MOLINE, Ill. — Mostly sunny, partly sunny, partly cloudy and mostly cloudy. These are just a few of the ways we describe what the current and expected sky is going to look like at a point in time. However, many of you over the years have noticed something. The use of partly cloudy and partly sunny. Do they mean the same thing? Or, is there indeed a distinct difference between the two? That's what Michelle from Davenport, Iowa asked. Let's dig in!
Definition of partly cloudy
According to the National Weather Service, partly cloudy indicates opaque clouds that cover between 3/5 and 5/8 of the total sky are forecasted. The term partly cloudy is usually more popular with most forecasters, as it covers both the daytime and nighttime conditions in one phase. Partly sunny, while having the same definition as partly cloudy, can only be used during the daytime for obvious reasons.
Sometimes meteorologists will also use phrases like, "a mix of sun and clouds," instead of partly cloudy. This differs from mostly cloudy which infers only a few peeks of sunshine are expected between a small amount of gaps within the clouds. Plus, it does sound a little bizarre to say partly moony, or partly clear at night when there is no moon present. Partly cloudy covers all the bases here no matter what time of day it may be.
Other definitions for cloud cover
The National Weather Service defines other types of sky cover in the following ways:
- Mostly Cloudy: When 3/4 to 3/8 of the sky is covered by clouds
- Mostly Sunny: When 1/8 to 1/4 of the sky is covered by clouds
- Fair: Less than 3/8 cloud cover, light winds, no extremes in temperature, dry conditions, and good visibility
The "fair" terminology is not used often because it can be confusing considering it pertains to more than just clouds.
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