Humidity, evaporation, and condensation

For cloud formation air must become saturated or cool to dew point. Then water vapour changes into droplets.

Sunshine raises temperature and fog droplets can turn back into water vapour. Therefore visibility improves.

Water vapour is an invisible gas and clouds are groups of condensed droplets or ice particles. In exams do not write clouds as only vapour.

When relative humidity is high, the air already contains a large amount of water vapour. It can therefore take in less additional vapour from sweat, so the rate of evaporation decreases and sweat dries slowly. Option B incorrectly links high humidity with lowering sweat below its freezing point. Exam tip: higher humidity generally means slower evaporation.

Absolute humidity tells the mass of water vapour in a fixed volume of air. It is linked with the actual amount of moisture.

Specific humidity indicates the amount of water vapour in an air mass. It is considered a useful and stable measure of moisture.

Condensation nuclei provide surfaces for water vapour to change into droplets. Formation of cloud droplets becomes difficult without them.

Evaporation is a process from the surface of a liquid. Its rate can increase when surface area increases.

Dew forms on surfaces and fog forms as tiny droplets in air. Condensation is the basic process in both.

Dew forms as droplets but below freezing water vapour can change into ice crystals. Therefore frost becomes more likely.

Clouds form by condensation and latent heat is released during condensation. This can affect atmospheric instability.

Relative humidity depends on the ratio to capacity. Warm air can hold more moisture so it may remain unsaturated even with high moisture.

High humidity prevents air from taking much extra water vapour. Therefore evaporation of sweat becomes slow.

Below freezing water vapour can deposit as ice crystals on a surface. This is the basis of frost formation.

The amount of water vapour changes with temperature water sources and weather. Therefore it is called a variable component.

Before rainfall water vapour must change into droplets or ice particles. This is done by condensation.

When warm moist air touches a cold surface, the air cools. If it cools to its dew point, the water vapour in it condenses into tiny water droplets, as seen on the outside of a cold glass. Option B is incorrect because air can hold more moisture when it becomes warmer, whereas the air here is cooling. Exam tip: cooling to the dew point leads to condensation.

High humidity reduces air ability to take more moisture and low temperature reduces energy. Together they reduce evaporation.

Moisture alone is not enough because cloud droplets and their growth are also necessary. Treat humidity as an initial condition for rainfall.

On a cool clear calm night grass surface can cool to dew point. Then water vapour settles as droplets.