Science Questions

Why don't clouds fall to the ground?

Mon, 3rd Oct 2016

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Question

Isabella asked:

Why don't clouds fall to the ground?

Answer

We put Isabella's question to Chris Smith... Above the clouds

Chris -   Now the reason that clouds stay up where they are is because there are strong winds pushing the clouds upwards. What do I mean by that?

 

Well, clouds are full of water and, in fact, if we weighed a cloud that was one kilometer by one kilometer by one kilometer, because scientists have measured how much water there is in a cloud it would weigh about five hundred tons. But the water isnít in one giant blob, itís in lots of tiny particles and ice crystals which are called hydrometeors, and these have a very big surface area compared to their size. And because the Earth is constantly being heated up by the Sun shining on the Earthís surface, warming the surface, the surface warms the air above the surface, and the warm air rises because it expands and become less dense. So thereís a column of warm rising air which is pushing upwards and this hits these tiny water particles which are trying to fall down under their own weight and it keeps them up there.

 

And thatís why clouds have a flat bottom, because thatís the point at which the tiny water crystals particles inside, which are trying to fall downwards, meet the warm air coming upwards and the two balance each other out, and the form the cloud base. So thatís why clouds stay up there and also why five hundred tons of water in one cloud doesnít come crashing straight down to the Earth. Good thing really, isnít it!

 

 

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They do. It's called rain, hail or snow if it happens in big lumps, and fog if they just drift into the mountainside. alancalverd, Tue, 4th Oct 2016

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.
Nilak, Sat, 26th Nov 2016

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