[thedoc (OP)]

Why don't clouds fall to the ground?
Asked by Isabella


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[chapter podcast=1001476 track=16.10.04/Naked_Scientists_Show_16.10.04_1005769.mp3]  ...or Listen to the Answer[/chapter] or [download as MP3]

[alancalverd]

They do. It's called rain, hail or snow if it happens in big lumps, and fog if they just drift into the mountainside.

[nilak]

The answer seems a bit more complicated to me. For answering this I think we should first analyse fog or even steams in the laboratory (or kitchen). The water droplets seem suspended in the air and not being pushed upwards by flow of air. They react very fast at any motion of air. Above a bowl boiling water they will rise fast then they stabilise. I didn't investigate this, but at first look, it seems that the droplets are so small that when they move through the air they create a drag force that almost stop their fall completely. The larger the droplets the faster they will fall. So my opinion is, the droplets (and hence the clouds) in the absence of a upward flow will fall slowly to the ground if they are small enough.
      Another effect is during descent or climb it may happen to evaporate so they appear at approximately the same level. In this case, larger droplets fall as rain, smaller droplets rise up.   At high altitudes there are also strong winds, but usually sideways. Build ups are indication of upward currents. Clouds may appear slow because they are very large but usually they move and change shape at high speeds.