The answer to both of your questions is actually the same. Clouds
aren't just random droplets of water. What they are is tiny droplets of
water that have condensed from water vapor in areas where the
conditions (temperature, air pressure and humidity) are exactly right.
Anywhere where you have the correct combination of these conditions,
you'll get a cloud. Anywhere that you don't, you don't.
Areas with this kind of conditions don't occur in random tiny
places. Instead, there will be a large area of one condition that
gradually shifts into another. (If this doesn't make sense, think of
just temperature. You don't get a different temperature at every spot
on the globe. Instead, you get a large area of warm air that gradually
shifts into a large area of cooler air. The same is true for air
pressure and humidity.) This is why you get a large area of 'cloud' in
one place and nothing at all in another. You can think of clouds as
being physical maps of the weather conditions in the sky.
This is also what holds the clouds up. In general, if a cloud is
high up in the sky, the conditions just below it aren't right for the
cloud to form. As the water droplets fall, they reach areas with the
wrong temperature or pressure and evaporate again. Only if the clouds
become densely packed enough for the water to form large enough
droplets can the droplets fall to the ground. Not all clouds form high
up, either... sometimes the right conditions can occur at groundlevel,
giving us fog.
(republished on 08/01/06)