MadSci Network: Earth Sciences

Re: why do all raindrops fall together during rain?

Date: Wed Nov 17 07:49:14 1999
Posted By: Carl Morgan, Forecaster, Meteorology, National Weather Service
Area of science: Earth Sciences
ID: 942680342.Es

You are correct. Raindrops do start falling once they exceed
a certain weight. Here is a description of how they form.

The processes that produce raindrops can occur under two conditions.

1) the clouds are warmer than 0 degrees C.

2) the clouds are 0 degrees C or colder. These processes occur more easily within maritime clouds
than they do in continental clouds, because maritime
clouds generally contain more water vapor.

As far as why the rainfall rate or dropsize changes, or the
reason for sudden starting or stopping of rain, there are a
couple of different possibilities.

First, the atmospheric conditions may change within a cloud.
Perhaps the updraft strengthens, and therefore the drops must
grow larger before they can fall through it. Some theories
suggest that a quick burst of heavy rain will follow a lightning
strike, because the electrification may cause drops to suddenly

Another possibility, which probably seems like an
oversimplification, is that clouds and storms are almost
always in motion. It is doubtful that a raincloud remains
stationary overhead throughout its lifetime. Take a typical
thunderstorm as an example. Looking at the storm on radar,
you can see that the heaviest rain is occurring in the center
of the storm, with lighter rain on the periphery. As the storm
moves overhead, you will experience a rapid onset of heavy
precipitation, followed by a steady light rain before the
rain ends. Considering that the typical lifecycle of a
thunderstorm takes about 1-2 hours, a storm may move 100
miles before it dies. And the people in the path of the
storm will all experience sudden changes in rainfall
intensity and dropsize.

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