Why do we get colorful sunsets in western Pennsylvania?

PITTSBURGH — We know that we can get some awesome sunsets here in western Pennsylvania, and we’ve had several in the past few days.

Check out the sunset from May 4. Decaying thunderstorms and some clearing made for a beautiful mix of pinks and purples in the sky.

Just a few days later, we saw this reddish-orange sunset.

To understand why the sky turns such amazing colors on certain days, it’s first important to know why the sky is blue.

The sky is blue due to something we call "selective scattering" of blue and violet light by air molecules. Scattering is simply the reflection of light by particles. This selective scattering is called "Rayleigh scattering."

Blue and violet colors are shorter wavelengths, whereas red is a long wavelength.

Clean air molecules can scatter the blue and violet light very well, which is why the sky is blue.

On the other hand, aerosols are larger particles and effectively scatter orange and red light. During sunset, sunlight takes a longer path through the Earth's atmosphere. Violets and blues are scattered out of the sunlight path and therefore, we see the sky as orange or red.

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So how do we know whether there are more air molecules or aerosols in our atmosphere?  Have you ever heard the sayings, “Red sky at night, sailor’s delight” or “Red sky at morning, sailor take warning?”

This has to do with high and low pressure in our atmosphere. High pressure is typically associated with calm weather while low pressure can bring rain and thunderstorms.

If we look to the west at night and the sky is red, then that would mean a high pressure is moving in.

Under high pressure, the air is sinking and therefore, there are generally fewer clouds. Sometimes there can be an inversion, where there is a layer of warmer air just above the surface. An inversion would tend to trap dust particles near the surface. We know that aerosols scatter the red colors better, so that’s why we see red. Therefore, the sailor would have nice weather moving in because our weather moves from west to east.

If we look to the east at sunrise and the sky is red, then that would mean that the high pressure is moving out and low pressure is moving in. Therefore, the sailor would need to take cover because bad weather is coming.

If there is haze in the air, the sky may appear pink or yellow.  In general, mid- and high-level clouds typically bring beautiful sunsets.

Cox Media Group