Sky Watching

-Colours of the Sky
-Why are Skies blue and Clouds white?
-Rainbows

The blue sky, white clouds and most of the colours of the sky result from light being sent off in different directions as the light collides with air molecules, other substances or the water and ice of clouds. More than 12 miles above the Earth, the sky begins turning dark because there are fewer molecules to send light off in different directions. From space, if you look toward the sun you can see its light; if you look elsewhere you see only black sky, stars and maybe the moon.

Why are Skies blue and Clouds white?

To understand why the sky is blue and clouds are white, you have to realize that the white light from the sun consists of all the colours of the rainbow. Light travels as waves of different lengths; each colour has its unique wavelength. In addition, light will travel in a straight line unless something sends it off in a different direction.

As it turns out, air molecules are just the right size to send the shorter wavelengths of light, mostly blue, off in different directions. Longer waves, such as red, are not scattered by air molecules. As sunlight enters the atmosphere the blue light is scattered by air molecules. The blue light waves spread all over the sky and down in all directions. No matter which way you look, blue light, which is better described as scatted blue light waves, is coming at you from that direction. That explains why the colour of the sky is usually blue.

The sky may also turn white or gray on hazy days while hazy air has plenty of moisture and condensation nuclei that scatter more wavelengths of sunlight. It also turns red, orange or yellow at sunrise and sunset because light is traveling through more air when the sun is low in the sky. The long path through the air means most of the blue and colours with shorter wavelengths have been scattered in different directions. Few reach your eye. You see the yellow, orange and red colours, which pass more freely through the air.

As for the white colour of the clouds, the concept is more or less similar. Since water droplets or ice crystals are big enough to scatter light of all wavelengths, which combine to produce white light. Clouds appear dark when they are in the shadow of other clouds, or when the top of a cloud is casting a shadow on its own base. Moreover, dark clouds are not necessarily rain clouds. Rainy or snowy days are often dark because clouds block sunlight.

Rainbows

Rainbows are formed when raindrops reflect sunlight toward the observer, dispersing the light into colours. This phenomenon can be explained as the prism effect of water.

As sunlight enters raindrops, different colours bend at slightly different angles. Some of the light reflects off the back of the drop and is bent again as it emerges. Drops at different angles send different colours to the observer's eye.

If the light is hitting the raindrops at the proper angle, a secondary, larger rainbow will form outside of the main rainbow. A secondary rainbow is fainter than the main rainbow because the light is reflected twice by each raindrop. This also reverses the colours of the secondary rainbow.

In the middle latitudes where storms travel from west to east, rainbows are usually seen when a storm is over. Whereas in the tropics, where storms travel from east to west, rainbows are often seen before a storm arrives.

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