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Henry Lind's Weather Watch
by Henry Lind

Weather Watch

A Daily Gazette news blog
Weather events in our region and why they happen

Diurnal clouds

The summer season is now officially recognized on the calendar but a better indication of one of the most pleasant of summer days was the magnificent cloud formations we have been seeing. If you spent any time outdoors this weekend – even if it happened to be pulling weeds – you may well have noticed the development of puffy white clouds as the day progressed. The pessimists among us may have thought, “It’s going to rain“, while the optimists would marvel at the beauty of the array. The weather enthusiast would describe them as diurnal cumulus thermal clouds. How did they get there and where did they go?

To understand how these form we need to remember that the air above us is cooler at higher altitudes. When the earth warms due to the sun beating down through a perfectly clear morning sky, surface air warms also and begins to rise because it is lighter than the surrounding air. The pockets of floating warm air are replaced by cooler air and the process continues, creating a conveyor of sorts with warm air rising until it reaches an area of the same temperature. You have no doubt observed birds, butterflies, or even gliders taking advantage of the energy in that process to circle gently upward without flapping wings.

But all good things must come to an end and in this case the air finally rises high enough that it collides with much cooler air and the moisture within condenses forming tiny droplets. This temperature is called the dew point and as the name suggests it is when dew forms as air is cooled. We see the tiny droplets aggregated as clouds. The amount of moisture contained in the air, and the difference between the temperatures in the various layers of air will sometimes contribute to these clouds forming rain or in some cases thunderstorms. The benign artistry of the past few days reached its conclusion before that because the air did not contain much moisture and they all disappeared with sunset – they are truly diurnal visitors.

Photo courtesy of the National Weather Service.

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