Humidity

Atmospheric humidity is a measure of water held in the air as a gas. Water can be solid (ice), liquid (water) or a gas (vapour). The vapour component makes up about 99% of all water held in the atmosphere. The air that we breathe is a mixture of gases - mostly nitrogen (78%) and oxygen (21%) with small amounts of carbon dioxide, argon and water vapour among other things.

Warmer air can carry more water vapour than cooler air, if there is plenty of water available. This is because it has more energy to evaporate water into vapour, and keep it in this state. The tropics are very warm and very humid - the air in the tropics contains lots of water vapour. There is very little water vapour over the very cold Arctic and Antarctic. Some very warm regions are also very dry (e.g. the deserts of the Sahara), because there is very little available water to evaporate into vapour, and at about 30 degrees north or south of the equator the air descends from above and is already very dry.

 

Types of humidity

The amount of water vapour in the air can be quantified in three different ways:

Relative humidity

Relative Humidity (RH) is the most common measure of humidity. It measures how close the air is to being saturated - that is how much water vapour there is in the air compared to how much there could be at that temperature. Warmer air can hold more water vapour because there is more energy available. If the RH of the air is 100% then it is fully saturated.

During a period of high temperatures, air with very high RH is very uncomfortable as the saturated air affects our body's cooling mechanism. The air cannot easily contain anymore water as a vapour and so cannot effectively evaporate the sweat from our skin.

In low temperatures, air with very high RH can make us feel cooler. This is because there is more water vapour close to our skin and since water is a much better conductor than dry air, the cold temperature of the air is conducted to our skin, making us feel cooler.

Specific humidity

Specific humidity and the mixing ratio measure the amount of water vapour in the air. They are very similar but the specific humidity is the mass of water vapour in a mass of air (including dry air and water vapour), whereas the mixing ratio is the ratio of the mass of water vapour to mass of dry air (not including water vapour).

Both the specific humidity and the mixing ratio are highest around the equator at around 20 g kg-1, where the air is warm and can contain more water vapour, and lowest (near zero) in the cold polar regions and high in the atmosphere.

Thermal humidity

'Dew point temperature' and 'wet bulb temperature' are also measures of humidity. These are both measures of how close the air is to being saturated. If they are equal to the actual air temperature then the air is saturated and RH is 100 %.

The wet bulb temperature is the traditional way of measuring humidity. It is measured by allowing the air to cool a thermometer exposed to water by evaporation.

The dew point temperature is measured by cooling a surface to the point at which the air condenses out some water vapour - this is the temperature at which the air has become saturated and is akin to the dew seen on grass in early mornings when the temperature has dropped over night.