24.6 Predicting the Weather

Figure 27 Moonlight reflecting off ice crystals in cirrostratus clouds can cause a halo to appear around the moon. Such a halo often indicates that precipitation is on the way.

An old folk rhyme includes this line: “The moon in halos hid her head….” The rhyme suggests that a halo around the moon is a sign that rain or snow should be expected. This bit of folk wisdom has some scientific validity. When you see a halo around the moon, what you're seeing is moonlight shining through cirrostratus clouds. Because cirrostratus clouds often move in before storm clouds, a halo around the moon might indeed indicate precipitation in the near future.

Now consider another bit of weather wisdom from long ago: If a cat scratches itself while sitting on a fence, expect rain before night. Would you look for rain if you saw a cat do that? Probably not. That's the trouble with folk wisdom. Some of it makes sense, and some of it doesn't. Luckily, there are now more reliable ways of forecasting the weather.

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