Humidity, evaporation, and condensation

Dew is a form of condensation. When water vapour in the air cools to its dew point on a cold surface, it changes into tiny liquid water droplets. In contrast, evaporation is the change of liquid water into water vapour. Exam tip: the change of water vapour into liquid water is called condensation.

Wind removes moist air above a water surface, so evaporation increases. Clothes dry faster in moving air.

A hygrometer measures the amount of water vapour in the air, that is, humidity. A barometer is a close distractor, but it measures atmospheric pressure, not humidity. An anemometer measures wind speed, while a seismograph records seismic waves. Exam tip: associate “hygro” with moisture or humidity.

Evaporation sends water into the atmosphere as vapour. It later connects with condensation and rainfall.

Plants release water vapour through leaves, called transpiration. It is a source of atmospheric moisture.

In cool air near the ground, water vapour can condense to form fog. It reduces visibility.

Water vapour is water in its gaseous state and occurs in the atmosphere as invisible moisture. Liquid water may form tiny droplets in clouds, fog, or rain, but water vapour itself is a gas. Exam tip: The term ‘vapour’ indicates the gaseous state of water.

The governing concept is condensation: when water vapour loses heat on or near a cold surface, it changes from a gas into tiny liquid water droplets. If these droplets collect on grass, leaves, windows, metal, or another exposed surface, the visible deposit is called dew. Hence option B is correct. A heat wave is a period of unusually hot weather, while dust and sand are solid particles and cannot be produced by the condensation of water vapour. The surface must become sufficiently cool, often near the dew point, for this process to occur. This example also shows that condensation is the reverse of evaporation: evaporation changes liquid water into vapour, whereas condensation changes vapour back into liquid droplets.

At a low temperature, the saturation capacity of air for water vapour decreases; therefore, air can hold less water vapour. When air cools, any vapour exceeding its reduced capacity may condense. ‘Unlimited’ is incorrect because the water-vapour-holding capacity of air is limited and depends on temperature. Exam tip: warmer air can hold more water vapour, whereas cooler air can hold less.

Seas and oceans have vast water surfaces, so much evaporation occurs. Hence they are major vapour sources.

When saturated air cools further, water vapour may condense. This may begin cloud or dew formation.

Relative humidity is generally expressed as a percentage. A higher percentage means the air is more humid.

Sunlight raises temperature and speeds up evaporation. Therefore clothes dry faster in sunlight.

Condensation occurs only when water vapour is present in air. Without vapour, droplets cannot form.

In high humidity, sweat evaporates slowly, so cooling is reduced. Link humidity with evaporation of sweat.

In high humidity, air is already moist and takes less vapour. This slows evaporation.

Water vapour is water in its gaseous, invisible form present in the air. The amount of water vapour in air determines its humidity. Dew, fog and snow are visible forms of water, so they are not correct. Exam tip: remember that invisible atmospheric moisture is water vapour, whereas dew and fog form after condensation.

Tiny cloud droplets form by condensation of water vapour. It is an early stage of rainfall formation.

In evaporation, liquid water changes into gaseous water vapour. It is an early link in the water cycle.

Condensation is the process in which water vapour in the atmosphere cools and changes into liquid water droplets. Therefore, it is a gas-to-liquid change. Liquid changing into gas is evaporation, while solid changing directly into gas is sublimation. Exam tip: dew, fog, and cloud droplets are associated with condensation.