Answer Two Science questions

Nassr Alsadeg

Extra Credit 4

Why does a blue sky have white clouds? Why do the clouds appear red at sunset? What is the “Green Flash seen in some sunsets in Hawaii??

The Earth's atmosphere is primarily made of Oxygen and Nitrogen. It happens that both these are made up of atoms of about the same size. Our Sun produces light which is actually a mixture of light of a lot of colors. These various colors are actually just different sizes of waves of radiation. Our eyes only recognize the total of all this light, and so it appears to us to be a bright yellow, almost white color. Clouds include little condensed droplets of water. These droplets are a lot bigger than atoms, and so they are not as transparent as oxygen or nitrogen gas. Therefore, all light colors tend to reflect off of those cloud droplets. During the day, that means that the light reflected off them appears bright white.

Sunsets are reddened because for sun positions which are very low or just below the horizon, the light passing at grazing incidence upon the earth must pass through a greater thickness of air than when it is overhead. Just before the sun disappears from view, its actual position is about a diameter below the horizon, the light having been bent by refraction to reach our eyes. Since short wavelengths are more efficiently scattered by Rayleigh scattering, more of them are scattered out of the beam of sunlight before it reaches you. Aerosols and particulate matter contribute to the scattering of blue out of the beam, so brilliant reds are seen when there are many airborne particles, as after volcanic eruptions.

The reason for a green flash lies in refraction of light in the atmosphere: light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light curves more than lower frequency light, so green/blue rays from the upper limb of the setting sun remain visible after the red rays are obstructed by the curvature of the earth. Green flashes are enhanced by atmospheric inversions, which increase the density gradient in the atmosphere, and therefore increase refraction. A green flash is more likely to be seen in clear air, when more of the light from the setting sun reaches the observer without being scattered. We might expect to see a blue flash, but the blue is preferentially scattered out of our line of sight and remaining light ends up looking green.