Lecture 14 — The causes in general of phenomena generated by the moist exhalation in the lower part of the air. Of rain, dew and frost.

Let us go on to treat of the region which follows next in order after this and which immediately surrounds the earth. It is the region common to water and air, and the processes attending the formation of water above take place in it. We must consider the principles and causes of all these phenomena too as before.

The efficient and chief and first cause is the circle in which the sun moves. For the sun as it approaches or recedes, obviously causes dissipation and condensation and so gives rise to generation and destruction.

Now the earth remains but the moisture surrounding it is made to evaporate by the sun's rays and the other heat from above, and rises.

But when the heat which was raising it leaves it, in part dispersing to the higher region, in part quenched through rising so far into the upper air, then the vapor cools because its heat is gone and because the place is cold, and condenses again and turns from air into water. And after the water has formed it falls down again to the earth.

The exhalation of water is vapor: air condensing into water is cloud. Mist is what is left over when a cloud condenses into water, and is therefore rather a sign of fine weather than of rain; for mist might be called a barren cloud.

So we get a circular process that follows the course of the sun. For according as the sun moves to this side or that, the moisture in this process rises or falls. We must think of it as a river flowing up and down in a circle and made up partly of air, partly of water. When the sun is near, the stream of vapor flows upwards; when it recedes, the stream of water flows down: and the order of sequence, at all events, in this process always remains the same. So if 'Oceanus' had some secret meaning in early writers, perhaps they may have meant this river that flows in a circle about the earth.

So the moisture is always raised by the heat and descends to the earth again when it gets cold. These processes and, in some cases, their varieties are distinguished by special names. When the water falls in small drops it is called a drizzle; when the drops are larger it is rain.

Some of the vapor that is formed by day does not rise high 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 vapor 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 vapor 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 vapor 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 vapor 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 vapor 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 vapor 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 vapor in a north than in a south wind. Only the north winds quench the heat before any considerable quantity of vapor 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.

94. After determining concerning phenomena caused from the hot exhalation lifted to the highest place of the air, the Philosopher here determines about phenomena caused from the moist exhalation.

First, about phenomena caused above the earth from the moist exhalation;

second, about those caused on the earth from the moist exhalation, at let us explain the nature of winds (349a12).

95. Regarding the first he does two things:

first, he shows what his intention concerns and says that we must now discuss the things which come to be in the region which, going downwards, is the second after the uppermost region of the air (where the phenomena already discussed take place), but which, going upward, is first, i.e., the region immediately around the earth: this region is the lower region of air. It is the region common both to water and to air; because in it air exists according to the natural order of the elements, and water is generated there from vapors borne aloft. Hence not only is it common to water and air, but also to the phenomena attending the generation of that water and air. These generations take place on high, when water is resolved into vapors (which pertain to the nature of air) and vapors are gathered into water. He also indicates the method for determining these matters and says that we should first take the common principles and causes of all these things that happen.

96. second at the efficient (346b20), he determines the proposition.

First, he posits what pertains commonly to the cause of all passions of this kind;

second, he determines concerning each of them separately, showing wherein they differ, at **so the moisture is always raised **(347a8).

About the first he does three things:

first, he sets down the efficient cause of these passions and says that all these passions have as their cause, in the sense of the movent, and primary, and first principle, the circle of the Zodiac in which the sun clearly moves, which both separates the vapors, by resolving them from earth, and unites them by its absence: for when coldness increases in the air on account of the sun's absence, clouds are condensed into water. Accordingly he adds that from its being at one time near us, and at another time away from us, the sun is the cause of generation and corruption. It gets near us, when by its own motion it approaches the northern signs; it is moved away from us, when it tarries in the southern signs.

97. second at now the earth remains (346b23), he shows the material cause of these passions and says that, since the earth is at rest in the center, the aqueous humor surrounding it is, both through the agency of the sun's rays and through other heat from the higher bodies, resolved into vapor and, being thus refined by the virtue of the heat, borne aloft.

