Network News

X My Profile
View More Activity
The new Washington
Post Weather website
Jump to CWG's
Latest Full Forecast
Outside now? Radar, temps
and more: Weather Wall
Follow us on Twitter (@capitalweather) and become a fan on Facebook
Posted at 11:00 AM ET, 02/14/2008

Partly Sunny, Partly Cloudy, Mostly Confusing

By Dan Stillman

Your input wanted: Help us define these terms

Pop quiz! And lucky for you, it's multiple choice:

What's the difference between "partly sunny" and "partly cloudy"?

A. "Partly sunny" is more sunny than "partly cloudy."
B. "Partly sunny" is less sunny than "partly cloudy."
C. There is no difference.

The answer ...

... is not so clear. In fact, there seems to be no shortage of confusion about these two terms, which are nonchalantly dropped into forecasts by meteorologists everywhere, including by us at Capital Weather Gang.

The National Weather Service Billings Weather Forecast Office in Montana has this to say: "Officially, there is no difference between partly cloudy and partly sunny in a forecast. It is up to the discretion of the forecaster to emphasize sun or clouds. Partly cloudy and partly sunny mean about the same thing: There is going to be a mixture of clouds and sunshine."

That definition is also echoed here and here.

So ... case closed, right? The answer is "C. There is no difference."

Not so fast.

Official definitions are great, except that the average weather forecast consumer doesn't carry a glossary of weather terms with them. Which means that "partly sunny" and "partly cloudy" are probably interpreted as meaning different things by different people.

The Roanoke Times summarized the debate in this article a few years back. While many sources take an objective approach and define the two terms as the same, the Times story recognizes a subjective component as well: "... there is also a widespread public perception that 'partly sunny' sounds more bright and cheery that does 'partly cloudy.'" The story goes on to quote a TV meteorologist as saying, "Before I studied weather in college, I always thought 'partly sunny' meant more sun, because you actually hear the word 'sunny.' "

Similarly, New Hampshire's WMUR takes the public perception angle into account in its explanation: "Partly sunny is similar to 'partly cloudy' but is used more frequently during the day to emphasize daytime sunshine. I find that when people hear 'partly cloudy' they think more clouds than sun and when they hear 'partly sunny' more sun than clouds."

Adding another element to the mix, KOMO in Seattle factors in how the weather is changing: "If the forecast is improving (as in, rain event is just ending and we'll 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."

The one thing everyone seems to agree on (thankfully) is that partly sunny is not appropriate for a nighttime forecast.

We address the matter here on our frequent questions page:

Different forecasters and forecast outlets have different answers to this question. Here's what we think makes the most sense: Partly sunny means a mixture of sun and clouds is expected, but more sun than clouds. Partly cloudy also means a mixture of sun and clouds is expected, but more clouds than sun.

But one visitor recently challenged us on this definition: "Why would partly sunny mean more sun than clouds if the weather also contains the terminology mostly sunny? Also it would to seem to make sense that if the sky was partly sunny then it would be predominantly clouds and part of all those clouds would be sun ... not that all this makes any difference. What it is, it is. What it is called makes no difference," he wrote in an email to CWG.

Should we change our definition? If so, to what? Let us know what you think by submitting a comment below.

By Dan Stillman  | February 14, 2008; 11:00 AM ET
Categories:  Education  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   Del.icio.us   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: A Lovely Change of Pace
Next: CommuteCast: Clear Skies and Diminishing Winds

Comments

I come down on the side that "partly cloudy" is sunnier than "partly sunny." It seems to me that the former implies that it will be sunny except for a few clouds, and the latter implies the opposite. Clear? Hey, that's a different issue.
Thanks all. Enjoy it no matter what the weather.

Posted by: Jody Schroath | February 14, 2008 11:12 AM | Report abuse

Couldn't NWS start trying to predict the proportion of sky (in tenths) covered by clouds? How about "two-fifths cloudy" or "nine-tenths cloudy" instead of those old canards "partly cloudy, mostly cloudy, partly sunny", etc.?

I believe they have to report the amount of cloudiness, in tenths of sky covered, on the Daily Weather Map and hourly airport reports.

They're doing a fine job reporting the amount of precipitation expected in rain or snow forecasts nowadays.

Posted by: El Bombo | February 14, 2008 11:15 AM | Report abuse

By the way, the British express their forecasts in terms of "bright intervals", "cloudy intervals", "periods of thundery rain or thunderstorms", etc. Maybe we could start following the Brits.

Posted by: El Bombo | February 14, 2008 11:19 AM | Report abuse

I like the term I heard in Vancouver BC, where it's usually (except in summer) gloomy: "sunny breaks." That puts the best spin on a mostly-dark day.

