WEATHER: Is it partly cloudy or partly sunny? Mostly cloudy, or mostly sunny? Is there really a difference?

We looked to The National Weather Service for answers.

Partly sunny can only be used during the day. And much as we suspected, partly cloudy and partly sunny mean the same thing.

The definition says “between 3/8 and 5/8 of the sky is covered by clouds” when it is partly cloudy or partly sunny. Forecasters might shake things up a bit and say “a mix of sun and clouds,” but it’s really all the same thing.

Now if that condition were to happen at night, it would be partly cloudy.

Mostly cloudy means more clouds than sun, or by definition, “when ¾ to 7/8 of the sky is covered by clouds.”

Mostly sunny is “when ⅛ to ¼ of the sky is covered by clouds,” and that condition at night would be known as mostly clear.

To get a bit more technical, we’ll refer to the Media Guide for the National Weather Service, where Appendix A: Anatomy of a Zone Forecast defines most terms one might see or hear in reference to weather.

The sky condition of cloudy or overcast is when opaque cloud coverage is 88 to 100 percent. If it were 87 percent, down to 70 percent coverage, that would be mostly cloudy or considerable cloudiness. Partly sunny or mostly cloudy (we guess it’s a “glass half full” kind of thing) would be cloud coverage of 51 to 69 percent.

Mostly sunny or partly cloudy means 26 to 50 percent cloud coverage. (We’re getting curious as to who calculates the percentage of sky covered by clouds.)

Sunny and mostly clear are 6 to 25 percent cloud coverage, while sunny or clear is zero to five percent coverage.

Confused? We didn’t even get into precipitation probability or prevailing wind direction or wind velocity.