Why is a green laser beam visible while a red or blue beam is not?

See this site for a graph of the visibility spectrum. Here is an excerpt:

“The actual visibility of the ray will vary depending on the color, power, moisture and dust content in the air.. Range can be miles depending on the absorption of the medium. Note that the brighter the ray, the less power at the impact point. This is why on a foggy night the ray can be strikingly visible but the impact point will be of limited range.”

It’s because of scattering. Some of the light in the beam must be deflected toward your eye for you to see it. Scattering along the beam will make it visible. Longer wavelengths are scattered less than shorter wavelengths. Red light has a longer wavelength. Little of the red laser light (at 650nm) is scattered so we can’t see the beam of a low-powered red laser pointer. Green light has a shorter wavelength. Much more of the green laser light (at 530nm) is scattered so we can see the beam of the low-powered green laser pointer. Moisture and dust play a role in scattering but the air molecules alone scatter light through Rayleigh scattering.