### 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
Message:
```
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
fall.

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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