Atmospheric Optics
Light is all around us, from both natural and artificial sources, during the day and the night. We think we understand it, and that what we see by it is an exact representation of what we are looking at. However we can be mistaken; the setting sun seen on the horizon has in fact already dropped below the horizon. Twinkling stars are also an effect of this same process, called refraction.
Light passing through a medium such as air or water can be absorbed and scattered by the molecules in the medium or refracted by changes in air density. Earth's atmosphere contains air, water and dust molecules that cause light rays from the sun to change direction as they pass through slightly different densities of air - this is known as refraction. The amount of refraction of light is dependent on the refractive index (a measure of how much a substance bends light, dependent on its density and the type of molecules) and the incident angle at which the light enters the substance. Denser substances such as water will bend the light more than a less dense substance like air, and light entering a substance at an angle will refract more than entering perpendicular to the substance's surface. Air itself can have different indices- air that is warm will be less dense and so will refract light less.
Looking up in the direction of the zenith, an observer will look through one air mass- ie the minimum amount of air that light from the sun will travel through to the surface. Light at an angle z from the zenith will pass through more air, so travels through an equivalently greater air mass at a greater incidence angle z. Roughly, the air mass varies with secant z,
as cos(z)=1 airmass / n airmasses in a first-order case,
although...
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...ed at the same time. There is also a chance that light from another point on the disk would be refracted into the line-of-sight so the intensity of light would not change.
An observer, therefore, can only be sure that what he is looking at is a true representation of an object if he is looking straight up to the zenith. Any view towards the horizon will be subject to increasing refractive and scattering effects, that can cause disparity in an object's position, changes in the colour of the incoming light and minute changes in the quality of light causing twinkling.
Sources:
David K Lynch and William Livingston; Color and Light in Nature, Cambridge University Press, 2nd Ed. 2001
Fraknoi, Morrison, Wolff; Voyages through the Universe, Saunders College Publishing, 2nd Ed. 2000
Scientific American: Ask The Experts
Astronomy Picture of the Day: Why Stars Twinkle
Newton, Henry, and William Winsor. "Spotlight on Colour: Flake White." Winsor&Newton. N.p., 2011. Web. 30 Apr. 2014.
There are three ways the sun's heat and energy are transferred throughout the atmosphere: radiation, conduction and convection. The sun's radiation heats the surface of the earth and the resulting heat is transferred to the atmosphere primarily by convection. Conduction is a minor contributor to the overall process since the transfer of heat via the air is a slow, inefficient process. Convection is the vertical process that carries warm air up from the ground to be replaced by cooler air, which in turn is warmed, and cycles upward again. On a global scale, convection is responsible for the atmospheric circulation which redistributes heat from the warm equatorial regions to the poles. The Coriolis effect, the apparent curvature of winds and ocean currents due to the earth's rotation, causes the atmospheric circulation to be divided into three convective zones per hemisphere: Hadley cells (tropical), Ferrell cell (temperate) and the Polar cell (UXL Encyclopedia of Weather and Natural Disasters). These convection cells along with horizontal advection are responsible for global wind patterns. On a smaller, local scale, convection currents are linked to the development of deep convective clouds and local storm systems. Because precipitation is central to the earth's energy balance, circulation, and water cycle, atmospheric scientists have focused their efforts on understanding how pollution effects the development and intensity of convection currents, cloud cover, precipitation, and thunderstorms.
Refraction is what happens to light when it passes from one medium to another. For example, things appear differently from the bottom of a swimming pool than on the top. Simplistically, refraction is the bending of light. The explanation for this phenomenon, however, can be described with light as rays and light as waves. No matter the case, it is important to remember that the speed of light is constant in every homogeneous medium, regardless of shape, size or form.
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would be absorbed and the earth would be extremely cold. When too many rays are absorbed, the
which can be up to 100ft high. The sun was now beginning to set. It
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Lynch, David K., and Livingston, William, Color and Light in Nature Cambridge University Press 1995