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Clear Sky Charts (called clocks until February 29, 2008) are web graphics which deliver weather forecasts designed specifically for astronomers. They forecast the cloud cover , transparency and astronomical seeing , parameters which are not forecast by civil or aviation forecasts. [ 1 ]
It is calculated as the fraction of the actual total solar radiation on the surface of the Earth during a certain period over the theoretical maximum (i.e., clear sky) radiation during the same period. [1] [2] The clearness index is a dimensionless quantity and can vary from 0 (sky is completely covered) to 1 (perfectly sunny). [3]
Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. [2] Okta is the usual unit for measurement of the cloud cover.
The aurora is expected to be bright and visible in multiple northern U.S. states Oct. 3 through Oct. 5 as well as from the lower Midwest to Oregon.
Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast). In addition, in the SYNOP code there is an extra cloud cover indicator '9' indicating that the sky is totally obscured (i.e. hidden from view), usually due to dense fog ...
A sky chart showing the location of Comet C/2022 E3 (ZTF) throughout January. (NASA) The easiest time to see the comet will be during the second weekend of February as it passes incredibly close ...
The Aurora Borealis, also known as the "Northern Lights", illuminate the sky as people gather to watch the annual Perseid meteor shower near the village of Borodinka in the Omsk region, Russia ...
Good seeing nights tend to be clear, cold nights without wind gusts. Warm air rises ( convection ), degrading the seeing, as do wind and clouds. At the best high-altitude mountaintop observatories , the wind brings in stable air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4".