98. third at but when the heat (346b26), he shows the way in which the things under discussion are generated. About this he does three things:

first, he states in general the way these passions are generated and says that the vapor borne aloft by the power of the heat is abandoned by the heat which bore it aloft. This happens in two ways: in one way by the fact that the finer and warmer elements in the vapor are raised higher still, to the upper region of the dry exhalation—consequently, the portion of vapor left behind remains cold; in another way by the fact that the heat in the vapor is quenched by having been lifted far from the earth, in the air above the earth where heat is feeble on account of the rays reflected from the earth being scattered far apart, as was stated above. Therefore, when the heat which warmed and elevated the moist vapor runs out, this vapor returns to its nature, with the coldness of the region condensing it; thus it becomes cool, and after being cooled, it is thickened, and once thickened, it falls to earth.

99. second at **the exhalation of water is ** (346b32), he shows what intermediates are involved in these transmutations. In the first transmutation, in which water is subtilized and raised up, the intermediate is vapor: for the very exhalation resolved from the water is called "vapor," which is intermediate between air and water. In that transmutation in which air is condensed into water, the medium is a cloud, which is a step toward the generation of water. But when the cloud is condensed into water, that which is left over in the cloud, i.e., whatever could not be condensed into water, is called the fog of mist. That is why mist is more a sign of clear than of rainy weather: for mist is, as it were, a barren cloud, i.e., devoid of rain, which is the natural effect of a cloud. However it sometimes happens that mist is carried up along with the vaporous exhalations before they are perfectly condensed into a cloud—and then mist can be a sign of rain.

100. third at so we get a circular process (346b35), he shows how the above-mentioned transmutations bear an analogy to the first movent cause, i.e., to the circling of the sun. For a certain circling is discernible in the above-mentioned transmutations, as water is refined into vapors which are condensed into clouds, and the clouds into water, which falls to earth. He says therefore that this circular transmutation imitates the circular movement of the sun—for the sun is changed to diverse parts of the heaven (for example, to the north and to the south); and that cycle is completed in the fact that vapors ascend upwards and descend downwards. But we should understand this flow of ascending and descending vapors as a certain circular stream common to air and water: for the resolving of water into vapor pertains to the air, while the condensing of clouds into water pertains to the water. When, therefore, the sun is near, this river of vapors flows upward; when the sun is away, it flows downward; and this goes on without interruption in the order described. From this he concludes that perhaps the ancients, in speaking of Oceanus as a certain river surrounding the earth, were cryptically speaking of this river, which, as was said, flows circularly around the earth.

101. Then when he says, so the moisture is always raised (347a8), he discusses the foregoing passions in detail, by pointing out their mutual differences. And it is divided into two parts:

in the first he determines about the generation of those phenomena whose cause is quite plain;

in the second about the generation of hail, concerning which there is greater difficulty, at from the latter (347b12).

102. About the first he does two things:

first, he determines about rain and says that when watery moisture is elevated through the power of heat and is again brought down on account of cooling, different names, based on varying characteristics, are given to these passions of air. For when the vapors condensed into water in small parts fall, then they are called psecades (347a8), i.e., drops, as occasionally happens, when small drops fall; but when the drops of a larger size generated from the vapors fall, this is called "rain."

103. second at some of the vapor that is formed by day (347a13), he determines about dew and frost.

First, he determines the manner in which they are generated. And he says that they arise from the fact that, when the sun is above the earth in daytime, something evaporates from the watery moisture because of the sun's heat, but this evaporation is not suspended or raised very high above the earth, for the simple reason that the fire, i.e., the heat raising this vapor, is slight in comparison with the watery moisture elevated. Consequently, when the air cools at night, the vapor elevated during the day condenses and falls to earth and is called "dew" or "frost." The approach and departure of the sun in its diurnal course has the same relation to the generation of dew and frost as, in the generation of rain, its proper motion has according as it approaches and departs in summer and winter.

104. second at **when the vapor **(347a16), he points out their difference and says that frost occurs when the vapor is frozen before it is condensed into water: for this reason it occurs in winter and in wintry, i.e., frigid, places. But dew occurs when the vapor is thickened into water and there is neither enough heat to dry out the vapor that has been raised, nor enough cold for the vapor to freeze. Hence dew must occur either during warm seasons or in warm places: because dew occurs always in temperate times or temperate places, but frost in times and places that are colder, as has been said. For since vapor is warmer than water (because some of the heat raising it is still in it) more cold is needed to freeze vapor than to freeze water: as a result, frost never occurs except where there is much cold.

