Humidity, evaporation, and condensation

Condensation changes water vapour into tiny droplets or ice particles forming clouds. It is an important stage before precipitation.

Warm air can hold more water vapour. Therefore more moisture is needed for saturation when temperature rises.

At night, the ground loses heat by radiation and cools the air in contact with it. If this air reaches its dew point, water vapour begins to condense, producing dew or, under suitable conditions, fog. Evaporation generally requires heat, so it is not the direct result of nocturnal radiational cooling. Exam tip: cooling to the dew point indicates condensation.

Condensation begins when air becomes saturated or cools to dew point. Saturation is the basic condition for condensation.

Dust and salt particles provide surfaces for droplet formation. Without them, formation of cloud droplets becomes difficult.

Warm moist air can quickly become saturated on cooling and form droplets. This can form clouds or fog.

One hundred percent relative humidity means air is saturated at that temperature. Cooling in this condition can cause condensation.

Evaporation is a surface process and can occur at suitable temperatures. Boiling occurs throughout the liquid at a specific temperature.

As temperature rises, the water vapour holding capacity of air increases. Thus the maximum limit of water vapour is linked to temperature.

During the day, solar heating raises the temperature of the surface and the dew droplets. On receiving sufficient heat, the liquid water droplets evaporate into water vapour and mix with the air, so the dew appears to disappear. Condensation is the opposite process: it forms droplets from water vapour on a cool surface. Exam tip: the change of liquid water into water vapour is called evaporation.

The cold surface cools nearby air to dew point, so water vapour changes into droplets. This is a simple example of condensation.

Water vapour is not visible in gaseous form, but condensed droplets can be seen. Clouds and fog are droplets, not vapour.

Clouds are made of condensed water droplets and ice particles. Since water vapour is invisible, do not call clouds vapour.

Evaporation and condensation are opposite but connected processes in the water cycle. One sends moisture upward and the other forms clouds.

Moist air rising along a mountain slope can cool to dew point. This causes condensation and cloud formation.

In temperature inversion, cold moist air stays trapped below, so fog clears slowly. This is common in winter.

The amount of water vapour changes with temperature, water sources and air conditions. Therefore it is considered a variable component.

Evaporation increases water vapour in air, and cooling or saturation causes condensation. This is the core idea of the subtopic.

Relative humidity tells how much moisture air holds compared with its capacity. In exams identify it by percentage.

Cool air has lower moisture holding capacity so relative humidity rises. Remember the temperature and humidity link.