Meteorology   

because the ratio of the fire that is raising it to the water that

is being raised is small. When this cools and descends at night it

is called dew and hoar-frost. When the vapour is frozen before it

has condensed to water again it is hoar-frost; and this appears in

winter and is commoner in cold places. It is dew when the vapour has

condensed into water and the heat is not so great as to dry up the

moisture that has been raised nor the cold sufficient (owing to the

warmth of the climate or season) for the vapour itself to freeze.

For dew is more commonly found when the season or the place is warm,

whereas the opposite, as has been said, is the case with hoar-frost.

For obviously vapour is warmer than water, having still the fire

that raised it: consequently more cold is needed to freeze it.

Both dew and hoar-frost are found when the sky is clear and there is

no wind. For the vapour could not be raised unless the sky were clear,

and if a wind were blowing it could not condense.

The fact that hoar-frost is not found on mountains contributes to

prove that these phenomena occur because the vapour does not rise

high. One reason for this is that it rises from hollow and watery

places, so that the heat that is raising it, bearing as it were too

heavy a burden cannot lift it to a great height but soon lets it

fall again. A second reason is that the motion of the air is more

pronounced at a height, and this dissolves a gathering of this kind.

Everywhere, except in Pontus, dew is found with south winds and

not with north winds. There the opposite is the case and it is found

with north winds and not with south. The reason is the same as that

which explains why dew is found in warm weather and not in cold. For

the south wind brings warm, and the north, wintry weather. For the

north wind is cold and so quenches the heat of the evaporation. But in

Pontus the south wind does not bring warmth enough to cause

evaporation, whereas the coldness of the north wind concentrates the

heat by a sort of recoil, so that there is more evaporation and not

less. This is a thing which we can often observe in other places