Powerful cold fronts move across North America at this time of year. These blankets of dry, cold air push away the clouds and haze, providing some amazingly beautiful blue skies. That color is produced by the interaction of sunlight with Earth’s atmosphere.

The Sun is classified as a yellow star because its energy output peaks at yellow wavelengths. And if we could see the Sun from a distance of a few light-years, where it would appear as only a pinpoint of light, it would have a yellow hue.

But from close range, the Sun is so intensely bright that we see its light as a mixture of all the colors of the rainbow. As a result, it looks white.

As the Sun’s light enters Earth’s atmosphere, it’s subjected to a number of effects. Most of the time, the most important effect is Rayleigh scattering. It’s named for a British scientist who studied the effect in the late 19th century.

Blue light waves are shorter than waves of red light. That makes them the right size to bounce off molecules of nitrogen and oxygen in the atmosphere. That scatters them in random directions. Since the blue wavelengths are scattered across the entire sky, the sky looks blue.

Molecules in the air actually scatter a lot of violet light as well. But our eyes are more sensitive to blue wavelengths, so we see the sky as distinctly blue – the frosty color of clear winter days.

Tomorrow: the oddball month of February.

Script by Damond Benningfield