How Do Clouds Float? - Page 3

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Jul 1, 2005

When any object falls through the air (or through water, for that matter), the aerodynamic drag force counters the force of gravity, and after a time/distance, the falling object reaches its terminal velocity. This is the rate at which the falling object thereafter descends toward the surface. The time it takes to reach the ground can be calculated from the terminal velocity.

For the typical cloud droplet of 10 micrometres (it takes about 15 million cloud droplets to form the typical raindrop), its terminal velocity is around 0.3 cm/s or about 10 metres per hour (30 ft/h). To fall from the cloud base at 3500 m at this rate would take 350 hours! An object moving that slow would appear to us as being stationary unless we observe it closely for some length of time. Only when the droplets congregate to form rain drops, 300 or more times larger, does the fall time drop to minutes.

This small fall velocity is countered by the rising air surrounding it so that many cloud droplets are actually rising at a net rate of perhaps tens of centimetres per second. Of course, a droplet caught in a downdraft will descend at the rate of the downdraft plus its terminal velocity. Now should a cloud droplet be in a region of descending air heading toward the surface, it must pass through the cloud base on its way to the ground. In passing, nature does a bit of a magical trick and makes the droplet disappear.

Ah, but no hocus-pocus here, just old reliable physics. You see, the cloud base it also the condensation level for the atmosphere surrounding the cloud and its neighbours. Above this level, rising air will have cooled to its condensation point and liquid water will be produced from the rising water vapour within the air. That is why all cloud bases are more or less at the same altitude. But reverse the process and the condensation level becomes the evaporation level and liquid water within the descending air will return to the vapour state...and that makes it disappear to our sight.

The combination of slow descent of cloud droplets and their quick evaporation below the cloud base gives us the appearance that the cloud is floating gently on air. Ah the tricks of Mother Nature!

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