The Grammarphobia Blog

Is “partly cloudy” the same as “partly sunny”?

Q: From the tiny weather thingy on the front page of a recent New York Times: “Today, mostly cloudy, then turning partly sunny.” Is there a difference?

A: First things first. That thingy—the box in the upper right-hand corner of the Times front page—is referred to by journalists as the “weather ear.”

As for your question, yes, there is a difference between “mostly cloudy” and “partly sunny.” More on this later.

Let’s take a moment for another divagation, a word favored by Angela Thirkell, one of our favorite writers.

Your question inspired us to ask a more puzzling question: Is there a difference between “partly sunny” and “partly cloudy”?

We like to think that the terminology of weather forecasting—if not the forecast itself—is fairly precise.

That’s why we always assumed there must be a subtle difference between “partly sunny” and “partly cloudy.”

Not so, it turns out. The choice of words seems to depend on the mood of the forecaster.

David Feldman pondered the question in his 1986 “Imponderables” book, Why Don’t Cats Like to Swim?

“While you might assume that a partly sunny sky should be clearer than a partly cloudy one, the two terms signify the same condition,” Feldman writes. “You have merely encountered a weathercaster who prefers to see the glass as half full rather than half empty.”

(Feldman’s book, by the way, addresses another burning question: Why do women open their mouths when applying mascara?)

If blogs maintained by newspapers, magazines, and broadcasting outlets are any indication, the “partly sunny/partly cloudy” business must be keeping plenty of people awake nights.

The only clear difference we can see, however, is that “partly sunny” isn’t appropriate at night.

Dan Stillman, a meteorologist at the Washington Post, addressed the question on the Post’s weather blog in 2008:

“What’s the difference between partly sunny and partly cloudy? Different forecasters and forecast outlets have different answers to this question. Here’s what we think makes the most sense: They both mean the same thing—a mix of sun and clouds. We tend to use partly sunny for daytime forecasts and, obviously, partly cloudy for nighttime. Sometimes we’ll use partly cloudy during the day to highlight a change from sunnier to cloudier skies.”

In expectation of further enlightenment (or perhaps encloudiment), we went to the National Oceanic and Atmospheric Administration website, where a meteorologist named Jeffrey Nesmith has written an article called “Weather Words.”

Nesmith says that in forecasting sky cover, the National Weather Service divides the sky into eight equal parts, then determines how many eighths are expected to be covered by opaque clouds.

(“Opaque” means you can’t see through them—no blue sky by day, no moon or stars at night.)

The Weather Service, Nesmith says, accompanies its forecast with the following “conversion table” for the use of local media:

“Cloudy: 8/8 opaque clouds;

“Mostly Cloudy, Considerable Cloudiness: 6/8 to 7/8 opaque clouds;

“Partly Sunny [day], Partly Cloudy [night]: 3/8 to 5/8 opaque clouds;

“Mostly Sunny [day], Mostly Clear [night]: 1/8 to 2/8 opaque clouds;

“Sunny [day], Clear [night]: 0/8 opaque clouds.”

As Nesmith says: “Different expressions can be used to depict similar sky conditions; e.g., partly cloudy and partly sunny can be used interchangeably to describe 3/8 to 5/8 opaque sky cover. Since some terms are more fitting for use during daylight hours (e.g., ‘partly sunny’ and ‘mostly sunny’), forecasters usually favor their use during the daytime over their nighttime equivalent.”

We all know, however, that plenty of radio, TV, and Internet weatherfolk go their own way, issuing daytime forecasts of “partly cloudy” instead of “partly sunny.” Maybe they just weren’t feeling perky that day.

KOMOnews.com, a Seattle network with radio, television, and Internet news outlets, acknowledged in a weather FAQ that there’s “a bit of psychological factor” involved:

“If the forecast is improving (as in, rain event is just ending and will clear up a bit), we’ll usually go partly sunny. If the forecast is deteriorating (as in, it’s sunny now, but rain moving in tomorrow), we might say mostly cloudy as the condition during the increasing clouds. We tend to save ‘partly cloudy’ for nighttime, since we can’t use partly sunny.”

And perhaps the need for variety plays a part too. “There really isn’t much of a difference,” said KOMOnews. “It just gives us a larger vocabulary to work with.”

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