Lecture 15 — On the place of the generation of hail and snow.

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 vapor, for the region from which and the time during which the vapor is collected are considerable. But of dew there is little: for the vapor 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 vapor 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 vapor 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.

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 air (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 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, provided that the freezing takes place in a shorter time than its fall. The nearer to the earth, and the more suddenly, this process takes place, the more violent is the rain that results and the larger the raindrops and the hailstones because of the shortness of their fall. For the same reason large raindrops do not fall thickly.

Hail is rarer in summer than in spring and autumn, though commoner than in winter, because the air is drier in summer, whereas in spring it is still moist, and in autumn it is beginning to grow moist. It is for the same reason that hailstorms sometimes occur in the late summer as we have said.

The fact that the water has previously been warmed contributes to its freezing quickly: for so it cools sooner. Hence many people, when they want to cool hot water quickly, begin by putting it in the sun. So the inhabitants of Pontus when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their reeds that it may freeze the quicker, for they use the ice like lead to fix the reeds. Now it is in hot countries and seasons that the water which forms soon grows warm. It is for the same reason that rain falls in summer and not in winter in Arabia and Ethiopia too, and that in torrents and repeatedly on the same day. For the concentration or recoil due to the extreme heat of the country cools the clouds quickly.

So much for an account of the nature and causes of rain, dew, snow, hoar-frost, and hail.

110. After determining about the generation of rain, dew and frost, the Philosopher here begins to determine about the formation of hail. About this he does three things:

first, he shows where hail is generated;

second, he enumerates certain phenomena accompanying hail that raise a difficulty about its formation, at but we must go on to collect the facts (347b34);

third, he assigns the cause of its generation, at some think that (348a14).

He says therefore first, that although vapor freezes in this lower air near the earth, yet water does not coagulate here to form hail as it coagulates in the region of the clouds. For three bodies condensed by cold come from that region: namely, rain water, snow and hail. But in the case of two of these bodies, certain proportionate things take place in the lower region near the earth, and which are generated from the same causes, but differ from rain and snow according to more and less, depending, namely, on whether they are produced more quickly or more slowly, and on largeness and smallness [of amount]. For snow and frost are proportionately the same, and likewise rain and dew: they differ according to abundance and scarcity. For rain comes to be from the cooling of an abundance of vapor—the cause of this abundance being a large and spacious region and a long time in which the vapor is being united and collected, and also a large place from which it is collected. For, since rains are generated on high, vapors collect there from many regions.

Dew, on the other hand, has little vapor, because the time is short during which it is collected (for the consistency of that vapor is ephemeral (347b12), i.e., of a single day), and the region is small in which it is collected, for it is collected close to the earth—and this becomes plain fog from the fact that the generating of dew is swift and its amount is small. And as it is with dew and rain, so with snow and frost: for when an entire cloud is frozen, snow comes to be; but when some small vapor near the earth freezes, frost comes to be. Therefore, both of them are a sign of cold weather or of a cold region—for, since some heat still abides in both the vapor and the cloud, they would not freeze, unless there were present a vast coldness overwhelming that heat: there being in the cloud still a large residue of the heat which made the watery moisture evaporate from the earth, and in the vapor still more. Thus, as rain and snow come to be in the upper region, so dew and frost in the lower. But although hail comes to be in the upper region, nothing below corresponds to it: the reason for this will be plain, when the cause of the generation of hail is explained.

111. Then when he says, but we must go on to collect the facts (347b34), he proposes certain phenomena that occur with respect to hail and create a difficulty as to its generation.

And he proposes two difficulties with respect to generation of hail;

the second one is mentioned at **again, there is a difficulty ** (348a4).

He says therefore first, that with respect to the generation of hail we must consider the facts which are thought to be reasonable, and not false. And first he proposes that hail is, as it were, *crystal *(347b34), i.e., water solidly frozen; he further proposes that water freezes especially in winter. But the contrary is seen to happen: for hailstorms occur mainly in spring and autumn; and after this, during the time of fruit, i.e., in summer and around the onset of autumn; but less often in winter, and, in that case, when the cold of winter has been less. Generally speaking, hail occurs in more temperate places; snow in colder places and times. From all this it would seem that hail, in which a larger amount of freezing is involved, ought rather to occur in places and times that are cold.

