On Sat, 6 Nov 1999 you wrote:

> What makes the sky blue? And why?
>
> Thanks,
> Chris

Hi Chris,

The sky is blue because molecules and tiny particles that make up Earth's
atmosphere scatter light from the sun. When sunlight hits a molecule, it
can bounce off in any direction. Due to a physical effect called the
Tyndall effect, when light scatters off of particles that are about the
same size as the light's wavelength, shorter wavelengths of light (blue
colors) bounce off more often than longer wavelengths of light (red
colors).

The sun emits light of all colors. So, when sunlight passes through the
atmosphere, the blue light gets scattered, making the sky blue, but the
other colors pass through. At sunset, the sunlight travels through so
much atmosphere that not only the blue light, but also the green and
yellow light, are scattered by the sky. This makes the sun appear red,
as only the orange and red light has not been totally scattered, and turns
the sky many different colors.

If the atmosphere is full of particles much larger than the wavelengths of
visible light, such as water droplets or dust, then all colors of light
are scattered. So, on hazy days the sky appears more white than blue.

Thanks for writing!

Sincerely,
Kurtis Williams

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