Re: Why does the sky change color with the seasons?

Hi Steanne!

The phenomenon you are describing sounds like Rayleigh scattering. Rayleigh scattering is the dispersion of electromagnetic radiation by particles that have a radius less than approximately 1/10 the wavelength of the radiation. It is named in honor of Lord Rayleigh, who published a paper in 1871 describing this phenomenon.

Sunlight spans a wide range of electromagnetic wavelengths. When they are all added up, sunlite looks more or less white. The Earth's atmosphere contains mostly Nitrogen and Oxygen which scatters short wavelengths. Nitrogen and Oxygen atoms are small enough to allow some of the wavelengths which fall in the visible spectrum to pass through with little scattering. What this means is that light that is more red (longer wavelengths) will follow a straight path to the Earth's surface, whereas more blue light (shorter wavelengths) will be scattered. This scattering makes blue light bounce around in the atmosphere, some of it eventually does make it to the Earth's surface.

That's essentially why the sky is blue in the daytime, but why does it change colors? As a consequence of Rayleigh scattering, more atmosphere means more scattering of blue light. In the morning and evening, the sun is closer to the horizon so sunlite must travel through more of the atmosphere before it gets to our eyes. What happens is that most of the blue light is scattered, leaving mostly red light. The end result is that the color of the sky changes depending on the angle of the Sun.

The angle of the sun changes throughout the course of a single day, so we get different colors from morning to evening. The angle of the sun also changes during the course of a year, that's what causes change of seasons. Depending on the latitude of where you live, you will see more or less of a change in the color of the sky for different seasons. I was born South India about 12.5 degrees North latitude. In the wintertime, the sun was still fairly high in the sky. The temperature didn't get much below 70 degrees in the winter and there wasn't much of a change in color of the sky. Now I live near Washington, DC which is about 39 degrees North latitude. The sun is much closer to the horizon in the wintertime, so it is colder and the sky color change is more pronounced. Of course, you will notice a faster change during Spring and Fall because the sun appears to move faster North or South.

There are other atmospheric effects that affect the color of the sky. Humidity and other particles in the air cause the sky to look more gray because the air is not as clear. Warm air is capable of holding more water (more humidity), so hot, humid days result in a grayer skies. Humidity is lower in the winter, so the sky is clearer.

I hope I've answered all of your questions, if not, please feel free to email me at madhu@madhu.com.