Have you ever felt hot and sweaty even when you haven't done any exercise? That might be because of humidity.

But what is humidity?

Humidity is a measure of how much water there is in the air. Not liquid water like rain, invisible water in the form of a gas called water vapour.

Humidity is a measure of how much water there is in the air. Not liquid water like rain, invisible water in the form of a gas called water vapour.

Water is constantly evaporating into the air and turning into vapour. Some of it condenses back into liquid as clouds and rain. Some of it stays hanging in the air as vapour.

Air temperature has a big impact on humidity. Warmer air can hold much more water vapour. So it takes much more vapour to make warm air feel sticky and humid. So when the temperature goes up, the humidity comes down.

The other thing that affects humidity is how much water there is around. It’s much more humid after it has rained, or when you are near the sea or a loch, because there is more water to evaporate into the air. Even a garden pond can increase the humidity!

Different parts of the world have very different kinds of humidity.

Here in the Sahara Desert in Africa, there’s very low humidity. The air is hot and can hold a lot of vapour. But there aren’t any rivers or lakes to supply it. Because there is so little water vapour in the air, the humidity is low. Clouds don’t form so it doesn’t rain or snow and the ground is very dry. This means there aren’t a lot of plants, people or other living things because they all need water to survive.

But not all deserts are hot. Did you know that Antarctica is the largest desert on Earth? It’s so cold that the air wouldn’t need much vapour to become humid. But because it’s so cold, there is no liquid water to evaporate into the air. The humidity is extremely low - the air is very dry, there is little snowfall and only a few things can live here.

Where there is plenty of water vapour in the air there is more chance of life surviving. A good example would be a rainforest. And here in the Amazon it’s very humid! And you can see that there is lots of life here!

When it’s warm, our bodies sweat to cool us down, but if the air is already very humid, then it is less easy for the sweat to evaporate from our skin, so we can’t cool down as easily.

Next time you’re out and about and feeling hot and sweaty, just touch your head - and if it feels warm and sticky that’s probably because of the humidity!