Okta

A unit of measurement for quantifying cloud cover

Scale of cloud cover measure in okta ( eighth ) with the meteorological symbol for each okta

In meteorology , anokta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. 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 or heavy snow.^[1]

When used in weather chart , okta measurement are show by mean of graphic symbol ( rather than numeral ) contain within weather circle , to which are attach further symbol indicate other measure datum such as wind speed and wind direction .^[2]

Although relatively straightforward to measure (visually, for instance, by using a mirror^[3]), oktas only estimate cloud cover in terms of the area of the sky covered by clouds. They do not account for cloud type or thickness, and this limits their use for estimating cloud albedo or surface solar radiation receipt.

cloud okta can also be measure using satellite imagery from geostationary satellite equip with high – resolution image sensor such as Himawari-8 . similar to traditional approach , satellite images is account do not account for cloud composition .

Oktas are often referenced in aviation weather forecasts and low level forecasts: SKC = Sky clear (0 oktas); FEW = Few (1 to 2 oktas); SCT = Scattered (3 to 4 oktas); BKN = Broken (5 to 7 oktas); OVC = Overcast (8 oktas); NSC = nil significant cloud; CAVOK = ceiling and visibility okay.^[4][5]

Cloud – cover symbol used on weather teleprinter and aviation report in the US^[6]

In the early 20th century, it was common for weather maps to be hand drawn. The symbols for cloud cover on these maps, like the modern symbols, were drawn inside the circle marking the position of the weather station making the measurements. Unlike the modern symbols, these ones consisted of straight lines only rather than filled in blocks which would have been less practical on a hand drawing. A reduced set of these symbols were used on teleprinters used for distributing weather information and warnings. These machines were 5-bit teleprinters using a modified version of the Baudot-Murray code.^[7]

Weather teleprinter keyboard and code

There are some symbol in Unicode which resemble those used for okta . However , some okta symbols is lack lack a similar – look unicode character . The use of Unicode to render okta depend on whether a font with these character is instal ; unicode symbols is be may be difficult to work with in software that does not render these character uniformly .

The Unicode set of related symbols includes: