D.1. Evaporation And Condensation
- The water vapour in the air is known as humidity and the actual amount of water vapour present in the atmosphere is known as the absolute humidity. The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as relative humidity.
- The amount of water vapour in the atmosphere is added or withdrawn due to evaporation and condensation respectively. Evaporation is the process by which water is transformed from liquid to a gaseous state. The transformation of water vapour into water is called condensation.
- Dew- When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in the air above the surface) such as stones, grass blades and plant leaves, it is known as dew.
- Frost – Frost forms on cold surfaces when condensation takes place below the freezing point (00C), i.e. the dew point is at or below the freezing point.
- Fog – When the temperature of an air mass containing a large quantity of water vapour falls. All of a sudden, condensation takes place within itself on fine dust particles. So, the fog is a cloud with its base at or very near the ground.