WATER IN THE ATMOSPHERE
Humidity essentially refers to the concentration of water vapour present in the air. Water vapour is the gaseous form of water, and its presence in the atmosphere influences many aspects of the weather, such as cloud formation, precipitation, and temperature.
There are two main ways to express humidity:
1.Absolute Humidity: This is the measure of the actual amount of water vapour in the air. If you were to take a cubic meter of air and measure the grams of water vapour present, you’d have its ‘absolute humidity’. But this value can be misleading because warm air can hold more water vapour than cold air. So even if two samples of air have the same absolute humidity, their relative humidity can differ significantly if their temperatures are different.
2.Relative Humidity: This compares the actual amount of water vapour present in the air to the maximum amount of water vapour the air can hold at that temperature. If air has 50% ‘relative humidity’, it’s holding half the water vapour it’s capable of holding at that temperature. 100% relative humidity means the air is fully saturated and can hold no more moisture at that temperature.
When air has all the moisture it can hold at a particular temperature, we say it’s ‘saturated’. This threshold is critical for several weather phenomena:
1.Dew Point: This is the temperature at which air becomes saturated, meaning it can’t hold all the moisture in gaseous form and some of it condenses into water droplets. You may notice dew on grass in the mornings; this is because the temperature overnight dropped to the ‘dew point’, causing moisture in the air to condense. The dew point is a good indicator of the atmospheric moisture content. A high dew point indicates high humidity, while a low dew point indicates dry air.