Weather In-Depth: Why is the sky blue?

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ROCHESTER, N.Y. —  We just had a period of three weeks in Rochester with essentially just bright sunshine across Western New York.

The big exception was Monday when we measured a significant amount of rainfall. But today it was back to the clear blue skies for a good portion of the day. In my opinion, when the atmosphere becomes very dynamic with lots of changes from day to day, we appreciate the nice weather that much more.

So on a day like Tuesday, this begs the question, “why is the sky blue?” One of our producers at News 10 NBC asked that very question recently and I thought that would be a good topic for our In-Depth weather segment.

Our explanation starts with incoming short-wave radiation that is coming off the sun. This is the sunshine we love so much. If you hold up a prism to that visible light, it will actually split the light into the colors of the spectrum.

As that light passes through space it will eventually come in contact with the outer atmosphere of the Earth. Well, guess what? Those molecules in the atmosphere are just the right size to split out and scatter just a small portion of the light and that fragmentation is the color blue. This is probably where the term “big blue marble” comes from when describing the Earth.

However, there are few of exceptions to this blue color. When the sun angle is very low on the horizon the visible light is cutting through a greater portion of the atmosphere. As a result, that will scatter a different portion of the spectrum and we usually see that beautiful red or orange sunset.

Another exception is when we have increasing amounts of pollution like smoke. Unfortunately, that is a color change we do not want to see.