Quote:
Originally Posted by yguy
What keeps clouds airborne?
The obvious answer is that they're less dense than the air beneath them, but I don't see how that's possible. I'm assuming H2O that's visible can only be in the form of droplets, which are denser than the surrounding air, so I don't see how a cloud can be anything but denser than the air beneath it. What's wrong with that assumption, or what else am I missing?
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Fair weather cumulus clouds (the cauliflower-shaped things you see on summer days) form at the top of rising columns of air (thermals).
The usual description is that air is heated by contact with the ground. It expands and is so less dense than the surrounding air and begins to rise. This draws in more air beneath, and for a few minutes a rising column, or sometimes a rising vortex ring of air exists. These so-called 'thermals' are used by soaring birds and glider pilots to gain height. When you see buzzards or gliders circling round and round, you can be sure that they've found a thermal and are using it to go up. These vertical winds often blow upwards at surprisingly high speed - ten to twenty miles per hour is not uncommon even in England on an otherwise calm summer's day. The air surrounding the thermals sinks back towards the ground, but the areas of 'sink' are usually much larger (and correspondingly slower-moving) than the areas of 'lift'.
Where a thermal is departing from near the ground, the surrounding air has to rush inwards to fill the void - this is what causes summer breezes - often you will notice the prevailing breeze change in strength or direction, sometimes completely reversing in direction for a minute or so, as a thermal passes nearby.
As the air rises, it expands and cools, but is still hotter than the surrounding air, so continues to rise. Eventually the rising air cools till hit reaches the 'dew point' - this is the temperature at which the water vapour in the air condenses out into tiny water droplets. On a particular day, the moisture content of the air and the temperature determine at what height this happens. This height is called 'cloudbase'.
Often cloudbase will be a couple of thousand feet in the morning, and then rises as the day warms up. In northern Europe, cloudbase is rarely higher than about six thousand feet, but in more tropical countries it may be fifteen thousand feet or so. When the air is moist enough or cold enough that cloudbase falls to zero feet, then we get clouds at ground level; we call this mist or fog.
So cumulus clouds aren't really 'floating' - they are just markers sitting on the tops of rising columns of air. As soon as the thermal 'dies' the cloud that sits atop of it begins to dissipate. When there are plenty of these clouds around, look at them carefully. You'll see that some of them have very flat lower surfaces - often they are a bit darker (because the cloud is higher and blocks more light). These are the clouds that are 'working' and marking the tops of still-active thermals. Other clouds that have a more ragged wispy appearance are the ones that were formed by now-extinct thermals - these clouds don't last very long. On a calm day, if you have the patience to watch one for ten minutes or more, you will see it evaporate away.
Other more layered types of cloud last much longer, but the principle is usually the same - the cloud is a dynamic thing - the misty water droplet laden air is heavier than clear air; it tends to fall, but that causes its temperature to rise to the point where the water droplets evaporate again. At the same time, any 'clear' air below the cloud which happens to rise is cooled as it rises to the point where water droplets form.
Sometimes there are enough tiny droplets to clump together and form rain. But most rain never reaches the ground - as it falls into warmer air at lower heights it evaporates back into vapour.
Water vapour, in the form of H
2O molecules is actually lighter than most other air molecules - N
2 and O
2.
There is also the complicating fact that when water vapour condenses out into droplets it releases its latent heat of vaporisation into the containing air; this tends to warm the containing air making it rise and therefore cool more - which causes yet more water vapour to condense. It's a runaway process until all the water vapour is used up, or the temperature falls to freezing point. When freezing point is reached (higher clouds are formed from ice crystals, not water droplets) the same thing happens again with latent heat being released to the air as water turns to ice.
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