Making Clouds
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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 mare¹s tails. How do they all form?

Hi, I¹m 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 sun¹s muffled light, caused by refraction from the crystals..

So what makes the clouds different? That¹s 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.