Why are the base of clouds flat?

Question

Have you ever looked up and thought the bottoms of clouds are flat? If you go up in a plane you actually go through a layer that clouds sit on making the bottom of the clouds flat. Why is this?

Answer 1

Because they're resting on a lower stratum of cold air.

The cold air below the lighter, gaseous clouds is denser and tends to act as a table top on which they rest, flattening them out on the bottom.

Answer 2

Well, clouds aren't always flat. Mammatus clouds (indicative of extremely dangerous weather) look like little bubbles on the bottom of clouds.

But, the reason for the typical flat cloud look is that clouds begin to form where the atmosphere is first saturated. Because the atmospheric conditions (pressure, temperature, etc.) in a region are relatively the same at a certain altitude, the base of the cloud will not be able to expand below that level (i.e. clouds cannot form below a certain altitude unless the conditions below are good for cloud formation).

If you have cumulus clouds, these clouds will expand upward because of buoyancy (basically thermodynamics and physics) and because the conditions above are usually cold enough to form clouds (until you reach the stratosphere). Stratus clouds are usually pretty flat all the way around. Cirrus clouds are not flat because they form from ice crystals.

Edit: I'm not actually sure about stratus clouds so I would factcheck that with someone else.

Answer 3

They are not ALL flat!
Stratus are flat at the bottom and at the top: they lay at a lower level, where the air is relatively stable.
As said above, the tempreature gradient is relatively constant, and the dew point (where water vapour condenses into a cloud) is at a relatively "flat" level: the bases of the clouds at that altitude will be flat.
Cumulus, a bit higher, form in an unstable atmosphere, where the air currents are irregular: hence, the cumulus have no flat base.
Strato-cumulus form at a low level: it is a combination of flat stratus and bubbly cumulus: they are not really flat, but more like the bottom of a "flying saucer".
Alto-cumulus form at much higher altitude and have no flat base: they are "bubbles" of cotton!
Only low clouds have flat base...
and the terrible cumulo-nimbus: they start at the dew point level (and are flat), and go up to 10,000 feet+, where they "bubble" into an anvil shape. If you fly, avoid them by 10 miles at least. At the center, there is a updraft that pulls the air up. Around, there is a downdraft that may prevent you to climb. In it, there is a storm. The base is not flat either: it looks like a funnel going up...

Answer 4

It is the lower convection clouds such as cumulus which have flat bases - not all clouds.
As the sun warms the ground, in turn the air immediately above the ground is warmed, causing it to rise. Is the warm air rises, it then cools. When it reaches a critical height, it has cooled so much that it it can no longer hold the water vapour. The water vapour condenses into droplets, and the cloud is formed.

The important thing to remember is the height at which this happens is quite uniform as far as we can see, because the temperature gradient of the air is also uniform, so the cloud base is flat.

Answer 5

Ok, well as you go up in elevation, the air cools. Well, clouds are simply condensation of the moisture in our atmosphere. In order for the water to condense, the air has to reach it's saturation point (100% relative humidity). Well as the temperature cools, the percentage of moisture inthe air relative to the to the specific amount of water vapor in the air increases, hence the relative humidity increases. Well, there's a specific point in the atmosphere where this temperature is achieved and that is the line that marks the bottom of the cloud and that's why it's flat.

Answer 6

Air gets colder at higher altitude. As air currents (carrying moisture as water vapors) rise, it cools down, and at one point it reaches an altitude where the temperature is low enough that the water vapors become saturated; rising further means further cooling. At lower temperature the water vapors are saturated at lower concentrations, and thus the excess water condensates. What you see as clouds is a huge number of extremely small droplets diffusing light (which is why you see them). The flat base is simply the altitude at which water vapors become saturated.

This explains the flat base of clouds in calm atmosphere. There is also another mechanism for cloud formation - at the weather front, where a mass of warm air collides with a mass of cold air. The warm air can carry higher concentrations of water vapors, and when it cools down at contact with cold air, it forms clouds. You may see frontal clouds with a well defined base, but this base is not parallel to Earth; it will have the shape of the contact area between the warm and cold air.

Answer 7

The flat bottom is just the boundary between the denser dry air below and the less dense wet air above.

Answer 8

because if they touched the ground it'd be fog

Answer 9

thermal layers.