Humidity and temperature

The amount of vapour that the air can contain depends greatly on its temperature: the hotter the air is, the greater the amount of water vapour that it can contain. On cooling a mass of air, it becomes over-saturated and the vapour condenses in the form of microscopic droplets of water. We can appreciate this phenomenon observing the behaviour of the air that we exhale: our breath contains a certain percentage of water vapour, that , at 37°C (our body temperature) is far from saturation point. If we breathe in cold surroundings, the exhaled air cools, becomes over-saturated and the excess vapour condenses in the form of little droplets of water (that we incorrectly call ‘vapour’, but which in actual fact is liquid water: water vapour is a transparent, colourless, invisible gas). If we exhale in warm surroundings, instead, this phenomenon does not occur, even though the quantity of vapour contained in the exhaled air is always more or less constant.

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