Why is the sky bluer in the winter than the summer?

Like the colors on the leaves, the sky also changes color with the seasons. In the winter, the sky has more of a blue hue. But in the summer, it looks milky white, even when free of clouds. Why is that?

Why is the sky blue to begin with?

The sun emits white light, which has all colors of the rainbow within in. But as those light waves approach the Earth, they begin to encounter air molecules–the Earth’s atmosphere. Air molecules, including the oxygen and nitrogen that make up about 99% of it, are just the right size to scatter the light of lower wavelengths and higher frequency. That light is the blues and violets. The other colors with higher wavelengths aren’t impacted as much by the atmosphere, so they continue onto the surface. The blues and violets are scattered around the air molecules in the atmosphere, giving us the blue color to the sky.

Types of light scattering

This phenomenon is specifically knowns as Rayleigh scattering. Air molecules are the perfect size to scatter the blues and violets. This is more prominent in a dry atmosphere, like the winter. Rayleigh scattering is responsible for the blue sky, as well as the orange and red sunsets when the sun angle is lower.

Mie scattering occurs with larger particles like water vapor, dust, and smoke. This is most common in a moist atmosphere, like in the summer. Mie scattering is responsible for the milky look to the sky even when there are no clouds. The smokey and hazy skies in the summer also appear milky due to Mie scattering.

If you were to look at a picture of a clear sky in the winter vs the same location in the summer, you would see the difference. Winter typically brings a drier atmosphere, dominated by Rayleigh scattering. Summer has more moisture, making Mie scattering the dominant feature.

The color of the sky is also impacted by the sun angle. Lower sun angles in the winter help to enhance the Rayleigh scattering, while higher sun angles in the summer help promote more Mie scattering.

So now if you look at the sky in the winter and notice the crisp blue hue to it in the winter, you will know why that is the case! And similarly in the summer, you can remember why Mie scattering causes the milky look to the sky.