation. It is commonly understood as a percentage.

At dew point, air becomes saturated with water vapour. Further cooling can lead to condensation.

Higher temperature helps water change into vapour faster. Link heat with faster evaporation.

Open water surfaces supply vapour directly to the air. Oceans are major sources of evaporation.

Humid air contains more water vapour. That is why humid weather can feel sticky.

Dry air can take in more water vapour, so evaporation is faster. Connect dryness with faster evaporation.

When relative humidity is high, air already contains much water vapour. So fresh evaporation becomes slower.

In condensation, water vapour cools to its dew point and changes into tiny liquid-water droplets. Therefore, cooling is correct. Heating generally promotes evaporation rather than condensation. Exam tip: clouds, dew and fog are common examples of condensation.

Clouds form when water vapour condenses into droplets or ice particles. Link clouds with condensation.

At night, surfaces cool and water vapour condenses as dew. Dew on grass in the morning is a common example.

Fog forms by condensation of water vapour near the ground. It can be understood as a cloud close to the ground.

Evaporation uses heat, so cooling may be felt. The body feels cooler when sweat evaporates.

Absolute humidity tells the actual amount of water vapour in air. Remember it as real vapour content.

Saturated air holds the maximum water vapour at its temperature. Further cooling may cause condensation.