Meteorology   

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 is concentrated by the

surrounding heat and causes the cloud to go over into water

suddenly. (For this reason rain-drops are much larger on warm days

than in winter, and showers more violent. A shower is said to be

more violent in proportion as the water comes down in a body, and this

happens when the condensation takes place quickly,-though this is just

the opposite of what Anaxagoras says. He says that this happens when

the cloud has risen into the cold air; whereas we say that it

happens when the cloud has descended into the warm air, and that the

more the further the cloud has descended). But when the cold has

been concentrated within still more by the outer heat, it freezes

the water it has formed and there is hail. We get hail when the

process of freezing is quicker than the descent of the water. For if

the water falls in a certain time and the cold is sufficient to freeze

it in less, there is no difficulty about its having frozen in the air,