WRAL WeatherCenter Blog

When rain clouds travel say from west to east, how does all that water stay in the sky for so long? If I were to throw a bucket of water in the air, gravity would cause it to fall back down. Why not rain?

Posted January 20, 2008

MIKE MOSS SAYS:    Ray,   There is a great difference in throwing a bucket of liquid water into the air and the processes by which clouds and rain form, and thoase differences are what allows clouds to remain in the sky for considerable amounts of time.

If you toss a bucket of water up, it will break into blobs and large droplets and gravity will quickly bring those back down to the ground. However, suppose instead of tosing the water up from a bucket, you sprayed it in the form of fine drops from a water hose. they would still fall back to the ground but would do so somewhat more slowly. To take it a step farther, what if you used an atomizer that broke the water into a fine mist rather than typical water hose-sized spray. Those tiny mist droplets may fall back to the ground as well, but they will do so quite slowly due to their light weight and the effects of drag from the surrounding air. In fact, if you swipe your hand through the mist you would create some turbulent swirls and see some of the mist droplets rising because of air moving upward at a faster rate than the terminal fall velocity of the droplets.

This is the secret to clouds remaining in the air. The droplets that comprise clouds are vastly smaller than the droplets that make up rain, and the clouds tend to form and exist in areas where the average movement of air is upward. This upward motion cancels out the fall speed of the droplets and keeps them aloft. Actually, the upward motion can lead to the formation of the cloud droplets in the first place, by causing the rising air to cool as it moves upward to regions of lower pressure. This cooling can lead to condensation of invisible water vapor molecules, which is where most of the cloud droplets come from in the first place.

Under the right conditions, a fraction of these cloud droplets can grow larger through various processes, including condensation or deposition of additional water vapor onto the droplet, or by the collision and coalescence of multiple droplets to form larger ones. When this subset of droplets reaches a size having a terminal fall velocity greater than the upward motion of the air column beneath it, they will fall to the surface as raindrops.


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