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Touch of Grey
Mon Jan 20, 2003
Listen in RealAudio 
Hi, I'm Bryan Yeaton for The Weather Notebook. For those of us who spend any time at all
looking at the sky, it's not difficult to notice that clouds come in a variety of shades and
colors. The colors are usually limited to the few hours right around sunrise and sunset. The
shades, though, are observed during the middle part of the day.
The shade of a cloud -- in other words, how white, black or grey it is -- depends on its age,
or more precisely, the age of the individual drops of water that make up the cloud. Cloud
drops are small -- several thousand times smaller than rain drops, which themselves are only a
few millimeters across. An interesting thing about cloud drops is that the smaller they are,
the more light they reflect. When light hits a very small drop, all of the light bounces off.
As the drop gets bigger, it absorbs some of the light. So clouds with big cloud drops are
darker.
The bigger the drop, the older it is. In the short life of a cloud drop, which varies from
several minutes to several hours, it either grows and falls, or grows and evaporates. Either
way, the cloud will be darker if it is full-grown. We are all correct in being a bit leery of
a dark cloud. A dark cloud has big drops, which could start to fall. Take some time to check
out a big puffy cumulus cloud. The top of the cloud will almost always be whiter, not
completely because it's in the direct light, but because the top is the youngest part of the
cloud.
The Weather Notebook is made possible by a grant from The National Science Foundation.
Additional support comes from Subaru of America, The Beauty of All-Wheel Drive. Thanks today
to the entire Weather Notebook staff, Doug Sanborn, Melody Nester, Sean Doucette and Peter
Crane.
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