Chapter 31: Clouds and precipitation

On clouds, mist, rain, dew, snow, frost, and hail.⁹⁰⁷

1 Anaximenes

Anaximenes says that clouds form when the air thickens further; when it is drawn still more closely together, showers are squeezed out. Snow forms when the falling water freezes, and hail when something airy is enclosed together with the moisture.⁹⁰⁸

2 Anaxagoras

Anaxagoras gives a similar account of clouds and snow. Hail forms when portions of frozen clouds are thrust toward the earth; as these cool during their descent, they become rounded.⁹⁰⁹

3 Metrodorus

Metrodorus says that clouds are formed by the air from the upward movement of watery matter.⁹¹⁰

4 Xenophanes

Xenophanes says that phenomena aloft occur through the sun’s heat as their initiating cause. When moisture is drawn up from the sea, the fresh portion separates because of the fineness of its particles, forms clouds as it becomes mist, drips down in showers under compression, and gives off winds as vapor.⁹¹¹ For he writes expressly:

The sea is the source of water.⁹¹²

5 Epicurus

Epicurus says that they arise from atoms. ⟨And⟩ hail becomes rounded, ⟨as does⟩ rain, molded in the course of its long descent.⁹¹³

5 continued: Rejected wording

[Empedocles: light falling into a cloud.]⁹¹⁴

5 continued: A fragment after a gap

* * and it produces wind by pushing the clouds aside, showers by dispersing them, hail by compressing them, and snow by enclosing some airy matter with them.⁹¹⁵

6 ⟨Arius Didymus⟩: The moist exhalation

⟨Aristotle’s view.⟩ From the moist and vaporous ⟨exhalation⟩ come rain, dew, frost, mist, clouds, snow, and hail.⁹¹⁶

6 ⟨Aristotle⟩: Rain and drizzle

Rain forms when the vapor rising from the earth gathers through condensation ⟨in⟩ the upper regions and turns into water. It then falls in a mass to earth because the heat that raised it has left it. When it is scattered into small portions, it is called drizzle.⁹¹⁷

6 ⟨Aristotle⟩: Dew and frost

When it descends before rising very high, because there is little of the fire that raised it, it receives the names dew and frost. It is frost when it freezes before gathering into water (this occurs in winter and in places with a more wintry climate); it is dew when the moisture that is forming cools during the night and settles on the earth at dawn.⁹¹⁸

6 ⟨Aristotle⟩: Where dew and frost gather

Hence dew and frost fall most abundantly around marshes, lakes, and rivers, and especially in places with channels and hollows. For the greatest amount of moisture rises from such places, and it cools more.

6 ⟨Aristotle⟩: Mist and cloud

A mass of rising vapor, when only slightly thickened, is called mist: a kind of thin cloud that bears no water, forming earlier than cloud and dispersing earlier, and serving as a sign of fair weather. When it rises farther and gathers together, it is called cloud.⁹¹⁹

6 ⟨Aristotle⟩: The burden carried by heat

All these occur near the earth because the vapor cannot rise very far. On the highest mountains, at any rate, none of them occurs. ⟨And⟩ the reason is that vapor rises chiefly from hollow, water-rich places: the heat that raises it is like something carrying a burden beyond its strength. It cannot lift it to a very great height, but, while it is still near the earth, lets it fall back upon it.⁹²⁰

6 ⟨Aristotle⟩: Rain and dew compared

From the region of the clouds three bodies come down to us: water, snow, and hail. Two of these have corresponding counterparts, since they arise from the same causes as the phenomena below, differing in degree and in greater or smaller quantity. For snow and frost are the same, as are rain and dew, except that the one is abundant and the other scanty. Rain forms from much vapor as it cools, whereas dew forms from little, since its gathering takes place within a day.⁹²¹

6 ⟨Aristotle⟩: Snow and frost compared

The same applies to snow and frost: snow is the freezing of cloud, frost the freezing of vapor. Hence snow is a sign of either a cold region or a cold season.

6 ⟨Aristotle⟩: Hail and the seasons

Hail, by contrast, occurs in milder regions or seasons. It occurs chiefly in summer and autumn, rarely in winter, and then when the cold is less intense. Hail is the freezing of water that is falling from the clouds.⁹²²

6 ⟨Aristotle⟩: The dry exhalation

From the dry and smoky exhalation, extending far and rising aloft, so that for this reason it is often ignited by the movement of the heavenly bodies, each of the things mentioned earlier is produced.⁹²³

7 ⟨Arius Didymus⟩: Chrysippus’s definitions

Chrysippus’s view. Chrysippus said that mist is dispersed cloud, or air possessing thickness; dew is moisture descending from mist; rain is a fall of water from clouds; and a downpour is a fall of violent and abundant water from clouds.⁹²⁴

7 Chrysippus: Frozen forms of water

Hail is the breaking up of frozen rain; snow is frozen cloud, or the freezing of cloud; water frozen on the earth is ice; and frost is frozen dew.⁹²⁵

8 Arrian: Two origins of mist

Arrian says of mist ⟨that⟩ some forms before cloud, before rising up, but much of it comes from cloud that has spread out and dispersed. These things occur if the sun, the other stars in the sky, and the sky itself do not prevail over them.⁹²⁶

8 Arrian: Drizzle and rain

From clouds that have formed without being too closely compressed, fine drops descend to earth; both mists and the less dense clouds dissolve into these. But clouds that have gathered in a great mass and changed into water produce rains from clouds.

8 Arrian: How high clouds rise

Mists generally settle upon the earth, since their vapor is still dispersed and has not gathered together. Clouds rise upward, though even these are not carried more than twenty stadia above the earth.⁹²⁷