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  1. ITV Report

Why is it so humid?

Enjoying the sun in Leeds Photo: MIKE NEWTON

Muggy. Close. Sticky. Tropical. There are many words to describe humid weather but why do we feel uncomfortable when it's humid? It's all to do with water vapour. Water can exist in three forms - Solid, liquid and gas. Solid water is familiar to us as ice. Liquid water of course comes out of the tap; we can feel it and see it. Water gas, or water vapour is not as familiar to us because it is invisible. Water vapour is around us all the time. The room you are sitting in in full of water vapour.

Enjoying the sunshine in York Credit: KEITH LAVERACK

The process of evaporating water into water vapour requires a lot of energy and our body's cooling mechanism utilises that process. When sweat evaporates from our bodies it takes energy out of us in the form of heat - in other words sweating cools us down, but if there is already a lot of water vapour in the air then that evaporation process is not as efficient. In other words if it's humid we don't cool down as much and we feel uncomfortable as a consequence.

Busy day at Scarborough Credit: PETE WINTON

A good way of improving the evaporation process is to introduce a continuously fresh supply of air, in other words a breeze. So when it's humid and still, it feels very pleasant when a breeze picks up because the evaporation of our sweat is speeded up and we cool down. Tomorrow (Wednesday) will continue to be humid but a fresher supply of air (that is one with a lower water vapour content) will move in from the west late Thursday into Friday. Phew!

JON MITCHELL