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Astronomical seeing, sometimes known as atmospheric seeing, is a measure of the extent to which the Earth's atmosphere disturbs a view of a celestial body. The effect, which is common in most parts of the world, is caused by the rapid movement and mixing of volumes of the atmosphere having different temperatures and densities. An observer sees moving, blurred, scintillating or twinkling images. Astronomical seeing is consequently a significant and unpredictable limitation when striving to achieve the best possible views of planets and the closer stars. No Earthbound observer can entirely escape its effects although there are some well known locations, such as Hawaii and Tenerife, where the atmosphere is normally quite stable and the effect is limited or rare. It is no co-incidence that many of the world's astronomical telescopes are found in these same locations.

Atmospheric turbulence occurs principally within about 150 metres of the ground, in the central troposphere (up to about 2 km), and in the upper troposphere (approximately 6 -12 km.) Each of these atmospheric layers exhibits its own particular characteristics.

Disturbance of the atmosphere is greatest close to the ground. The ground is heated by the Sun during the day and then radiates the heat unevenly from areas of different densities and topographies at night, and consequently generating local convection currents. Turbulence generated in the central troposphere is typically dependent upon local topographical features upwind of the observing site. Features such as mountain ranges and large cities disturb the atmosphere many miles downwind of the feature itself. With this effect in mind, it is better to observe when prevailing winds have passed over uniform terrain for a considerable distance before reaching the observation site. Disturbance in the upper troposphere is likely to be a consequence of rapidly moving streams of air - known as jetstreams. These can cause images to appear fuzzy and stripped of fine detail, although still relatively stable.


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