Different behaviour

If the air is dry, the majority of solar radiation reaches the ground, which gets heated and which in turn, as a result of a conduction mechanism, heats the air it comes into contact with. The heated air in turn loses heat to the air surrounding it, by means of a convection mechanism, and in this way the heat of the Sun is redistributed through the entire atmosphere.
Things work out differently if the air is humid: the water vapour heats up, in fact, because it is capable of absorbing great amounts of solar radiation directly. In this way the atmosphere gets heated directly by the solar radiation and it, in turn, releases heat to the ground it comes into contact with. Atmospheric dust behaves in the same way, and so do carbon dioxide, methane and those that are commonly referred to as ‘greenhouse gases’. In the same way as they absorb heat emanating from the Sun, these components of the atmosphere prevent the heat originating from the ground in the form of infrared radiation to move away, thus contributing to the heating of the lower levels of the atmosphere, a process that has become well-known as the “greenhouse effect”. This property can be easily verified during the night: we all know that, observing a winter sky bright with stars, we are bound to have a cold night with possible nocturnal frost, while an evening with a cloudy sky will definitely be warmer. The greenhouse effect is therefore a natural process: what is not natural is the great amount of ‘greenhouse’ gases that human activities release in to the atmosphere.

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