Atmospheric refraction, the bending of light as it passes through the Earth's atmosphere, is primarily affected by variations in air density and temperature.
Understanding Atmospheric Refraction
Atmospheric refraction occurs because light travels at different speeds through air of different densities. Denser air has a higher refractive index, meaning light bends more as it enters it. This bending causes objects, especially those viewed near the horizon like the sun or stars, to appear higher in the sky than they actually are.
Key Factors Influencing Atmospheric Refraction
Here's a breakdown of the main factors:
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Air Density: Density is the most important factor. Higher density air refracts light more strongly than less dense air. Air density decreases with altitude, leading to a gradual bending of light rays as they travel through the atmosphere.
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Temperature: Temperature significantly influences air density. Warmer air is less dense than colder air. Therefore, temperature gradients (changes in temperature over distance) cause variations in air density and, consequently, variations in the amount of refraction. Temperature inversions, where warmer air sits above colder air, can lead to unusual refraction effects.
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Altitude: As altitude increases, atmospheric density decreases. Because the air is thinner, there are fewer air molecules to refract the light. The effects of atmospheric refraction are most prominent when viewing objects close to the horizon where the light must pass through more of the atmosphere.
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Humidity: Humidity has a small, but measurable, effect. While counter-intuitive, humid air is slightly less dense than dry air at the same temperature and pressure. This is because water molecules are lighter than the average of nitrogen and oxygen molecules that make up most of the atmosphere. Thus, increased humidity generally decreases refraction, albeit subtly.
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Air Pressure: Higher air pressure leads to higher density, which then leads to more refraction. However, pressure gradients in the atmosphere are usually less significant than temperature gradients in their impact on refraction.
Impact and Examples
These factors combined cause interesting phenomena:
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Apparent Flattening of the Sun: As the sun sets, the lower edge appears to be refracted more than the upper edge, causing the sun to appear flattened.
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Stars Appearing Higher: Stars always appear slightly higher than their actual positions due to atmospheric refraction. This effect is greatest near the horizon.
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Mirages: Mirages are caused by extreme temperature gradients near the ground, which lead to strong refraction.
In summary, atmospheric refraction is a complex phenomenon primarily driven by air density variations, which are influenced by temperature, altitude, humidity, and pressure.
