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