Why is the sky so blue in September?

On a clear day light entering our atmosphere collides with various particles, mainly nitrogen and oxygen. These collisions then scatter shorter wavelengths turning the skies blue.  The sky appears a deeper blue on days when there are fewer particles in the atmosphere to scatter the sun’s light, such as a clear day with low humidity.

On the other hand, compare that to a hot summer day when it is very humid. There are more water vapor particles for the sunlight to be reflected off of, making the sky appear milkier, or hazy.

If you have ever visited the Rocky Mountains, you may have noticed the brilliant blue skies. There is less atmosphere between the ground and the sun at higher elevations, equating to less sunlight being reflected.