What Are Clouds Made Of?

Today we are going to talk about clouds… what are they made of? How are they formed?

Before that, I offer you a very simple little experiment!
The experience
Material
A glass, a freezer, or a fridge

The experiment consists of taking a glass from your cupboard, placing it in the fridge or freezer for a few minutes. What do you observe when you take it out?

You observe that after a few seconds, it is covered with mist. The same thing happens when you get out of the shower and look at the bathroom mirror.

What happened?

  • Initially, the glass that you placed in a very cold place cooled itself.
  • When you place it in a place at room temperature, the water vapor in the air in the room condenses on the cold walls of the glass into tiny droplets. (see here the explanations on the changes of state of matter)
  • It is exactly the same phenomenon at the origin of the appearance of clouds in the sky.
  • A story of humidity and temperature.
  • Clouds are collections of small water droplets and/or ice crystals in the atmosphere in concentrations large enough to be visible. It is therefore not water vapor as is often believed …


Yes, but why do they form in certain places, at certain altitudes? To answer this question, we must take a closer look at the interactions between air and water vapor present in the air, as well as the influence of temperature.

Air humidity

Air is made up of a number of gases including oxygen (luckily for us), nitrogen, and other components like water vapor. The quantity of water contained in the ambient air in the form of vapor defines the humidity or even the hygrometry. Its value depends on the environment in which we are (near a water point for example) and the weather (sunshine, wind)

Definition of temperature

Temperature, you know, it’s a bit like the agitation in the matter.

Thus, at high temperatures, the air molecules move very quickly in all directions. At low temperatures, the movement is much less intense and the more the temperature drops, the more the particles slow down.

Air humidity and temperature

  • With this definition, it is easy to understand the influence of temperature on air humidity and condensation phenomena.
  • The higher the air temperature, the more molecules move and have more energy to occupy a large space (hence the expansion of the gases when they are heated): there is therefore more room to accommodate the water vapor.
  • On the contrary, the colder the air, the less water vapor it can store.
  • When the water molecules run out of space, they group together: the droplet appears!

Some values:

-At 0 ° C, the air can contain up to 4.8 g of water vapor per m3

– At 10 ° C, the air can contain up to 9.4 g of water vapor per m3

– At 20 ° C, the air can contain up to 17.2 g of water vapor per m3 of air

The values ​​given are a maximum, we say that we then have a “Saturated” air. Below the maximum, relative humidity is then defined. A figure of 50% relative humidity means that the air could contain twice as much vapor before condensation occurs.

What about the impact of altitude?

When we go up in altitude, the pressure decreases (the column of air above our heads decreases), and the temperature also decreases. So the amount of water that the air can hold decreases. From a certain altitude, the temperature is too low: the molecules group together in the form of small droplets or in the form of ice crystals (the liquid or solid state depends on the temperature encountered and the presence of “dots”. ‘grip for crystals’, called “condensation core such as micro dust or aerosols …).

The rain

Who says cloud, says rain but not always! In fact, in a cloud, the droplets are so small that they float in the atmosphere and fall extremely slowly.

Water droplets or ice crystals need to be magnified considerably in order to reach sizes large enough to fall as rain or snow.

1 million water droplets in a cloud = formation of a raindrop.

What about the fog?

  1. It is purely and simply a big cloud that is low to the ground. At night, the ground or objects usually cool faster than air loaded with a certain amount of water vapor, so there is a temperature inversion.
  2. The air near the ground quickly becomes saturated with moisture and all the excess condenses: hence the appearance of fog. Fog also appears over streams, lakes which, by evaporation, saturate the air.
  3. If the ground and the objects present are at a temperature below 0 ° C, the tiny droplets of water that will settle there will form frost: this is called freezing fog.
  4. Fog dissipates when heating (sun, human activity) allows water droplets to evaporate.

Other clouds: the contrails of the planes

It is exactly the same phenomenon: aircraft exhaust contains water vapor. The exhaust trails result from the water vapor saturation of the atmospheric layers traversed by the airplane; this saturation giving rise to the condensation and then to the solidification of a sufficiently large quantity of water vapor.

Have you ever created a cloud yourself?

You have certainly observed that you yourself made clouds. In particular, you must have noticed it when you went out in cold weather. The water vapor that you release with your exhalation arrives in a very cold environment. The molecules condense and you form small clouds!

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