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 by changing how effectively the body can regulate its temperature. First, let us talk a bit more about the moisture part of our atmosphere - water vapour.
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 but 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, which can easily be moved around the atmosphere. The water vapour eventually condenses back into water, releasing energy as heat, and this heat is a key component in the formation of storms.
Humidity is essentially the amount of water vapour in the air, so if air has a lot of moisture in it, and it's a warm day, it will feel very close and muggy, making it difficult for us to keep cool.
The amount of water vapour also plays a role in the amount of rainfall we see. This is because rainfall depends on the energy balance in the atmosphere and the availability of small particles for raindrops to form around.
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.
The higher the humidity the greater the water vapour, and the more rain we're likely to see. This is why tropical rain showers are often more frequent and intense than the showers we tend to have in the UK.
Humans and animals
Humidity can affect human health because it affects our thermal comfort - in other words, whether we feel too hot or too cold. When the weather is warm and humidity is high, the body finds it difficult to keep cool because it's harder to remove heat via evaporation of sweat into the air which already contains a lot of moisture.
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.
In a similar way to humans, 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.
Humidity therefore is a very important part of the weather, governing how heavy a rain shower is or how comfortable it feels on a warm day.