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In meteorology, an okta is a scale 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 through to 8 oktas. In addition, in the SYNOP code there is an extra cloud cover indicator '9' indicating that the sky is totally obscured, usually due to dense fog or heavy snow.
来源:维基百科