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The dawn brought one of those "lambish" March mornings. The clear sky turned robin's egg blue as the dawn twilight arch gave way to morning, and all through the event the winds blew lightly - if at all. In a few hours, the warm sun would break the morning chill, and by mid-morning the perfectly clear sky would begin to lose its fair complexion as small buds of whiteness pop up hither and yon. So started a day when cumulus humilis, the most basic of the cumulus cloud family, would fill the skyscape. This cloud type often is the forerunner to larger, more developed cumulus varieties that fill the afternoon sky. But today, these fair-weather cumuli dot the sky within the large high-pressure cell that sat over the region.
Early spring is often a good time for cumulus humilis formation because the warming sun still has limited strength to heat the ground surface, particularly if snow and ice cover the soil. High pressure systems in early spring often retain some of their winter weight, and the excess pressure sinks the colder air toward the surface, warming it by compression to form a rather strong temperature inversion. In the inversion, a warm air layer at altitude caps a cold air layer that during the night reached the surface. Now with heating from the mid-morning sun, a shallow layer of warm air begins to form at the cold air's base. Forming ThermalsOnce a spot on the ground surface has been heated to a greater temperature than the air immediately above it, a buoyant bubble or air, or thermal, will usually form. Thermals take shape most readily over dark soil, rock or pavement and less easily over snow- and ice-covered surfaces. The thermal, being lighter than its surroundings, ascends into the colder air. As the thermal or updraft ascends, the bubble of air expands and thus cools. As the ascent and expansion continue, the thermal's core temperature drops. When its temperature approximately equals that of the surrounding air, the ascent stops. Across the local landscape, many such thermal plumes may be rising at any given time. You can see something similar to this by watching a pot of water begin to boil on the stove. Around the pot, you will see trails of rising bubbles as the water heats toward rapid boiling. (For more on buoyancy updrafts, see my article "What Goes Up".) When the wind field around the thermal is light (like the condition in the pot), the thermal rises almost vertically. If the wind is moderate, the thermal rises at an angle tilted in the downwind direction. But if the wind if very strong and gusty, the thermals may be broken apart, quickly mixed with the surrounding air and thus have only a limited ascent.
The copyright of the article Cumulus Humilis: A Fair Weather Cloudscape in Meteorology is owned by Keith C. Heidorn. Permission to republish Cumulus Humilis: A Fair Weather Cloudscape in print or online must be granted by the author in writing.
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