Stratus or Cumulus?

 Photo by Craig Johnson, Copyright 2018

Photo by Craig Johnson, Copyright 2018

What kind of cloud is this? It looks heaped which suggests it is a type of cumulus. On the other hand it also has some stratus characteristics - layered and areas that are flat. Meteorologists combine the two types into one - stratocumulus. This type has both cumulus and stratus elements. In this case the stratocumulus formed in seasonably cold air flowing into a departing low pressure area. The air above this cloud is sinking. Sinking dry air warms at 5.5 degrees F for each 1,000 feet it descends due to the higher air pressure at lower levels. The warming creates a layer of relatively warm air (but not necessarily warm) above the cool air below. The layer is stable - the lower air cannot rise through the upper layer. The result is a nearly continuous cloud with a flat layer expanse.

At the same time the cumulus shape forms when moist air near the surface rises during the heat of the day. The rising air cools and water vapor condenses into puffy cumulus cloud elements. In the end the combination of rising and sinking motion create a cloud that looks both cumulus and stratus at the same time - stratocumulus. By late afternoon the surface heating weakens as the Sun sinks lower in the sky. The cumulus elements disappear leaving a dissipating flat cloud that evaporates leaving a clear chilly night.