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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
too。 Wells； for instance； give off more vapour in a north than in a
south wind。 Only the north winds quench the heat before any
considerable quantity of vapour has gathered； while in a south wind
the evaporation is allowed to accumulate。
Water； once formed； does not freeze on the surface of the earth；
in the way that it does in the region of the clouds。
11
From the latter there fall three bodies condensed by cold； namely
rain； snow； hail。 Two of these correspond to the phenomena on the
lower level and are due to the same causes； differing from them only
in degree and quantity。
Snow and hoar…frost are one and the same thing； and so are rain
and dew： only there is a great deal of the former and little of the
latter。 For rain is due to the cooling of a great amount of vapour；
for the region from which and the time during which the vapour is
collected are considerable。 But of dew there is little： for the vapour
collects for it in a single day and from a small area； as its quick
formation and scanty quantity show。
The relation of hoar…frost and snow is the same： when cloud
freezes there is snow； when vapour freezes there is hoar…frost。
Hence snow is a sign of a cold season or country。 For a great deal
of heat is still present and unless the cold were overpowering it
the cloud would not freeze。 For there still survives in it a great
deal of the heat which caused the moisture to rise as vapour from
the earth。
Hail on the other hand is found in the upper region； but the
corresponding phenomenon in the vaporous region near the earth is
lacking。 For； as we said； to snow in the upper region corresponds
hoar…frost in the lower； and to rain in the upper region； dew in the
lower。 But there is nothing here to correspond to hail in the upper
region。 Why this is so will be clear when we have explained the nature
of hail。
12
But we must go on to collect the facts bearing on the origin of
it； both those which raise no difficulties and those which seem
paradoxical。
Hail is ice； and water freezes in winter； yet hailstorms occur
chiefly in spring and autumn and less often in the late summer； but
rarely in winter and then only when the cold is less intense。 And in
general hailstorms occur in warmer； and snow in colder places。
Again； there is a difficulty about water freezing in the upper region。
It cannot have frozen before becoming water： and water cannot remain
suspended in the air for any space of time。 Nor can we say that the
case is like that of particles of moisture which are carried up
owing to their small size and rest on the iar （the water swimming on
the air just as small particles of earth and gold often swim on
water）。 In that case large drops are formed by the union of many
small； and so fall down。 This cannot take place in the case of hail；
since solid bodies cannot coalesce like liquid ones。 Clearly then
drops of that size were suspended in the air or else they could not
have been so large when frozen。
Some think that the cause and origin of hail is this。 The cloud is
thrust up into the upper atmosphere； which is colder because the
reflection of the sun's rays from the earth ceases there； and upon its
arrival there the water freezes。 They think that this explains why
hailstorms are commoner in summer and in warm countries； the heat is
greater and it thrusts the clouds further up from the earth。 But the
fact is that hail does not occur at all at a great height： yet it
ought to do so； on their theory； just as we see that snow falls most
on high mountains。 Again clouds have often been observed moving with a
great noise close to the earth； terrifying those who heard and saw
them as portents of some catastrophe。 Sometimes； too； when such clouds
have been seen； without any noise； there follows a violent
hailstorm； and the stones are of incredible size； and angular in
shape。 This shows that they have not been falling for long and that
they were frozen near to the earth； and not as that theory would
have it。 Moreover； where the hailstones are large； the cause of
their freezing must be present in the highest degree： for hail is
ice as every one can see。 Now those hailstones are large which are
angular in shape。 And this shows that they froze close to the earth；
for those that fall far are worn away by the length of their fall
and become round and smaller in size。
It clearly follows that the congelation does not take place
because the cloud is thrust up into the cold upper region。
Now we see that warm and cold react upon one another by recoil。
Hence in warm weather the lower parts of the earth are cold and in a
frost they are warm。 The same thing； we must suppose； happens in the
air； so that in the warmer seasons the cold