Clouds

After considering how vapors, by expanding, cause winds, we must now see how, by condensing and contracting, they form clouds and fogs.

As soon as they become significantly less transparent than pure air, if they extend to the surface of the earth, they are called fogs; but if they remain suspended higher up, they are called clouds.

What makes them become less transparent than pure air is that when their movement slows down and their particles are close enough to touch each other, they join and gather into various small clusters, which are drops of water or particles of ice.

For as long as they remain completely separate and floating in the air, they can hardly obstruct the passage of light; but when they are assembled, even though the drops of water or the particles of ice they form are transparent, the surface of each drop reflects part of the rays that strike them, as stated in the Dioptrics about all transparent bodies. These surfaces easily become numerous enough to reflect almost all the rays.

Water droplets form when the subtle matter surrounding the small particles of vapor no longer has enough force to make them expand and repel each other, but still has enough to make them fold and subsequently join and accumulate into a sphere.

The surface of this sphere becomes immediately smooth and polished because the parts of the air that touch it move differently from its own parts, and the subtle matter in its pores moves differently from that in the pores of the air, as previously explained when discussing the surface of seawater.

For the same reason, the droplets also become perfectly round: as you often see, river water swirls and forms circles where something prevents it from moving straight as its agitation requires.

Similarly, the subtle matter flows through the pores of other bodies, in the same way as a river flows through the spaces between the plants growing in its bed, moving more freely from one part of the air to another, and from one part of the water to another, than from air to water, or vice versa, as noted elsewhere.

It must swirl inside the drop and also outside in the surrounding air, but in a different manner inside than outside, thus arranging all the parts of its surface in a circle.

The water, being a liquid body, must obey these movements. Without doubt, this is enough to explain why water droplets must be perfectly round, in the sense that their sections are parallel to the earth’s surface; there is no reason for any part of their circumference to move closer to or further from their centers more than others, since they are equally pressed by the surrounding air on all sides, assuming the air is calm and still, as we must suppose here.

However, considering them from another perspective, one might wonder, when they are so small that their weight is not sufficient to force them to divide the air and descend, if this does not make them somewhat flatter and less thick in their height than in their width, like T or V.

They have air around their sides as well as underneath them; and if their weight is not sufficient to make the air below them give way and let them descend, it is equally insufficient to make the air on their sides move aside and let them become wider.

Conversely, one might wonder if, when their weight makes them descend, the air they divide does not make them somewhat longer and narrower, like X or Y.

Again, it should be noted that, being surrounded all around, the air they displace as they descend must rise above them to fill the space they leave, and it can only do so by flowing along their surface, where it finds a shorter and easier path when they are round than if they had any other shape. Everyone knows that of all shapes, the round shape is the most efficient, meaning it has the least surface area relative to the volume it contains.

Therefore, in whatever way one might consider it, these droplets must always remain round, unless the force of some wind or some other specific cause prevents them.

As for their size, it depends on how close the parts of the vapor are to each other when they begin to form the droplets, as well as on how agitated they are afterward, and on the amount of other vapors that can join them. Initially, each droplet is composed of only two or three small parts of the vapor that meet.

But soon after, if this vapor is somewhat thick, two or three of the droplets that have formed from it will meet and join into one, and again two or three of these will join into one, and so on, until they can no longer meet.

While they are suspended in the air, other vapors can also join them and increase their size until, finally, their weight causes them to fall as rain or dew.

Ice Particles

Ice particles form when the cold is so intense that the vapor particles cannot be folded by the fire-aether among them.

If this cold sets in only after the droplets are already formed, it freezes them all round unless accompanied by a strong wind, which may flatten them slightly on the side it hits.

Conversely, if the cold sets in before they begin to form, the parts of the vapor only join lengthwise, creating fine ice filaments.

However, if the cold arrives between these two stages, which is most common, it freezes the vapor parts as they fold and pile up, without giving them enough time to join perfectly to form droplets.

Thus, it creates small knots or clusters of ice, which appear white because they are composed of many filaments that remain separated and have distinct surfaces, even though they are folded over one another.

These knots appear fuzzy or hairy all around because there are always several parts of the vapor that, unable to fold and pile up as quickly as the others, attach straight against them, forming the tiny hairs that cover them.

Depending on whether the cold arrives gradually or suddenly, and whether the vapor is dense or sparse, these knots form larger or smaller, and the hairs or filaments that surround them are thicker and shorter, or finer and longer.

Vapors Converting to Water or Ice

There are always 2 things required to convert vapors into water or ice:

1. Their particles are close enough to touch each other
2. There is enough cold around them to make them join and stay together when they touch.

For it would not be enough for the cold to be very great if the vapors were spread so far apart in the air that they did not touch at all, nor would it be enough for them to be very close and compressed if their heat, that is, their agitation, were strong enough to prevent them from joining.

Thus, clouds do not always form high in the air, even though the cold is always sufficient for this effect.

It also requires:

• a westerly wind, opposing the usual course of the vapors, to gather and condense them where it ends; or
• 2 or more other winds, coming from different directions, to press and accumulate them between them; or
• one of these winds drives them against a cloud already formed; or
• they gather against the underside of a cloud as they rise from the earth.

Fog forms only when the cold air and the abundance of vapors coincide.

This often happens in the evening or at night after a warm day, especially in spring more than other seasons, and even more than in autumn. This is because the temperature difference between day and night is greater.

Fog forms more often in marshy or coastal areas than in places far from water or on water far from land. This is because water cools faster than land, chilling the air, which condenses the vapors produced abundantly by warm, moist land.

The largest fogs, like clouds, form where the course of two or more winds ends.

These winds drive many vapors to these locations, where they thicken into fog if the air near the ground is very cold or into clouds if it is only cold enough to condense them higher up. Note that the water droplets or ice particles that make up fog must be very small; if they were even slightly larger, their weight would quickly cause them to fall to the ground, so we would not call it fog but rain or snow.

There can never be any wind where fog forms, as it would quickly disperse it, especially when composed of water droplets. Even the slightest air movement causes these droplets to join and grow larger, eventually falling as rain or dew.