dimecres, de desembre 01, 2010

Colors in the Sky

Atmospheric optics is a collective name for everything having to do with light and color in the atmosphere, which includes subjects on the ground. Of all this, everybody knows the rainbow; other examples are the sunset colors, the flattening of the low sun, the occasional solar rays, and the light curtains of Aurora. But there is a vast number of phenomena which you may or may not have heard of, like rainbows, mirages, halos, coronas, sun pillars, air glow, green flashes, sunsets and auroras. Those here are the most typical but not the only ones.

The path of light can be obstructed in several ways:

- It is reflected from the surface of large particles such as raindrops and ice crystals
- It refracts gradually as it travels through the atmosphere, or abruptly as it passes between water and air.
- Light is scattered in all directions when it passes microscopic aerosol particles or air molecules
- It diffracts into a field of patterned waves as it skirts around tiny cloud droplets.

Each manner of obstruction produces different optical phenomena.
- Halos and rainbows are formed when ice crystals and raindrops refract and reflect light.
- Extreme refraction results in mirages.
- Scattering causes crepuscular rays, and the blue color of the sky during daytime, or red at sunsets.
- Coronas and iridescent clouds are produced by light that is diffracted by tiny cloud droplets.

Halo (it occurs when light travels through high thin clouds containing millions of tiny ice crystals. Because these crystals have a similar elongated hexagonal shape, light exiting through the cloud refracts 22 degrees -the radius of the Sun Halo!)

Coronas (the glow of the corona is a million times less bright than that of the photosphere, so it can only be seen when the disk of the Sun is blocked off in a total solar eclipse, or by using a special instrument, a coronagraph).

And scattering, refraction, diffraction or absorption that varies with wavelength determines and modifies the colors of all phenomena. Isn't that beautiful?