Ask A Met: Why Is The Sky More Blue Today?

This week, Morning Brief reader Collette wrote in to ask, "There may be blue sky, but what makes for a deeper blue in one area and a lighter blue in another?"

Meteorologist Jonathan Belles: It all comes down to optics, everything between you and the sun. In Earth's case, that's a whole lot of atmosphere.

When the sun is straight up at noon, you see blue sky because all the other colors in the spectrum up until blue and purple are scattered away. This is known as the Rayleigh effect.

We usually don't see purple because our eyes are in tune to see blue and red. With red being scattered away, we end up seeing blue.

The things that do the scattering for those other colors of the spectrum can be anything from ash to moisture particles. Our atmosphere is made up of particles. Nitrogen is in there, oxygen is in there.

But when you add bigger particles, we start to get away from blue. That would be ash, soot or anything else, including pollution that we put into the atmosphere.

We see red when the sun goes down because we're no longer looking straight up. We're looking at an angle, and we have to look through all the particles that we've added. That can be many times more particles.

That's part of the reason when we get those strange colors in the skies from wildfires and smoke plumes.

So, the atmosphere literally rises taller during the afternoon and during the summer, which means there's more particles up there. Of course, that’s with the added caveat that there is more moisture during the summer, too.

So, on a warm afternoon, the sky looks a little bit more white or washed out.

During the wintertime, when you look straight up, the atmosphere is quite a bit more thin. There are fewer particles for light to bounce off. That’s when you get those deep, dark blue skies.

Sometimes in the Arctic sky or Antarctic sky, you'll see almost a purple color because of that temperature thinness.

Depending on where you are, elevation relative to sea level can change your perception of the sky as well. If you were, for example, standing on Mount Everest, on that rare day with clear skies and without a storm, you'll see the same dark blue or purplish blue sky, because you're already above the bottom portion of the atmosphere.

As we head into cooler fall temperatures, we’ll start to see deeper blue skies.

That clarity is also true of night skies. The middle of the night in the middle of January is probably the best time for sky gazing because the atmosphere is so thin. People hate when I tell them that, though, because no one likes standing in dark, 20-degree weather to go look at the stars.

Do you have a question to ask the meteorologists at Weather.com? Write to us at [email protected] and we’ll pick a new question each week from readers to answer.