WRAL WeatherCenter Blog

I noticed when I took a trip recently and at other times in the past that the clouds on top are billow and fluffy but underneath they are flat. I really notice it more when I go to Arizona. Almost as if they are sitting flat on a table. What causes this?

Posted October 10, 2007 10:04 a.m. EDT

MIKE MOSS SAYS:        Jackie,    You were noticing an effect caused by something called the "convective condensation level." Vertical motions caused by heating of the surface lift buoyant parcels of air upward within confined shafts, while air sinks in compensation outside those shafts. Where the air rises, it cools at a known rate called the adiabatic lapse rate (9.8 degrees C/Km) due to the decrease in pressure on the parcels (and resulting expansion) with increasing altitude.

At some point, the rising air will become cold enough to become saturated with respect to water vapor, and any further upward motion will cause condensation to occur, resulting in cloud droplet formation. The height at which this will occur depends on how much water vapor the air near the surface contains, so that if the air near the ground is rather humid, the cloud bases will be lower, and if the air is drier, the cloud bases will be higher. Of course, if the air is very dry it may be possible to remain cloud-free.

Because the air near the surface is usually rather well mixed due to convective and mechanical turbulence, the height at which condensation occurs will tend to be just about the same for all of the cumuliform clouds within your local field of view, and they will all have flattened bases that correspond to the convective condensation level. On the other hand, the upper portion of the clouds will tend to be lumpy and rounded, because of variations in the upward velocity and momentum of the updrafts within the cloud, and also because at this upper boundary and along the sides of the cloud, some of the surrounding and overlying drier air is being mixed in (entrained) due to turbulence and wind shear, causing some of the cloud droplets that were formed as the air initially passed through the condensation level to evaporate again.