Half Dome (in California’s Yosemite National Park) under a bright blue sky.

To understand why the sky is blue, we need to understand a little about light. Light is a form of electromagnetic energy, which can propagate through empty space. We describe this energy by wavelength – the distance between successive crests of the wave. Our eyes are sensitive to light with wavelengths between approximately 0.4-0.7 microns (one micron is a millionth of a meter or one one-hundredth the diameter of a human hair). Blue colors have wavelengths between about .455 and .492 microns, while red colors have longer wavelengths between .622 and .780 microns.

When light beams interact with particles suspended in air, the energy can be absorbed or scattered. Scattering causes the light path to change direction. The amount of scattering is a function of the size of the particle relative to the wavelength of the light falling on it. Particles that are tiny compared to the wavelength of the light scatter selectively according to wavelength.

While air molecules scatter all colors, they scatter violet and blue the most. That’s why, no matter which direction we look, it’s more likely that blue light has been scattered back toward us. Why not violet? Two reasons. The sun emits more blue than violet light, and our eyes are also more sensitive to blue.

At sunset and sunrise, sunlight must pass through more atmosphere than during the day when the sun is higher in the sky. More atmosphere means more molecules are scattering violet and blue light. If the path is long enough, all of these colors are redirected out of our line of sight, while much of the yellow, orange and red colors continue straight from the sun to the eye. This is why sunsets often are composed of yellow, orange and red light.

Scattering affects the color of light coming from the sky, but the details are determined by the wavelength of the light and the size of the particle. The short-wavelength blue and violet are scattered by molecules in the air much more than other colors of the spectrum. This is why blue and violet light reaches our eyes from all directions on a clear day. But because we can’t see violet very well, the sky appears blue.

Scattering also explains the colors of the sunrise and sunset, Ackerman says.

“Because the sun is low on the horizon, sunlight passes through more air at sunset and sunrise than during the day, when the sun is higher in the sky. More atmosphere means more molecules to scatter the violet and blue light away from your eyes. If the path is long enough, all of the blue and violet light scatters out of your line of sight. The other colors continue on their way to your eyes. This is why sunsets are often yellow, orange, and red.”

And because red has the longest wavelength of any visible light, the sun is red when it’s on the horizon, where its extremely long path through the atmosphere blocks all other colors.

Thunderstorms, which can be the home of tornadoes, usually happen later in the day, when the sun is approaching the horizon. That creates a reddish tinge in the sky, as any fan of sunsets knows. But light under a 12-mile high thundercloud is primarily blue, due to scattering by water droplets within the cloud. When blue objects are illuminated with red light, Bachmeier says, they appear green.

Green is significant, but not proof that a tornado is on the way. A green cloud “will only occur if the cloud is very deep, which generally only occurs in thunderstorm clouds,” Bachmeier says. “Those are the kind of storms that may produce hail and tornadoes.” Green does indicate that the cloud is extremely tall, and since thunderclouds are the tallest clouds, green is a warning sign that large hail or a tornado may be present.

If this explanation is confusing, Bachmeier offers some alternative folk wisdom for the color change: that tornadoes sucked frogs and grasshoppers into the sky.

“The water droplets are usually about 10-20 micrometers across, or about one-twentieth of the diameter of the period at the end of a sentence,” Petty says. “A volume of air the size of a sugar cube may contain many thousands of droplets.”

Clouds are important for many reasons. They reflect sunlight and trap heat, and thus influence global warming. They are also the source of rain and snow, and the site of thunderstorms, hurricanes and tornadoes.

Clouds can suddenly appear in the sky, Petty says. “When humid air rises, it cools until the invisible vapor it carries is forced to begin condensing as droplets, just like vapor from your shower condenses on your cold bathroom mirror.”

At extremely high altitudes and extremely cold (-40 F and below) temperatures, clouds may contain only ice crystals. These clouds usually look wispy or hairy. The clouds that produce rain and snow are warmer, and commonly contain a combination of water droplets and ice crystals, Petty says.