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Red Sky at Night

People have been predicting weather since long before we had Doppler radar and satellite photos, and many of these prediction strategies have survived as popular folk sayings.

For example,"Red sky at night, sailors' delight. Red sky at morning, sailors take warning." This common phrase is supposed to predict whether or not a storm is likely, but how well does it work?

Actually, it's not immediately clear what this saying means in the first place. Since the sky is black at night and light in the morning, we can assume that the saying refers to sunset and sunrise. However, aren't sunsets and sunrises always somewhat red?

The key to this saying is to look at the part of the sky where the sun is not rising or setting. In other words, look at the eastern sky at sunset, and the western sky at sunrise.

The sunlight itself at sunset or sunrise is always reddish because it has to pass through more of our atmosphere, removing most of the other colors. In order for the sky to be reddish away from the sun, however, there needs to be moisture in the air; moisture that is possibly associated with a storm.

Most storms move from west to east, so red in the eastern sky at night means the storm has already gone past, delighting our sailors. Red in the western sky at dawn means the storm is on the way.

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