Logically I agree that "partly cloudy" implies fewer clouds than "partly sunny" does. I'd prefer "mostly clear" or "mostly cloudy" or "mixed clouds and sun."

Posted by: Novafp | February 14, 2008 11:34 AM | Report abuse

I totally agree with Jody. Applying the logic to another example, if you say your glass is "partly full" of water, then you mean there is more empty space than water. If you say your glass is "mostly full" of water, then there is more water than empty space. So "partly cloudy" would indicate there is more sun and a bit of cloudiness.

Posted by: WNDLZR | February 14, 2008 11:39 AM | Report abuse

Does partly cloudy = mostly sunny? Mix of sun and clouds seems to be sufficiently ambiguous to satisfy most people.

Posted by: Lester Burnham | February 14, 2008 11:49 AM | Report abuse

Good comments folks. Keep 'em coming. Also, I should throw one more wrench into this discussion:

How do the forecasts of "partly to mostly sunny" or "partly to mostly cloudy" fit into all of this?

Posted by: Dan, Capital Weather Gang | February 14, 2008 12:01 PM | Report abuse

On a totally unrelated note, the active weather of this week has prevented us from calling much attention to the National Weather Service-provided "At A Glance" forecast we launched on Monday at the top of the left sidebar. We'll have a post in the near future explaining some of the behind the scenes of this feature.

In the meantime, please spread the word that the snapshot forecast is now back (we know a lot you loyal visitors from the old site were missing this vital piece of information).

Posted by: Dan, Capital Weather Gang | February 14, 2008 12:07 PM | Report abuse

I remember reading about this in an old Bob Ryan almanac and he basically said there is no difference...(paraphrase) partly sunny should be used during the day, partly cloudy used at night......simple enough!

Posted by: Mike from the Blue Ridge | February 14, 2008 12:08 PM | Report abuse

Partly sunny, partly cloudy , or mostly cloudy are generic terms that are better left to the interpretation of the individual. To suggest a more specific prediction by the NWS only predisposes these folks to be incorrect more often.

As I illustrated in the previous post by Josh from 5 am this morning, the NWS already often demonstrates an abysmal performance west of the Blue Ridge. Josh talked to folks this morning at 5 am by saying "On Sunday increasing clouds will signal the approach of a large area of low pressure from the southwest." "rain is a possibility from the latter part of Sunday into Monday". That communicates in real terms what will likely occur based on the latest info. at Josh's disposal.

What did the folks west of the Blue Ridge receive from the NWS this morning??

They first said at 4:16 in their forecast discussion (which less than 10% will hear or read) "Precip. may start briefly as a mix", "but should see a rapid warmup for an all rain event".
12 minutes later at 4:28 they issued the early morning forecast which for my area stated for Sunday night: Snow, sleet or freezing rain likely with lows in the low 30's, P.O.P. at 70%. Does anyone recognize the disconnect from the discussion to the forecast? The forecast discussion from NWS, which is close to Josh's realm of thought is likely more accurate but instead, probably thousands of folks west of the Blue Ridge are thinking about or preparing for another winter storm Sunday night, that will not materialize. CWG is practicing a style of communication that the NWS could learn from.

There is much room for the NWS to focus on improving the basics before tinkering with trivialities.

Posted by: Augusta Jim | February 14, 2008 12:10 PM | Report abuse

I read somewhere (I think weather.com) a while back that there was a percentage difference between the terms. 20% clouds meant Partly Sunny. 30% clouds meant partly cloudy. Something like that.

Posted by: weatherdudeVA | February 14, 2008 12:22 PM | Report abuse

shame on weatherdudeVA for reading anything on weather.com...

Posted by: bernydoode00 | February 14, 2008 12:28 PM | Report abuse

It's one of those things I don't pay much attention to. If you ask me, partly sunny should be for the day time, and partly cloudy should be used at night, because obviously it wouldn't be partly sunny.

Posted by: Havoc | February 14, 2008 1:04 PM | Report abuse

Partly Cloudy=Mostly Sunny
Partly Sunny=Mostlly Cloudy

AccuWeather=Weekly Blizzards..;0..jk

Posted by: StormChaser | February 14, 2008 1:13 PM | Report abuse

about 20 years ago a local weatherman described "partly sunny" during the day and "partly cloudy" at night...makes mostly sense to me

Posted by: BRIAN | February 14, 2008 1:47 PM | Report abuse

The confusion doesn't really stop with explaining the difference between "partly sunny" and "partly cloudy" either. Throughout the years there have been forecast terms that make you wonder what exactly the weather people are talking about. What about "variable cloudiness" and "considerable cloudiness"? What's the difference between "cloudy" and "overcast"? How about "clear" and "fair"? Fair skies has always befuddled me; does it mean "a few clouds", a very attractive sky or a sky that is less attractive than, say, "good skies"?

