Humidity, evaporation, and condensation

Air containing a large amount of water vapour is humid, so such weather is called humid. Dry weather has little water vapour, while cold or stormy weather does not necessarily indicate high water vapour. Exam tip: the amount of water vapour in air is related to humidity.

When surface area increases, more molecules of the liquid are exposed to air at the same time. Hence, more molecules can escape from the surface and the rate of evaporation increases. Increasing surface area alone does not make evaporation decrease or stop. Exam tip: Clothes dry faster when spread out because their exposed surface area is larger.

Dew and fog form by condensation of water vapour. Remember both with moisture and cooling.

At dew point, air becomes saturated and condensation may begin. It helps in forming dew or clouds.

Humidity is directly related to the amount of water vapour in air. This is a basic idea of the chapter.

In clouds, water vapour condenses into droplets. Larger droplets may fall as rainfall.

The main sources of water vapour are evaporation and plant transpiration. Both are important in the water cycle.

When fog forms, tiny water droplets spread in air and visibility reduces. It is linked with condensation.

Drying of sweat is evaporation. Evaporation requires latent heat, which sweat absorbs from the skin and the body surface. As heat is removed, the skin temperature falls and the body feels cool. Cooling is caused by evaporation, not by sweat freezing. Exam tip: evaporation always absorbs heat from its surroundings.

The governing concept is condensation, the change of water vapour into liquid water when air or a surface cools sufficiently. During a clear, cool night, grass loses heat by radiation and its surface temperature may fall to the dew point. Air touching the grass is then cooled; if it becomes saturated, some water vapour changes into tiny liquid droplets on the blades. These droplets are called dew, so option D is correct. Grass does not manufacture water, air does not disappear, and smoke is unrelated to the process. Dew is different from rain because it forms directly on a surface rather than falling from clouds. The exact amount depends on moisture in the air, cooling, wind, and the surface temperature.

Evaporation is liquid to gas and condensation is gas to liquid. Treat them as opposite processes.

Water vapour can condense to form clouds and rainfall. Therefore it is an important element of weather.

Humidity tells the amount of water vapour in air. Exam tip: connect humidity directly with water vapour.

In evaporation, liquid water changes into vapour and enters the air. Exam tip: remember evaporation as an early step of the water cycle.

Condensation changes cooled water vapour into tiny water droplets. Exam tip: connect clouds and dew with condensation.

Saturated air cannot hold more water vapour. Exam tip: remember saturation as maximum holding capacity.

Warm air can hold more water vapour. Exam tip: remember the direct relation between temperature and holding capacity.

Relative humidity is expressed in percentage. Exam tip: associate percentage with relative humidity.

Strong wind removes moist air near the surface and increases evaporation. Exam tip: remember wind as a factor that increases evaporation.

Cooling reduces the water vapour holding capacity of air and condensation begins. Exam tip: link cooling with condensation.