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Making Clouds
Tue May 03, 2005
Listen in RealAudio 
Clouds. They can be big fluffy things, long stringy things, flaky things, and things that
cover the sky. Veils, caps, heaps, layers, mackerels, anvils, and mare's tails. How do
they all form?
Hi, I'm Bryan Yeaton for The Weather Notebook.
Most clouds form in exactly the same way. Rising air. Air can rise due to its being
warmer, and thus lighter than surrounding air, or can be forced upward by geographic
features, such as mountains. When this rising air reaches its dewpoint, invisible water
molecules combine to form visible droplets. But even these are tiny, a million are
needed to make one raindrop.
If the air is cold enough, these droplets will freeze, and the highest clouds we usually
see, cirrus, are made up of ice crystals. If you see a thin layer of high clouds covering
the sky, look near the sun‹you might spot a halo surrounding the sun's muffled light,
caused by refraction from the crystals.
So what makes the clouds different? That's where it gets a bit complicated. Upper air
patterns, moisture at different levels, wind, cold fronts and warm fronts all contribute to
variations. Fluffy cumulus clouds are caused by radiational heating of the ground, or
when a cold front burrows under warmer air, sending it upward toward the
dewpoint.
When a warm front displaces a colder air mass, the warmer air rises over the colder,
before pushing it away. The interaction zone,again, the rising warmer air, is where we
get the layered stratus clouds, which can not only cover the sky, but can cause it to rain
for days, something many parts of the country are wishing for right now.
The Weather Notebook is produced at the Mount Washington Observatory, with major
support from Subaru, the Beauty of All-Wheel Drive and the National Science
Foundation.
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