Atmospheric optics phenomena such as sunset mirages and green flashes occur due to the fascinating phenomenon of refraction caused by air layers with different temperatures, densities, and refractive indices. While the differences in refractive indices may seem insignificant, they have a cumulative effect on the path of sunset rays as they traverse large distances through the atmosphere.
Understanding Air Temperature Variation
Air temperature typically decreases smoothly with increasing height. This pattern is primarily a result of two factors: heat transfer from the ground or sea, and the decrease in air pressure with height. When warm air rises due to buoyancy, it encounters lower pressure and expands. As it expands, it performs work and consequently loses heat energy, causing its temperature to drop. This process continues until the rising air reaches a layer with the same temperature, establishing equilibrium. As a result, the upper part of the air column is cooler than the lower part. The rate at which temperature decreases with height is known as the lapse rate and is approximately 6.5°C per kilometer (3.6°F per 1000 feet). However, it can vary depending on local conditions and the moisture content of the air.
Temperature Inversions and their Impact
In certain cases, a temperature inversion occurs, disrupting the typical temperature profile. A temperature inversion refers to the presence of layers of warmer air above colder air. This phenomenon often arises when warm inland air moves over colder marine air. For example, along the Californian coast, warm Santa Ana winds from inland can overlay colder air influenced by the cold Californian ocean current. When one is positioned above an inversion layer, sunlight from space enters and leaves the layer before reaching the observer's eye. This specific condition creates ideal circumstances for observing mock mirage sunsets and M-Mir green flashes.
The Influence of Ground Heating
When the ground or ocean surface is intensely heated by solar radiation, it can generate an abnormally hot layer of air directly above it. These conditions are responsible for the formation of inferior mirages observed over hot roads. Similarly, they give rise to "Etruscan vase" sunset mirages and I-Mir green flashes. In these scenarios, the path of light is bent due to the sharp gradient in temperature and density between the heated air layer and the surrounding cooler air.
To summarize the key points discussed above:
By exploring these fascinating atmospheric optics phenomena, we gain a deeper understanding of the complex interplay between air temperature, refraction, and the mesmerizing visual effects that can be observed during sunset and sunrise. Next time you find yourself gazing at the horizon as the sun dips below, keep an eye out for the subtle yet captivating mirages and green flashes that may grace the sky.
Sunset mirages and green flashes result from unusual refraction by air layers at different temperatures and hence density and refractive index. The refractive index differences are minute but their effects accumulate as sunset rays travel large distances through the atmosphere.
Air temperature falls smoothly with height. This usual state of affairs arises primarily because (1) the air is heated by heat transfer from by the ground or sea and (2) because air pressure, produced by the weight of overlying air, decreases with height. Imagine a pocket of dry warm air rising by buoyancy. As it rises it encounters lower pressure and so it expands. However, to expand it must do work - expend energy. The work is at the expense of its heat content and so its temperature falls. Air pockets ascend until they reach air at the same temperature. When equilibrium is reached the air column will be found to be cooler at its top. The temperature drop - lapse rate - is about 6.5°C/km (3.6°F per 1000ft) but depends much on local conditions and especially the air moisture content. A temperature inversion is the presence of a kink in the normal temperature profile whereby layers exist that are warmer than usual, warm air overlays colder air. Inversions can form when warm air from inland blows over colder marine air. This happens off the Californian coast when warm Santa Ana winds from inland overlay air cooled by the cold Californian ocean current. When you are above an inversion layer, so that sunlight from space enters and leaves it before entering your eye, the conditions are right for a mock mirage sunset and M-Mir green flash. Mock mirage ray paths here. Ground or the ocean surface heated by solar radiation can produce an abnormally hot air layer above it. These are the conditions for the inferior mirages seen over a hot road and they similarly produce "Etruscan vase" sunset mirages and I-Mir green flashes. Mirage ray paths.
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"Air Temperatures, Mirages & Green Flashes". Atmospheric Optics. Accessed on November 30, 2023. https://atoptics.co.uk/blog/air-temperatures-mirages-green-flashes/.
"Air Temperatures, Mirages & Green Flashes". Atmospheric Optics, https://atoptics.co.uk/blog/air-temperatures-mirages-green-flashes/. Accessed 30 November, 2023
Air Temperatures, Mirages & Green Flashes. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/air-temperatures-mirages-green-flashes/.