Why Does the Sun Change Colors at Sunset?
Sunsets have always captured the imagination with their brilliant colors--even more so when people discover that sunsets vary depending on location, or when they discover an unusual or especially colorful sunset. Sunsets in the desert tend to be very orange, whereas the sunsets in urban areas tend to be muted and may have other colors like yellows and reds. But, whether in the deserts of Arizona or the high rises of New York City, all sunsets have two things in common: light and atmosphere.
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Light
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Light is electromagnetic energy that travels in waves and has frequencies both visible and invisible to the human eye. The visible light range is known as the ultraviolet (UV) spectrum. To the naked eye, UV light appears white, but it is actually made up of several different wavelengths of color. These colors are arranged in order from shortest to longest and blend from one to the next, like a rainbow. The order of the wavelengths are violet, blue and green, yellow, orange and red -- which means violet has the shortest wavelength; and red, the longest.
Atmosphere
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The atmosphere is the mixture of gases, water, dust, and other materials surrounding the Earth. The closer the atmosphere is to the Earth, the thicker it is; and this is due to the Earth's gravitational pull. As we travel further away from the Earth, the gravitational pull becomes weaker and the atmosphere becomes thinner until it dissipates into space. The Earth's atmosphere is not exactly the same all over. There are variations in composition based on location, weather patterns and human factors. For example, the atmosphere over the ocean will have significantly more water and salt than the atmosphere over the desert. And the atmosphere over a city may have more of certain types of gases, while the atmosphere over a volcanic island will have more ash and soot. These variations in atmospheric particles are what determine the how light behaves in the atmosphere.
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Light & Atmosphere
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Light travels in a straight line until it encounters the particles in our atmosphere, at which point the light reacts, but the reaction depends on the individual light wave lengths and the type and size of the particles. Water particles are larger than light waves, so when light hits a water molecule, the water acts as a mirror and bounces (or reflects) all the light wavelengths at once. As a result, the light continues to appear white to the naked eye. When light hits smaller particles, like dust and gas, the smaller particles absorb some of the light while other light passes through. When the particles absorb the light, they begin to radiate in the same color that was absorbed. So, if a bunch of gas molecules absorb yellow light, they will radiate yellow, and that color will be visible to the naked eye. Any color of light can be absorbed but the shorter wavelengths (like blue) are absorbed more often than the longer wavelengths. The process of absorbing and radiating (or scattering) light is called Rayleigh scattering.
The Sun During the Day
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The position of the sun has a direct influence on the way light enters the atmosphere. During the day, the sun is positioned high in the sky and emits light at a relatively short angle to the earth. Because of the shorter angle, the longer light waves pass straight through, while the shorter waves get "hung up" on particles in the atmosphere. As the shorter blue, green and violet waves are absorbed and scattered, the sky appears blue to the naked eye. Meanwhile, because the longer yellow and red light makes it through, the sun appears white or yellow against the scattered blue light of the sky.
Sunrise, Sunset
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As the sun rises and sets, it's in a lower position, which increases the angle at which light enters the atmosphere. Because of the longer angle, the shorter light waves never reach the lower areas of the atmosphere so that only the longer yellow, orange and red light is visible. As a result, the sunlight appears darker as more of the longer light waves are revealed. The major difference between sunrise and sunset is that sunsets tend to be more colorful. This is because most Earth inhabitants are active during the day and kicking dust and debris into the air. In the evening, these increased dust particles act as a filter, reflecting more colors into the sky.
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