Avoiding these terms or substituting "plain" language can even make the forecast even more perplexing. "Starlit skies with a hint of a chill in the air tonight" or "Bright sunshine with gentle breezes this afternoon" add almost an amusing touch of poetry to the forecast but do they actually convey the message? I guess we're more apt to describe severe weather with precision than benign or uneventful weather, and if so, that's a good thing. I certainly wouldn't want to be in the path of a "partly severe thunderstorm", not knowing what part I would get.

Posted by: Steve Wasko | February 14, 2008 1:54 PM | Report abuse

The National Weather Circus sez 'partly cloudy' and 'partly sunny' are the same animal. Opaque clouds covering 3/10 - 6/10 of the sky. 'Sunny' is used during the day and 'cloudy' is used at night. Vanilla is as vanilla does.

Consider the following...
The forecast can call for a 'sunny'...'mostly sunny'...or 'partly sunny' sky. If 'partly sunny' means 'more sun than clouds'...does that mean 'partly sunny' is sunnier than 'mostly sunny?

Or how about this...
The forecast can call for a 'cloudy'...'mostly cloudy'...or 'partly cloudy' sky. If 'partly cloudy' means 'more clouds than sun'...does that mean 'partly cloudy' is cloudier than 'mostly cloudy?'

If it's 'party sunny' during the day, then why not 'partly moony' at night?

Posted by: TQ | February 14, 2008 2:09 PM | Report abuse

Here are some other ways to describe mixed sky - these from the BBC forecast:

"Staying dry with bright or sunny spells"

"Most places will stay dry with variable amounts of cloud"

"Rather cloudy, just a few clearer spells, in the east with some patchy mist and low cloud developing"

Not saying these are better - just different

Posted by: MDScot | February 14, 2008 2:15 PM | Report abuse

what does "fair" mean?

Posted by: rjm | February 14, 2008 2:27 PM | Report abuse

To me, partly sunny means more clouds than sun and partly sunny means more clouds.

Posted by: Arlington Rich | February 14, 2008 2:55 PM | Report abuse

"Fair" indicates few or no clouds below 12,000 feet with no significant weather and/or obstructions to visibility.

Posted by: TQ | February 14, 2008 2:56 PM | Report abuse

I would say (with much pomposity) that you should throw out the "partly sunny".

It would make more physical sense if you only focused on the amount of clouds present. The sun/moon/stars are always there, but the clouds are the interlopers, and they are more interesting in terms of what the weather is going to be like.

Well, good, now that that's fixed, maybe you guys could get to work on that whole drive on the parkway, park on the driveway thingy.

I did notice that the weather at a glance box only has 6 days instead of 7. Was giving up a full day of weather knowledge part of the Faustian deal that brought you to the Post? I always knew that CapWx would sell us all out.

Posted by: doug | February 14, 2008 3:09 PM | Report abuse

------------------------------
shame on weatherdudeVA for reading anything on weather.com...

Posted by: bernydoode00 | February 14, 2008 12:28 PM
------------------------------


LMAO! Sorry, won't do it again ;P

I agree with stormchaser on the party/mostly thing. And the Accuweather weekly blizzard thing. LOL

Posted by: weatherdudeVA | February 14, 2008 3:11 PM | Report abuse

doug -- Good call. Nothing says "sell out" like losing that oh-so-accurate 7th day ... Good thing I know who you are and can sense your email sarcasm :)

Posted by: Dan, Capital Weather Gang | February 14, 2008 3:34 PM | Report abuse

What does "X% chance of rain" mean?

Is it the straight forward, "Given 100 forecast periods exactly like the current one, and it'll rain during X of them?" Or is it something more complicated?

Posted by: jyindc | February 14, 2008 4:10 PM | Report abuse

jyindc -- Good question. See the answer on our Frequent Questions page.

Posted by: Capital Weather Gang | February 14, 2008 5:22 PM | Report abuse

a, "Partly sunny" is more sunny than "partly cloudy."

Posted by: sam | February 14, 2008 5:36 PM | Report abuse

Regarding the comment "What does "X% chance of rain" mean?"...CapWx might want to read this short paper on the subject and consider correcting the FAQ.

http://ams.allenpress.com/archive/1520-0434/5/2/pdf/i1520-0434-5-2-354.pdf

Posted by: TQ | February 16, 2008 10:16 PM | Report abuse

The comments to this entry are closed.

 
 
RSS Feed
Subscribe to The Post

© 2012 The Washington Post Company