|MadSci Network: Earth Sciences|
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.
Under this condition, the process that forms raindrops is known as collision and coalescence. Basically, cloud droplets of varying sizes coexist. The larger droplets fall through the smaller droplets, collide, stick together and grow. This continues until the drops are large enough to fall to the ground. It is more difficult than you might think for collision to occur. Cloud droplets make up less than one-millionth of the volume of a cloud!2) the clouds are 0 degrees C or colder.
This depends on the formation of ice in a supercooled liquid cloud. At the same sub-zero temperature, the relative humidity will exceed 100 percent for the ice particle, while being only 100 percent over the liquid surface. For example, at -10 C, if the air were saturated with respect to liquid water (100% humidity), it would be supersaturated (110% humidity) with respect to ice. Under these conditions, the ice particles grow rapidly by deposition, depleting water vapor from the cloud. The vapor is replaced as water evaporates from the liquid droplets, which are trying to maintain 100% humidity. When the ice particle grows to a certain size, it will fall to the ground. The temperature profile of the atmosphere determines the state of the particle when it reaches the ground (rain, snow, sleet, etc). Under special conditions during a thunderstorm, hailstones may form, although the process is a bit different.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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