MadSci Network: Physics

Re: How do the particles of water in the clouds stay in the sky?

Date: Thu Jan 28 16:54:13 1999
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Physics
ID: 910120636.Ph

The particles in clouds do fall, as other particles do, but very,very 
slowly. Air currents often move them upward far quicker than they could 

Usually, the particles in clouds are ice rather than liquid water. And they 
are very small. The size range is roughly between 1 and 10 microns (i.e. 
millionths of a metre.) In American units, that means between about 40 
millionths of an inch, and one 2000th of an inch.

When a small object falls, it reaches a "terminal velocity" when the forces 
of gravity are matched by the forces of air resistance (which get larger as 
the object moves faster). Physicists have a formula for calculating this 
terminal velocity for spherical particles, which would work for falling 
water droplets. Ice crystals are not spherical. They would fall a fair bit 
slower than the formula says -- probably about half as fast.

Here are the results the formula gives:

particle diameter 10 micron:	terminal velocity 2.8 mm/s     = 10 m/hr
particle diameter 1 micron:     terminal velocity 0.028 mm/s   = 10 cm/hr

Let us now compare these velocities with the velocities of typical air 
currents. Suppose air blows in off the sea at a steady 5 knots (a gentle 
breeze -- roughly 2.5 m/s) and climbs to pass over a range of hills 2000 ft 
high (600 m) during the first 5 miles (8 km). Even if the flow is perfectly 
steady, with no turbulence, the air must rise at an average velocity of 
around 190 mm/s, or 670 m/hr.

So it is not hard to see that what really keeps cloud particles up is air 
currents. The speeds involved in even gentle air currents are way larger 
than the speeds at which cloud particles could settle downward.

Clouds are always changing. Sometimes air currents go downward rather than 
upward. If you think about it, they would have to, or we would lose all of 
our air down at the surface! What usually happens when the air currents 
carry cloud particles downward is that the air gets warmer, and the ice 
crystals simply evaporate. 

And of course the other thing that can happen in clouds is that the 
particles can join together to form larger ice crystal masses or water 
droplets, which fall faster, and produce snow or rain showers.

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