Office of the Washington State Climatologist

WA State Cloud Cover Tool

The cloud cover data below was derived from observations of hourly cloud cover from 25 weather stations, which are often located at an airport. Prior to the inception of the Automated Surface Observing System (ASOS) and the Automated Weather Observing System in 1980’s, cloud cover was determined by a human observer. The ASOS/AWOS systems, measure cloud cover in oktas (8th’s) using a ceilometer which is accurate to 12,000 ft.

The tables below are a monthly average from 1973-2000* of hourly cloud cover between sunrise and sunset that show the average cloud cover (in oktas) and the mean number of days clear, partly cloudy, and cloudy. Clear denotes an average sky cover of 0-2 oktas, partly cloudy denotes an average sky cover of 3-5 oktas and cloudy denotes an average sky cover of 6-8 oktas.

For each station, daily and monthly cloud cover data is also available for download. Information on the format of these files is available here and a station list is available here.

* In most cases cloud cover data is not available prior to 1973 and few stations may have shorter records.

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