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Can't stand the heat? It's the humidity you really hate

David Hawgood via Wikimedia Commons

David Hawgood via Wikimedia Commons

Jen Bartram
Digital News Editor

Thursday, July 17, 2014, 13:15 GMT -

Lots of forecasters this week have talked about the rising temperatures, with some areas of the UK expected to hit 30C on Friday.

However, it’s high temperatures combined with high humidity that cause those really sticky, muggy days and uncomfortably hot nights.

But what is humidity and what part does it play in our weather forecasts?

Humidity is a measurement of water content in the air, in the form of an invisible vapour. The air around us can only hold a limited amount of water vapour before it condenses into clouds or fog. If enough water vapour condenses, then water falls out of these clouds as precipitation.

When a weather forecaster refers to humidity, they generally mean relative humidity – this is the actual amount of water vapour in the air relative to how much moisture the air can hold at that temperature. This figure is given as a percentage.

The higher percentage of relative humidity, the warmer you will feel. But why?

In hot weather, the body needs to sweat to cool down and maintain a safe temperature. Sweat leaves the surface of the body by evaporation, and it’s this process that requires energy in the form of heat, and, in turn, cools the body.

If the relative humidity of the air is high, there is less room in the air for the water vapour from sweat to evaporate, in other words, it’s more difficult for the sweat to leave the body: we feel hot, sticky and stifled. That’s why on days with high temperatures and high humidity, the weather can feel unbearably hot and muggy.

However, if the relative humidity of the air is lower, more sweat will evaporate from our skin and this can have the effect of making things feel cooler than they are on the thermometer.

When the temperature is at or above 18C, relative humidity is taken into account to calculate a ‘feels like’ temperature, which you can see on the forecast page for your area. For example, on a sunny day, a temperature of 26C may feel like 31C.

On the other hand, when temperatures are lower, wind chill is used to calculate a ‘feels like’ temperature, which will appear cooler than the thermometer temperature when the wind is strong.

Over the next few days, the humidity levels are expected to climb along with the temperatures, which means that there’ll be some rather sticky days ahead.

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