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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 ]
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 half full of clouds would have a circle that was half white and half black. Below the shape indicating sky cover, the station model can indicate the coverage of low clouds, in oktas, and the ceiling height in hundreds of feet. The ceiling height is the height at which more than half the sky is covered by clouds.
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.
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]
One type of map shows where VFR (visual flight rules) are in effect and where IFR (instrument flight rules) are in effect. Weather depiction plots show ceiling height (level where at least half the sky is covered with clouds) in hundreds of feet, present weather, and cloud cover. [27] Icing maps depict areas where icing can be a hazard for flying.
The Atlas Coeli covers both hemispheres with 16 charts. The coordinate system is referred to equinox 1950.0 and the scale is 1° = 0.75 cm. There are six charts of the equatorial regions on a rectangular graticule, covering declinations from +25° to -25°; four charts for each hemisphere with straight, converging hour circles and concentric, equally-spaced declination circles covering ...
SIGWX is a Significant Weather Chart defined by ICAO. Weather charts [1] being issued by World Area Forecast Centres [2] (from meteorological offices in London and Washington), presenting the most important meteorological phenomena relevant especially for air traffic transport. WAFC publishes them in two formats BUFR code; PNG images; Charts ...