Humidity, evaporation, and condensation

Highly humid air cannot take much more water vapour, so evaporation slows down. Evaporation is faster in dry air.

In condensation, cooled water vapour changes into water droplets or ice particles. This process is important in cloud formation.

Small particles like dust or salt act as condensation nuclei on which droplets form. Remember the role of nuclei in cloud formation.

At night, when the surface cools to dew point, water vapour changes into droplets. Calm and clear nights favour dew formation.

Fog is a mass of tiny water droplets in air near the ground. It can be called a cloud near the ground.

Fog is denser than mist and reduces visibility more. In exams, consider visibility as the main basis.

Frost forms when the surface cools below freezing and water vapour changes directly into ice crystals. Distinguish it from dew.

Evaporation needs latent heat for water to become vapour. That is why evaporation can produce cooling.

During condensation, water vapour releases latent heat while forming droplets. This is important for atmospheric energy.

Rising air expands under lower pressure and cools, which can cause saturation and condensation. This process appears in mountain rainfall.

A large open water surface exposes more water to air, so evaporation increases. Treat surface area as a key factor.

Solar heat gives energy to water molecules so they change into vapour. Evaporation is generally faster at higher temperature.

High humidity slows evaporation of sweat and causes sultriness. This is a useful example from human experience.

Evaporation of sweat takes heat from the body, producing cooling. In high humidity, this cooling is reduced.

Relative humidity is found from the temperature difference between dry and wet bulbs. This instrument is called a psychrometer.

A large difference shows more evaporation from the wet bulb and relatively dry air. Therefore relative humidity is low.

When the difference is very small, evaporation is low because air is nearly saturated. This means relative humidity is high.

The sea is a huge water source, so continuous evaporation occurs from it. Coastal air usually contains more moisture.

Deserts have few water sources and high temperature, so air moisture remains low. Low humidity gives air higher evaporation capacity.

Evaporation carries water from the surface to the atmosphere. It is a key early step in understanding the water cycle.