Weather Whys; Cumulus Clouds

This is weather-wise. Satellites show forecasters the comma-shaped swirls of approaching storms in the Pacific Ocean. Usually those huge systems will leave mostly clear conditions in their wake, as cooler air and higher pressure move in behind the cold front. But sometimes a dappling of small white clouds is evident in the rear of the storm. You may have heard television forecasters say that those clouds indicate especially cold air at upper levels of the atmosphere. But in deference to those viewers who might not care, they usually don't spend precious airtime explaining why. It may have been an especially puzzling concept to the avid weather-watcher who knows that those puffy, cumulus clouds tend to form on warm days. The clouds develop when bubbles of warm air rise from the earth, cooling and expending along the way. Eventually, the water vapor in that bubble of air condenses and forms a cloud. Cumulus clouds don't usually form in the region of cold air behind the front, since that

region is generally very stable. Colder air tends to sink, and so the air at the ground may even be colder than the layer of air above it. Warmer air on top and colder air at the bottom is a very stable situation. As in that case, the air will tend to stay pretty much where it is. But if the air at upper levels is especially cold, even chilly air at the ground will seem warm by comparison. So the air at the ground will tend to rise. And although that cold air will generally contain very little moisture, if as it rises it cools and expands sufficiently, some puffy, cumulus clouds may be the result, perhaps sweating our appetite for summer. Weather-wise is made possible by a grant from the National Science Foundation and is a service of the University of Oklahoma. For weather-wise, I'm Drew Barlow.