How Rain Forms

Clouds that are composed only of water droplets fall as rain either:

• by their own weight, when their droplets are large enough; or
• by the air below, retreating, or the air above, pressing on them, gives them the opportunity to lower themselves
• several of these causes concur together.

When the air below retreats, the finest rain is made.

It is so fine that it is not said to be rain, but rather a mist that descends.

Rain becomes very heavy when the cloud only lowers because it is pressed by the air above, for the highest of its drops descending first, meet others which make them larger.

Someimtes in summer, during a calm weather accompanied by a heavy and stifling heat, heavy rain began to fall even before any cloud had appeared.

This is caused by a lot of vapors in the air. These were pressed by the winds from other places, as the calm and the heaviness of the air testified, the drops into which these vapors were converted became very large as they fell, and fell as they formed.

For fogs, when the earth cools down, and the air in its pores contracts, giving them a way to lower themselves, they turn into dew if they are composed of water droplets, and into drizzle or hoarfrost if they are composed of already frozen vapors, or rather which freeze as they touch the earth.

This happens mainly at night or in the morning, because it is the time when the earth cools down as it moves away from the sun. But the wind also very often knocks down the fogs, coming to the places where they are: and even it can transport their material, and make of it dew or hoarfrost, in those where they have not been seen: and we then see that this frost does not stick to the plants, except on the sides that the wind touches.

For the dew, which never falls except in the evening, and is only known by the colds and headaches it causes in some countries, it consists only of certain subtle and penetrating exhalations, which being more fixed than vapors, only rise in quite hot countries and on fine days, and which fall back as soon as the heat of the sun leaves them. whence it comes that it has different qualities in different countries, and that it is even unknown in many, according to the differences of the lands from which these exhalations come.

I do not say that it is not often accompanied by dew, which begins to fall from the evening on, but that it is by no means it that causes the evils of which it is accused.

It is also exhalations that compose manna, and other such juices, which descend from the air at night; for as for vapors, they cannot change into anything other than water or ice. And these juices are not only different in different countries, but also some only attach themselves to certain bodies, because their parts are undoubtedly of such a shape that they do not have enough hold against the others to stop there.

If the dew does not fall, and in the morning we see the fogs rise high and leave the earth all wiped away, it is a sign of rain.

This rarely happens except when the earth, not having cooled down enough at night, or being extraordinarily heated in the morning, produces a quantity of vapors, which, pushing these fogs back towards the sky, cause their drops to meet, grow larger, and prepare to fall in rain soon after.

It is also a sign of rain to see that our air, being heavily laden with clouds, the sun nevertheless appears quite clear from morning. for this means that there are no other clouds in the air near ours towards the East, which prevent the heat of the sun from condensing those which are above us, and also that it does not raise new vapors from our earth which increase them.

But this cause only taking place in the morning, if it does not rain before noon, it can judge nothing of what will happen in the evening.

I will not say anything about several other signs of rain that are observed, because they are for the most part very uncertain.

The same heat which is usually required to condense clouds and draw rain from them can also, on the contrary, dilate them and change them into vapors.

These vapors sometimes:

• are lost in the air imperceptibly
• cause winds, according to whether:
  • the cloud particles are pressed, scattered,
  • this heat is accompanied by humidity
  • the air around them expands or condenses