112. Then when he says, again, there is a difficulty (348a4), he presents a second difficulty, about which he does three things.

First, he presents the difficulty and says that it appears inconsistent that water should freeze on high—for it cannot freeze before it is formed, nor after it is formed, can it remain aloft, since it falls immediately. Hence it does not appear that there can be time for it to freeze and generate hail.

113. second at nor can we say that the case is like that (348a7), he gives an apparent solution of this difficulty. For one could say that the water, separated into minimal parts, remains in the air as though mingled with it, and does not fall at once, but abides in the air. This is what happens when psecades [droplets] (348a7) fall, about which we spoke earlier. And a like thing is true of earth in respect to water, which is to earth, as air is to water: for often gold or earth float on water because of the minuteness of their particles; but if those particles of earth or gold coalesced, they would fall to the bottom of the water. Hence as a result of the small particles that abide in the air congregating, large drops are formed and, in this way, the drops are brought down. In like manner, one could say that it is not impossible for water permeating the air to freeze and form hail.

114. third at this cannot take place (348a11), he rejects this solution and says that what happens to occur in droplets does not occur in the case of hail. For bits of frozen water, if small, could not consolidate to form something large like hailstones in the way that the parts of water, as moist, coalesce: for hard objects, such as ice, do not unite the way more moist things do. Hence a quantity of water equal to the size of the hailstone would have had to be hanging above in the air without falling: this is evident, for it could not be the size it is after freezing, if it had not been that size before freezing—since from many small things cannot be formed many large continua. But that such an amount of water should remain above without falling seems impossible.

115. Then when he says, some think (348a14), he assigns the cause of the generation of hail.

First, he presents the opinions of others;

second, his own opinion.

About the first he does two things:

first, he presents the opinions of others and says that it seems to some that when a large quantity of heat pushed a cloud into the upper region, which is very cold because the rays reflected from the earth do not reach there, the water arriving there is frozen on account of the region's being cold. And therefore the reason why hail occurs during the summer and in warm regions is that vast heat greatly pushes clouds into the upper region far from earth.

116. second at but the fact is (348a20), he attacks this position on three scores.

First, we see that hail does not occur on lofty mountains—but it should, if hail is formed by vapor lifted into very high places, just as we see snow, which is generated on high, on lofty mountains.

117. He gives the second argument at again (348a23), and says that clouds are often seen moving with a great noise close to the earth so that some people hearing it fall down in terror, as though it augured a great prodigy. At other times, when such clouds are seen near the earth without any noise, a great hailstorm occurs, with hailstones of incredible size and non-round shape. Now, these phenomena, namely, that the hail is not round in shape and that the stones are large, are due to the fact that the freezing of the hail took place near the earth; consequently its fall is of short duration—for if it had fallen a long time, the size of the hail would have been reduced and the shape rounded, because the descending motion would especially have dissolved the angled edges cleaving the air very strongly and offering it greater resistance. Therefore, it is not true that hail is generated very far from the earth.

118. He gives the third argument at moreover (348a30), and says that it is necessary that the size of the hail be due to the vigor of the cause freezing it: because hail is something congealed after the manner of crystal, as everyone can see. But the size of hail is greater when the stones are not round; from which it can be concluded that hailstones which are not round have vigorous cause of their freezing. But the very fact that the stone is not round is a sign that it froze close to the earth; because if the stones came from a great distance, their surface would have been worn down all over because of motion from a distance, and thus they would be round in shape and smaller in size. Hence he concludes that the freezing of hail is not due to vapors being pushed up into a highest cold region far from the earth.

119. Then when he says, now we see (348b2), he assigns the cause of the generation of hail.

First, he excludes one difficulty raised earlier;

second, he excludes another one, at we get hail (348b17).

He says therefore first that from frequent experience we observe that the hot and the cold being contrary surround each other and cause gathering. And this is evident in the case of the earth. For in the hot weather the interior of the earth is cold, due to the fact that the heat of the air surrounds earth's coldness; hence it congregates together within. On the other hand, when it is cold, the interior of the earth is warm, because the cold encloses inside the warmth which was in the earth. That is why in summer the water from fountains is cool but warm in winter. The same thing must be supposed to take place in the upper region also. Therefore, when the weather is warm, the cold, shut in by the contrariety of the warmth surrounding, acts with greater force—as a result, the coldness sometimes can form water very quickly from a cloud. And, for this reason, much larger drops are formed on hot days than in winter, and the rains are *labroterae *(348b2), i.e., more violent. The size and violence are due to the fact that a whole sheet of rain descends very suddenly, which is due to the rapidity of condensation.

