The visible part of the electromagnetic spectrum is often abbreviated ROYGBIV based on the first letters of each color’s name (red-orange-yellow-green- blue-indigo-violet).
The Appearance of the Sky
The colors we see in the sky come from sunlight that is scattered by molecules in the atmosphere. This process is called Rayleigh scattering. Nitrogen and oxygen make up most of the molecules in our atmosphere, but any gas or aerosol suspended in the air will scatter rays of sunlight into separate wavelengths of light. Consequently, when there are more aerosols in the atmosphere, more sunlight is scattered, resulting in more colorful skies.
Why Is the Sky Blue?
Sunlight is made up of all the colors of light. It appears as white light when all the colors are together. Sunlight travels as waves of energy, and different colors of light have different wavelengths. Red light has long wavelengths, while blue light has short wavelengths. Light bounces off of air molecules in the Earth's atmosphere, scattering in all directions. Blue light is scattered more than other colors of light because of its shorter, smaller waves. Because blue light is scattered more than other colors of light, the sky appears blue.
Blue isn’t the dominant wavelength in the visible part of the spectrum, but our sky appears blue on a clear day because Earth’s atmosphere scatters shorter, blue light wavelengths more effectively.
Why Are Sunrise and Sunset so Colorful?
The angle of sunlight as it enters the atmosphere also affects the color of the sky. During sunrise or sunset, when the Sun is close to the horizon, the light must travel through more of the atmosphere than it does when the Sun is overhead. This results in more scattering of light, including longer wavelengths such as yellow, orange, and red, which creates colorful sunrise and sunset skies.
Why Does the Sky Look Different Throughout the Year?
Certain times of the year tend to have more aerosols in the air than others, which leads to more colorful skies. In certain climates, the air tends to be dustier during the summer months, due to hot, dry weather. In the fall, when many farmers harvest their crops, more dust is suspended in the air. The full moon in the fall, called the Harvest Moon, often appears orange because of the extra dust particles in the sky. As with red sunsets, the unusual Moon coloration might be pleasing to the eye, but it is a sign of poor air quality.
Sundogs
Peggy LeMone
Sundogs are brightest in winter because ice crystals are more common, but can be seen at other times of year as well. A sundog (also called a mock sun or parhelia) is a brightly colored spot that appears along one, or both, sides of the Sun. Sundogs most often form when there are cirrostratus clouds covering the sky and the Sun is near the horizon. The cirrostratus clouds are made of hexagonal ice crystals which bend light, creating sundogs and other atmospheric optics such as the 22-degree halo.