105. Then when he says, both dew and hoar-frost are found (347a26), he shows the conditions of disposed air under which dew and frost occur.

First, he shows this generally as to both;

second, specially for dew, at everywhere (347a35).

About the first he does two things:

first, he shows what he is proposing;

second, he gives a sign of the aforesaid, at the fact that (347a30).

He says therefore first, that both dew and frost occur when the air is clear, without clouds and rain, and calm, without wind. Because if it is not clear, vapors cannot be lifted up during the day due to the lack of sufficient heat: if it is not calm, but windy, the vapors could not be condensed to form dew—for the wind, in agitating the air, prevents the vapors from massing together.

106. Then when he says, the fact that (347a30), he gives a sign to support what he posited earlier about generation of dew and frost. And he says that a sign of the fact that dew and frost are caused by the fact that vapor is not lifted far above the earth is that frost does not appear on mountains, whereas it would seem that, on account of the cold there present, it should appear there even more. There are two reasons for this.

First, because the vapor from which dew and frost are generated is raised from low, moist, places, from which many vapors are generated and lifted up: hence the heat which elevates them, bearing, as it were, a burden too much for its power, cannot raise them to a very great height; hence, while still near the lowest places, the heat leaves the vapors, and dew and frost fall. That is why frost cannot form on high mountains.

The second cause is that, as previously stated, the upper air above the mountains flows along, as though carried by the course of the heaven; and therefore, by its flowing it dissolves such gatherings of vapor that cause dew and frost. Now, more motion is needed for scattering large amounts of vaporous matter than small amounts: but the matter of rain and snow coalesces in large amounts, whereas the matter of dew and frost is, absolutely speaking, small, although it is large in relation to the small amount of heat elevating it: hence neither rain nor dew nor frost fall on the highest mountains, because of the greater flow of air; but on mountains that are not so lofty, rain and snow, but no dew or frost, fall, because of the lesser flow of air.

107. Then when he says, everywhere (347a35), he shows under what circumstances dew in particular occurs. About this he does three things:

first, he proposes a truth and says that dew forms in all places with south winds that are not strong enough to prevent vapors from gathering. It does not form with the blowing of the north wind except in the region of Pontus, which is very cold: there the opposite takes place, for dew forms there with northerly weather but not with southerly weather.

108. second at the reason is the same (347b1), he assigns the cause of what generally happens and says that the cause of this is akin to what has been said: namely, that dew forms in temperate times but not in winter, i.e., in very cold times. And he shows the point of similarity: the south wind brings mildness, but the north wind winter and cold, for it is a cold wind; and therefore, because of the winter, i.e., because of the coldness, the warmth is extinguished from exhalations, and, as a result, the vapors cannot be elevated for dew to be formed.

109. third at but in Pontus (347b4), he assigns the cause of what happens in Pontus. The reason is that, because of the great cold there, the south wind is unable to produce sufficient mildness for vapors to be elevated: therefore no dew forms there during the time of the south wind. But the north wind, because of its coldness, collects the warm matter existing in damp places, making a kind of anti-surrounding, i.e., by surrounding the warm matter with a certain contrariety—for when cold matter surrounds warm matter, if the former cannot completely quench the latter, it concentrates it. Thus, by the concentration of the hot, its effect is strengthened—and, as a result, more vapor is resolved. This occurs not only in Pontus but is frequently observed in other places: wells evaporate more with a north wind than with a south wind, because of the heat trapped in them by the surrounding cold. However, in other places the coldness of the north wind quenches the vapor's heat before any can be concentrated in amounts large enough to generate dew; but when there are south winds, they do not impede the accumulation of vapors needed for generating dew. Even in Pontus there are times when, because of the north wind, the heat of the vapors is quenched and, therefore, do not rise; but sometimes, because of the greatness of the cold, much warmth is enclosed within the earth and a large amount of vaporous exhalation forms, in such a way that, for a brief time, it resists the air's coldness, but only long enough for an amount sufficient for the generation of dew to be accumulated.