What is the difference between “partly sunny” and “partly cloudy” when used in weather forecasts?

Dear Tom,

What is the difference between “partly sunny” and “partly cloudy” when used in weather forecasts?

Jacquie Steuer,
Waukegan

Dear Jacquie,

For starters, “partly sunny” is a daytime term and “partly cloudy” can be used either during the day or at night. Technically, “clear” and “sunny” refer to a sky that is absolutely cloud free or covered by no more than one tenth of clouds; “mostly sunny” refers to a sky condition when the sky is cloud-covered from two tenths through five tenths; “partly sunny” refers to cloud coverage between six and nine tenths; and “cloudy” to a complete or nearly complete overcast. “Partly cloudy” refers to cloud coverage from two through five tenths; “mostly cloudy” is used in reference to six through nine tenths cloud coverage; and “cloudy” to a complete or nearly complete overcast.

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