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Name: Jean C.
Status: student
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Hi! I have a fair idea how clouds are formed. Correct me if I am wrong. Air rises due to convection, and as it does it cools. At some point along its journey upward its temperature becomes low enough that the moisture in the air condenses to become a cloud, that determines the height of the ceiling. So, my question is, how do we get different layers? I saw at least three levels of clouds out this evening, with 3 different ceilings? How does that happen? Jean,

There are two main processes that produce clouds and thus two general types of clouds. The first, that you have mentioned, are convective clouds, the second are stratiform clouds.

Convective clouds form in the way that you describe. They can result in our pretty fair weather cumulus clouds that do not extend to a great height or they can become tall cumulus congestus (but not quite thunderstorms), or they can become thunderstorms if the instability of the atmosphere is great enough.

The other form of cloud, stratiform, occurs because of cooling to the condensation point also, but it occurs as an entire layer of the air rises, not just a small convective cell.

These two types of forms can also occur together, such as in stratocumulus (low level), altocumulus (mid level) and cirrocumulus (high level ice clouds), where the layer is being lifted at the same time that convective motions are present. The convective structure of stratocumulus is most easily seen from above, but alto and cirrocumulus are often easily observed from the ground.

The atmosphere can have several layers that are fairly well isolated from each other and can have separate dynamics, including lifting and convection, although convection above the lower level of the atmosphere is normally much weaker. The atmosphere also has a fairly complex temperature structure, so that temperature is increasing with height in some layers (thereby squelching the production of clouds) and decreasing with height in other layers (where clouds are more likely to form because air can more easily rise if heated some).

Different characteristics of the air in the different levels can therefore produce different clouds or the same kind of cloud in the different layers, depending on the dynamics of the layer.

There are probably a number of sites on the internet that give descriptions of cloud types and more details on how they are formed. You may want to poke around a bit on the web to see them.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


Jean - Clouds form when water condenses out on condensation nuclei. This happens when the air reaches saturation. One way this can happen is as you described - rising air cools adiabatically. It can also happen by air being pushed up along a frontal boundary, blowing up on a hillside, cooling over the ground, or cooling when air currents eddy (perhaps in the area of a wind shear). In the latter, when air swirls is accelerates... as it accelerates its pressure drops (Bernoulli's principal)... and when the pressure drops, the temperature drops. If it drops to saturation (i.e., the dew point) a cloud may form. This effect can be seen occasionally when air rolls over the wing tips of an aircraft and a long skinny cloud forms.

Yes, clouds do form in rising columns of air, but also from many other causes.

Larry Krengel


Dear Jean-

The process you described concerning cloud formation was "convection," which is only one of the ways that clouds may be formed. To create clouds, the air must contain sufficient water vapor, and then the air must be cooled in some way in order for the vapor to condense into cloud droplets.

Convection is one way.

Another way is for the air to move over a cold body of land or water, and be cooled by contact or radiation to its dew point. Low clouds, called stratus clouds, form under these conditions.

A third way is for air to be forced aloft due to a physical barrier, such as a mountain range. Air flowing perpendictular to a barrier such as mountains, has to rise, and cools in so doing. If the the air cools to the dew point, clouds will form at that altitude.

Air masses of different densities do not mix easily, and the warmer, less dense air is usually lifted over a cooler air mass when they meet. Again, as the air is forced aloft over the denser air mass, it cools, and clouds form.

You might check out this link for a good basic description of cloud formation...

http://www.scoutingresources.org.uk/weather_clouds2.html

Many other links dealing with clouds and how they form can be found by searching the internet, using a search engine, such as "google.com"...and typing in "cloud formation."

Wendell Bechtold, MeteorologistForecaster, National Weather Service Weather Forecast Office, St. Louis, Missouri


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