Weather's Ups and Downs
(The Precipitation Ladder)

Cumulus Congestus, Willamette Valley, Oregon

Weather is cyclical: flood -- drought; hot --cold; windy --calm; hazy -- clear; high pressure --low pressure.

These polar opposites can all be related to "upness" and "downness" of the atmosphere. But there are apparent contradictions.

We feel up when it is cloudless, sunny and warm. Good things are happening. The grapes are ripening in the September sun The weather is good for golf, for football and for other outdoor sports. It is wonderful for gardening and house repairs.

Yet, it is the result of atmospheric downess. Up results from down.

Last week, it was cold and rainy. The weather was the opposite of all the above. People were grousing about the early wetness. The weather was producing a downer. Yet, atmospheric-wise it was a period of upness.

Down came from up. How can that happen?

There is a logical answer to this apparent contradiction. This is best seen through a sequence of atmospheric events summarized by the co-called precipitation ladder. The ladder is simply a mental hook for remembering which step follows which.

The Precipitation Ladder

In the thin envelope of air that surrounds the earth, gravity compresses the air molecules as closely as possible to the surface. As a result, half the weight of the atmosphere is contained within only about four miles. Thus the pressure exerted by the air drops very rapidly with elevation. Any bubble of rising air must expand to come to equilibrium with its peers at the new level; and work is required for the expansion.

This manifests as a drop in temperature at the rapid rate of 1 degree Celsius per 100 meters. This rapid cooling of ascending air results in humidification of the rising air. When saturation is reached, tiny airborne particles of dust and salt become centers on which water molecules stick, and grow, becoming cloud droplets. This is how 99 percent of all clouds are formed. With the growth of cloud particles, raindrops and snowflakes are produced. Up leads to psychological down.

Conversely, downward motion in the atmosphere leads to compressional warming which leads to drying which leads to clear skies. Air motion from the continent to the coast is sinking air.

Down leads to psychological up. Confusing, isn't it?

See Under-the-Weather Man for more on the up-down effect.



Droplet growth








Water vapor

"Dirty Air"

Words on the Weather

London Fog | Weather's Ups and Downs | Yin/Yang of Weather | Clouds from Prison

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