Partly Sunny, Partly Cloudy?

From WGNTV:

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.