To understand why the sky is blue during the day, you must first understand light. Light on Earth comes mainly from our star, the Sun, which keeps Earth from being a very dark world.

Light travels from the Sun at 186,282 miles per second in a vacuum. Now, that's fast! At that speed, the Sun's light reaches Earth in a little over eight minutes after traveling around 100 million miles. Now that's really fast! However, when light hits our atmosphere, it slows down to only 186,225 miles per second. Moonlight is light from the Sun that is reflected from the surface of the Moon to Earth. It reaches us in just three seconds! So as you can see, light is some pretty fast stuff!

The white light we see is only a small part of a broader band of radiation from the sun called the electromagnetic spectrum. All electromagnetic radiation, including visible light energy, travels in waves. Each type of wave has a unique wavelength and frequency. By using a prism, visible light can be broken into a rainbow of six colors: red, orange, yellow, green, blue, and violet. Each color in the visible spectrum has its own wavelength and frequency; red has the longest wavelength and the lowest frequency and violet has the shortest wavelength and the highest frequency.

As light enters the atmosphere, it usually strikes air molecules and aerosols, tiny particles of dust and drops of water vapor and the bounces in all directions.This bouncing is called scattering. The shorter the wavelength, the more light is scattered. Because blue light has a shorter wavelength, it is scattered ten times more than red light. Even though violet has a shorter wavelength than blue, our eyes are not as sensitve to violet and the sky looks blue. When light finally reaches our eyes, the blue light has been scattered all over the sky, and that is why the sky looks blue.

In early morning or late evening, light has to travel a greater distance and through a much thicker part of the atmosphere. The atmosphere is thicker (has more aerosols) closer to the ground due to gravity, and combined with the fact that light has to travel a greater distance, even more blue light is scattered. With most of the blue light scattered, sunrises and sunsets look quite red. When aerosols increase in the atmosphere from forest fires, volcanic eruptions, desert storms, or other pollutants, sunrises and sunsets will become even a more brilliant red than usual. Check out your sunrises and sunsets to see if this is happening.

Sometimes, the entire sky is not the same shade of blue. It is usually a lighter blue near the horizon and is almost white, at times. Reflections of light from the ground are part of the reason why this happens. The sky is a darker color when you are near the tops of mountains. Astronauts can see the sky darken through windows of their spacecraft as they launch into space.

Keep your eye on the sky. You never know when you might see something you have never seen before!