April Showers
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April showers bring May flowers. So the old saying goes.

They begin with a tip-tap, then a pitter-pat, and finally a steady whoosh of sky tears. For about fifteen minutes, distant trees and buildings disappear behind an aqueous curtain. Then, sunlight bursts out, reflecting in multicolored hues from the oil-water melange on the pavement below.

Hi, I'm Bryan Yeaton and this is the Weather Notebook.

These April showers are a common yet distinctive precipitation form. Their precipitation -- rain or snow -- falls from convective cumulus clouds and is characterized by rapid changes of intensity and the suddenness with which it starts and stops.

April is a transitional month for the character of precipitation over much of the continent. Winter's vigorous cyclonic activity is waning, and its steady frontal rains become less frequent.

In April, the upper atmosphere still remembers its winter temperatures, but below the earth is nearly bare of snow and likely very moist. The sun's growing heat warms the surface and evaporates soil moisture. Warm air below with cold air above produces rapidly rising, moist air bubbles that become cumulus clouds at the atmosphere's condensation altitude.

If conditions are right, these rising bubbles produce tall clouds with raindrops forming in the ascending air. When updrafts are strong, raindrops may grow large. Eventually, the updraft collapses and the drops fall to earth as rain. Because the collapse can be sudden, the onset of rain is also sudden, thus giving the rainfall its showery character.

Thanks today to contributing writer and meteorologist Keith Heidorn of Victoria, British Columbia. The Weather Notebook is a production of the Mount Washington Observatory and is supported, in part, by the National Science Foundation. Thanks today to assistant producer, Doug Sanborn.