Wet Outdoor
In the early morning, the ground outdoor is usually wet, although it didn't rain. Why?
Air always has some water vapour in it. The usual measure of this is called Relative Humidity (RH). It is expressed as a percentage of the maximum amount of water vapour the air can hold. If RH is less than 100%, any exposed area of water will evaporate (and so will sweat which is mainly water). If RH is 100%, liquid water is in equilibrium with water vapour, no evaporation occurs.
The maximum amount of water vapour that air can hold is temperature dependant. The lower the temperature, the less vapour. So if a body of air at RH 100% is cooled, some of the vapour has to condense to form liquid water. This why you get beads of moisture on a cool glass on a warm day. The air in immediate contact with the glass is cooled sufficiently to force the RH to 100%. This may not happen if the air was sufficiently dry to start with, eg in a desert.
During the night, the temperature of the air can fall to where the RH reaches 100%, then fog can form. Fog is really just a cloud at ground level. The fog then wets things it comes in contact with, and if you are a late riser the fog has gone but the moisture remains.
On a clear still night, the temperature of the ground can fall below the temperature of most of the air. It does this by radiating heat into space. On a cloudy night the heat gets bounced around by the clouds and on a windy night the air is pushed over the ground enough to keep the ground and the air at the same temperature. When clear, still conditions occur, the layer of air in contact with the ground cools more than the bulk of air, and as a consequence can hit 100% RH and give up moisture as dew or frost.
Note that when frost forms, the water is never in liquid form. It goes straight from vapour to solid (ice).
Note also that although we say the air holds the water vapour, this is not strictly true. On mars where there is very little atmosphere, water behaves pretty much the same way. At the poles in winter the 'humidity' gets to 100% and frost forms.
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