Absolute humidity
Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 10 "Water in the Atmosphere"
Meaning
Absolute humidity is the actual weight of water vapour in a fixed volume of air. It is measured in grams per cubic metre (g per cubic metre). The vapour comes from evaporation, which is water turning into vapour from seas, lakes and rivers, and from transpiration, which is plants releasing vapour through their leaves. Absolute humidity changes from place to place. It is highest over warm tropical oceans, where there is plenty of water and heat for evaporation. It is lower over dry land and cold regions. It also changes when air expands or is squeezed, because the same vapour then fills a larger or smaller volume.
Example
In summer, the air over the warm Arabian Sea and the Kerala coast around Kochi holds many more grams of vapour per cubic metre than the air over the Thar Desert in Rajasthan. That is why clothes dry slowly in humid Kochi and quickly in dry Jaipur.
Don't confuse with
- Relative humidity: this is the vapour present as a percentage of the most the air could hold at that temperature. If air warms while its vapour stays the same, relative humidity falls. Absolute humidity is a plain weight and does not depend on how much the air could hold.
- Specific humidity: this is the mass of vapour in a unit mass of air, in g per kg. It stays the same when air expands, warms or cools. Absolute humidity changes when the volume of the air changes.