Partly Cloudy vs. Partly Sunny

There are lots of terms in meteorology that are very similar and that's part of what leads to the confusion that exists out there.  The two terms in the title of this week's blog post are a great example.  Partly cloudy and partly sunny are two terms that pop up in weather forecasts a lot, but what's the real difference between the two?

To try and answer this, let's start by looking up the definition given by the federal organization that sets the standards for the field of meteorology in the United States: the National Oceanic and Atmospheric Administration (NOAA).  In the Federal Meteorological Handbook, NOAA describes five categories of cloud cover that should be documented at all weather observing stations.  The list includes the following:

Clear = No visible clouds
Few = 1/8 - 2/8 of sky covered by clouds
Scattered = 3/8 - 4/8 of sky covered by clouds           
Broken = 5/8 -7/8 of sky covered by clouds
Overcast = 100% cloud cover

Though these five terms are all the description NOAA deems necessary to describe sky cover, there's a lot of room for interpretation.  A big problem is that unless you're in the meteorology field, most people don't look at the sky in eighths and the difference between scattered and broken could be large.  To try and relate to the public better and keep forecasts more interesting, forecasters at the National Weather Service (which is a part of NOAA) decided to come up with their own categories for sky cover.  Here's the list they use:

 

Cloudy = 90-100% sky cover
Mostly Cloudy = 70-80% sky cover
Partly Cloudy/Partly Sunny = 30-60% sky cover
Mostly Clear/Mostly Sunny = 10-30% sky cover
Clear/Sunny = 0-10% sky cover
Fair = Less than 40% cloud cover, no
precipitation and no extreme weather

They too only have five categories (not counting fair), but they added different terms to describe the same conditions.  As far as the NWS is concerned, partly cloudy and partly sunny are the same thing.  The only difference is that partly sunny is used to describe daytime cloud cover and partly cloudy is used to describe nighttime cloud cover.  Same goes with mostly sunny/mostly clear and clear/sunny.

So case closed right?  Not so fast.  Because the terms are so similar, the nighttime and daytime terms often get used interchangeably by most forecasters.  If skies are expected to be partly sunny for the next five days, it's highly likely the forecaster will switch between saying partly sunny, partly cloudy, and fair just to spice up the forecast some.

The difference between partly cloudy and partly sunny though has really taken on a life of its own; so much so that many forecasters now separate the two terms to have a total of six categories of sky cover instead of five.  The most common order of these six categories of sky cover, from least to greatest, is as follows:

Sunny = 0-10%
Mostly Sunny = 10-30%
Partly Cloudy = 30-50%
Partly Sunny = 50-70%
Mostly Cloudy = 70-90%
Cloudy = 90-100%

This is the scale I personally go by, but even this is not agreed upon by all meteorologists.  Some argue that partly sunny and mostly cloudy mean the same thing while others use partly sunny when cloud cover is expected to dissipate later and partly cloudy when cloud cover is expected to increase later. 

The bottom line is that it depends on the person who made the forecast for what partly cloudy or partly sunny actually means.  While you're more likely to see more clouds when the forecast calls for partly sunny skies compared to partly cloudy skies, get to know your favorite source for weather and see what the sky looks like when it says partly cloudy or partly sunny.  You should be able to figure out what category the meteorologist who made the forecast falls in!

Big thanks to "The Media Mind" who provided the inspiration for this post after asking about this in the comments of my last blog post.  If you have a weather question you want answered, comment below and I'll do my best to answer it in a future post!