Therefore, what happens is the very contrary of Anaxagoras' theory, which held that this happens when the vapor, from which rain forms, rises into air which is very cold; but we, on the contrary, maintain that this happens when the vapor descends into warm air—and so much the more so according as the warmth is greater. Therefore, from the hot which surrounds and gathers together the cold, are due the large drops and the violence of the rain. But when the cold is still more gathered together as a result of being surrounded by an external warmth, not only are the clouds condensed into water suddenly but what is more, the water is frozen by the vehement power of the trapped coldness: then hail is formed. From this is plain the solution to the first difficulty: namely, why is water congealed into hail in warm weather more than in winter?

120. Then when he says, we get hail (348b17), he solves the second difficulty. About this he does three things:

first, he solves the difficulty;

second, he gives an explanation for the season in which hail is formed, at hail is rarer in summer (348b26).

third, he lays down something that contributes to the speed with which hail is formed, at the fact that (348b30).

The second difficulty arose from the fact that there did not seem to be able to be any time for the water on high to freeze into hail—for as soon as water is formed, it falls, and it cannot freeze before it has formed. To solve this difficulty, therefore, he says that the generation of hail occurs when the freezing of water is more rapid (on account of the vigor of the cold gathered together) than the downward motion of the water. That this is possible he now shows. For since every instance of local motion is in time, it is plain that the rain water is traveling downward for some definite period of time; but it happens that the coldness, because of its vigor, freezes the water in less time than the time of descent. Hence there is nothing to prevent the freezing from occurring in less time than it takes the water to descend, if the cold is very vigorous and intense.

This is why, the nearer to us the generation of water or hail takes place, the faster it freezes, since the heat is stronger near the earth and more vigorously expels and encloses the cold. In these circumstances the water of the rain must become more violent, and both the drops of rain and the hailstones larger, because they travel a shorter distance and less is dissolved away. For the same reason the raindrops do not fall thickly, for, since they suddenly condense into large drops at one time, they are not divided into many parts, and also fall very quickly—and that is why the stuff of rain and hail does not fall so thickly.

121. Then when he says, hail is rarer in summer (348b26), he assigns a reason for the time when hail is generated, and says that hail falls less in summer than in spring and autumn, but more than in winter: less in summer than in spring and autumn, because in summer the air is drier; but in spring the air is still moist on account of the preceding winter, while in autumn, it is already beginning to grow moist. Hence in summer there is not as much moist vapor for generation of hail as in spring and autumn, although there is more heat. But in winter, although there is an abundance of material, there is lack of heat to concentrate the cold that generates hail. Also hail occurs at harvest time, i.e., in late summer, for the same reason: because the heat is then still potent and the air is beginning to get moist.

122. Then when he says, the fact that (348b30), because he had solved the difficulty raised above by citing the speed with which hail is generated on account of the vigorous cold, he now posits something else that contributes to this same speed. And he says that a contribution to the speed of the coagulating process is the fact that the water is pre-heated (the vaporous stuff aiding the season's heat) and therefore freezes more quickly, because the cold acts upon it more vigorously and can penetrate farther into water that has been rarefied by the heat. This is the reason why many people; when they want to cool warm water, first place it in the sun. And fishermen in the region of Pontus, when they make huts during the ice season to fish (they catch the fish through holes cut in the ice of the sea or river), pour hot water around the poles they use for fishing, so that it will freeze faster; in this way they use the frozen water, as though it were lead, to keep the poles firmly fixed. But in warm regions and in warm seasons, hot water cools quickly, because it gets dense quickly, for the reason given. Hence, in Arabia and Ethiopia, rains occur in summer and not in winter, because the vapors are quickly cooled by reason of the contrariety of the heat surrounding them, since this region is very hot.

Finally he summarizes what has been said — and this is plain in the text.