|MadSci Network: Physics|
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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