People often mention humidity in relation to weather forecasts, but why do we need to know about it and why is it important?
Humidity has an important influence on water vapour and rainfall. It also affects human and animal health.
Water vapour is one of the key greenhouse gases. There is more water vapour than any other greenhouse gas in the atmosphere.
Without water vapour, we would not be able to live on Earth. As a greenhouse gas, water vapour lets energy from the Sun pass through and warm the Earth. The Earth then radiates energy away but some of this energy is trapped on the way out by greenhouse gases adding extra warmth. The average temperature of the Earth at the surface is 14.5 °C. Without any greenhouse gases, it would be around -18°C.
Water vapour is important for moving energy around the Earth. Energy from the Sun evaporates water into water vapour. This water vapour can easily be moved around the atmosphere. The water vapour eventually condenses back into water, releasing energy as heat. This heat is a key component in the formation of storms.
The amount of water vapour plays a role in the amount of rainfall. This is because rainfall depends on the energy balance and the availability of small particles to develop raindrops. During very heavy rainfall, where all of the water in the atmosphere is often rained out, the water vapour content governs the amount of rainfall.
A period of heavy rainfall from air that has lots of water vapour in it is likely to give more rainfall than from air which only contains a little water vapour.
Humans and animals
Humidity can affect human health as it affects our thermal comfort - in other words, whether we feel too hot or too cold. In warm, high humidity situations the body finds it hard to keep cool because it's harder to sweat and remove heat through evaporation into the already moist air.
This can lead to dangerous levels of overheating. Severe illness or even death can result from heat stress, and it can trigger other problems with breathing, heart attacks or strokes.
Humidity also affects animals. For example, high humidity in warm conditions can be a problem for dairy and beef cattle. Such conditions can put the animals under stress because they cannot regulate their body temperature as effectively. This stress can result in a reduced diet, problems with reproduction and even higher mortality rates.