Why are cloud bases flat, and why do they float?

Why are cloud bases flat, and why do they float? 

Clouds are often depicted as fluffy, billowing entities in children’s drawings and even in modern works of art.  However, usually not drawn by crayon or paint is the base of the cloud, which is often flat in comparison to the sides and top. This phenomenon is often missed simply because it is difficult to see a flat cloud base directly overhead from the ground, where people typically observe them from.

How Clouds Form

Clouds form in our atmosphere when gaseous water vapor in the air is condensed into liquid, similar to a glass filled with ice water on a hot day; the outside of the glass is dry initially, but soon fogs up and begins to bead with water. Barring that the glass has a leak, this liquid water must come from the water vapor in the air. In fact, about 1% of the total atmosphere is water vapor, but this amount is highly variable from area to area.

When a parcel of air is lifted by some method, the temperature begins to drop, since air pressure lowers as the parcel ascends. This process is called adiabatic cooling, since there is no actual heat being added or taken away. Cooler air has the capacity to hold less water vapor than warmer air, and in order to adhere to this limit, water condenses into its liquid form as it is lifted, thus forming cloud droplets.

You’ll see this happen frequently when you open your freezer door on a muggy day. Cold air comes rushing out, coming into contact with the warmer air in the kitchen, and the water vapor in the warmer air forms a short-lived hazy cloud.

This process continues more and more as the air parcel is lifted higher and higher, creating the larger clouds we see in the sky, and is actually visible if you look carefully in a very unstable environment where strong lift is present, like the beginning of a thunderstorm. Other processes are also at work in aiding cloud growth, but are far more complex on the microscale.

Lifting Condensation Level

So why are cloud bases flat then, especially given the strong upper level flow and turbulence present in the atmosphere? Well, as the air rises, it reaches a level where the air parcel has been cooled adiabatically enough to condense water. This level is called the Lifting Condensation Level, also referred to as Lifted Condensation, or LCL for short. It is a level or altitude in the atmosphere where a parcel of air begins to condense liquid water because the temperature and dewpoint equal each other; when the relative humidity is 100%.  The air parcel cannot hold any more gaseous water vapor, so some must leave as liquid water. 

Here, a parcel of air is lifted by moving up a mountain slope, a process called orographic lift.


Examples of cumulus clouds with flat bases

No air parcels below this level will condense water, since they have not cooled sufficiently, so we cannot see any clouds begin to form. As a result, clouds only condense after an air parcel reaches this level, forming flat cloud bases. LCLs vary in height, but on the scale that we see them, they appear fairly uniform across a large area. The billowing sides and tops can now be easily explained. Since those areas of the cloud are well past the LCL, they are free to condense in any direction they want.

Clouds without flat bases

Of course, there are several exceptions when other factors are present. Clouds sometimes do not have flat bases. Examples like this are mammatus clouds, which are the udder-like bulges on the bottom of a cloud, usually seen during thunderstorm development. They occur when strong downdrafts are pushing down from within the cloud, heavy hydrometeors are falling, or from warming, sinking air. Another type of non-flat cloud base is virga, which occurs when precipitation falling from a cloud evaporates before it hits the ground, creating long strands that drape from the bottom of the cloud. 

Mammatus (left) and Virga (right)

Why Clouds Float

Another interesting phenomenon is the reason why clouds float. Clouds are not above the laws of gravity; they experience the same downward acceleration we feel every day, the same acceleration that makes rain and snow fall. Yet we don’t see clouds falling to the ground every day. That's because clouds are not singular entities of mass.

Clouds are comprised of tiny water droplets, spread out for miles.  Each droplet is subjected to the force of gravity, which can be expressed in terms of Newton’s second law, Force=mass*acceleration. The acceleration in this case is the gravitational constant, 9.8 m/s^2, directed towards the ground (this is effectively the weight of the droplet). The mass of a droplet is very, very small, so the resulting downward force on the drop is very, very small.

Take a look in your house as the sun comes in through a window. You will see tiny dust particles floating in the air each with their own individual movement. Small air currents that you can’t feel contribute to their erratic movement, keeping them from dropping straight down because they are stronger than the force of gravity on the dust. In the same way, turbulence and updrafts in the atmosphere keep individual cloud droplets from falling. However, once these droplets gain enough mass through the various cloud growth processes, gravity finally begins to win, and the larger droplets fall as rain.


By Michael Hansen, Student Volunteer


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