Weather Blog

'Partly sunny' vs. 'mostly cloudy' -- yes they mean something different

'Partly sunny' vs. 'mostly cloudy' -- yes they mean something different
A partly sunny day over Tacoma and Mt. Rainier? Or mostly cloudy? (Probably "partly sunny"). Photo: Renee Fields Photography‎.

It's a surprisingly common question we get around here: "What's the difference between 'partly sunny' and 'mostly cloudy'? Isn't it if it's one, it's also the other?"

Yes and no. At least for the forecasts written by the National Weather Service, there are very specific definitions in their zone forecasts that are broken down into each region.

(You've likely seen those forecasts -- for instance, they're the ones in all caps you see if you've ever been a fan of "Weather on the 8's" on a certain dominant national weather channel.)

The weather service forecasters create a "gridded forecast" -- essentially using computer models and a dash of human local expertise to draw up a forecast with expected weather conditions over a map. A computer then translates that forecast into text for each region, with each specific forecast condition tied to very specific language in its output.

So, obviously somebody had to program the computers to know the difference between mostly cloudy and partly sunny. And it's there where we can find a definition!

For instance, if a sky is expected to be 70% covered in clouds, the forecast is "mostly cloudy", but clear away just, say, 3 percent of those clouds and now it's a "partly sunny" day -- unless it's at night, in which case it's still mostly cloudy (since "partly moony" would sound weird?)

On the other hand, it's a sunny day up until the sky has just over 1/4 of it covered by clouds, at which point we drop down to just "mostly sunny."

Here are the official National Weather Service definitions:

Sky Condition Day

Percent Cloud Cover

Sky Condition Night

Percent Cloud Cover

Cloudy

88-100%

Cloudy

88-100%

Mostly Cloudy

70-87%

Mostly Cloudy

51-87%

Partly Sunny

51-69%

Partly Cloudy

26-50%

Mostly Sunny

26-50%

Mostly Clear

7-25%

Sunny

0-25%

Clear

0-6%

So partly cloudy is less cloudy than partly sunny. (Stump your friends at the bar or water cooler with that bit of trivia! And then bookmark my blog on your phone for proof!)

Oh, and a partly sunny day instantly turns mostly cloudy -- not partly cloudy -- at sunset. But otherwise I think those terms make sense.

"Windy" versus "Breezy"

It doesn't just stop at cloud cover. People also wonder what makes a "breezy" day a "windy" day? Well, technically they can be both, but 26 mph sustained wind makes it a windy day -- unless it gets to 30 mph (sustained, not gusts) in which case it gets to be a "very windy" day.

Even stronger? Forecasters then get some choices to plug in like "strong winds" or "damaging winds" or "High Winds" (alas "Extra Layer of Hair Spray Required" is not one of them.)

Here is their official list (again, sustained wind speeds -- gusts can be higher. So in a typical wind storm where we might have 20-30 mph winds gusting to 45 mph, we still only get "windy" not "very windy.")

Sustained Winds / Term Used:

  • 0-5 mph: "Light", or "Light & variable wind" (no wind direction given)
  • 5-15 mph: Just compass direction, like "North wind 5-10 mph"
  • 15-25 mph: "Breezy", unless it's cold and then they can use "brisk" or "blustery"
  • 20-30 mph: "Windy"
  • 30-40 mph: "Very Windy"
  • 40-73 mph: "Strong" "dangerous" "damaging" "High Winds" (or if I ever hacked into their forecasts: "Extra Layer of Hair Spray Required")
  • 74 mph or greater: "Hurricane Force"

OK, now it gets complicated

Here is where Seattle forecasters get away with making the frequent rainy days sound different in the forecasts. Otherwise with over 150 days of rain a year, it'd start to sound pretty monotonous.

That is how we end up with forecasts with these gems: 

.TUESDAY...CLOUDY WITH A CHANCE OF SHOWERS. HIGHS IN THE LOWER 50S..

.THURSDAY NIGHT...MOSTLY CLOUDY WITH A CHANCE OF RAIN. LOWS IN THE LOWER TO MID 40S.

First a primer on "rain" versus "showers" -- sounds the same but it's a bit different and could probably do an entire blog on that. (Oh wait, I did!). "Rain" is a more widespread, longer-lasting event, where as "showers" are brief and passing. So a "chance of rain" means a possibility of a steady rain over a wider area, whereas a "chance of showers" is a possibility of a passing rain shower. Got that?

But then there's "chance" of rain, or "rain likely" or just "rain". And like our cloud cover, each has a meaning.

Percent Chance Expressions of Uncertainty Equivalent Areal Qualifiers
10 – 20 % Chance Isolated
30 – 40 – 50 % Chance Scattered
60 – 70 % Likely Likely
80 – 90 – 100 % No hedge: "Rain" or "Snow" No hedge, it's gonna happen

So if there is a 30 percent chance of showers over 30 percent of the area, you get a "chance" of "scattered" showers. If here is a 70% chance of showers, you get "showers likely" or a 70% chance of rain is "rain likely" but if that goes to 80% it's just "rain" in the forecast; no qualifiers.

Weather forecasters are allowed to change some text to like "Rain at times" or "Occasional Rain" or "Periods of Rain" to give some nuance to something a bit more extended rain periods than showers, but not quite non-stop rain.

What's an "upper 50"?

Last but not least, there's temperature. Forecasts give a small range in the text.

  • "Near 50": 48-52
  • "Around 55": 53-57
  • "Lower 50s": 50-53
  • "Mid 50s": 54-56
  • "Upper 50s":57-59
  • "50s" - 50-59 -- useful in the Midwest when there are big temperature swings, or when you really have a high uncertainly in your forecast (or your dart fell off the dart board and didn't stick to a number, rendering the forecast officially moot.)

So it all makes sense now, right?

Now you can see how this pieces all together: A weather forecaster creates a grid forecast over Seattle that shows 55% cloud cover, with a 30 percent chance of showers and sustained southerly winds of 22 mph and a high of 55, the computer will spit out this forecast: PARTLY SUNNY, BREEZY WITH A CHANCE OF SHOWERS. HIGHS IN THE MID 50S.

The forecasters are allowed to nuance the text as needed -- many times they'll move "Windy" to the front for emphasis.

.MONDAY...WINDY...RAIN. HIGHS IN THE MID 50S.

So there you have it. Go forth and win those bar bets.

(Want even more details? Check out this reference page from the National Weather Service.)

Warm winter bringing out the tulips early at Skagit Valley Tulip Festival

Warm winter bringing out the tulips early at Skagit Valley Tulip Festival »Play Video
File photos of the tulips at the Skagit Valley Tulip Festival. (Photo: Brendan Ramsey)

Walk around the Puget Sound area and you'll notice trees starting to bloom and perhaps the whirr of a lawn mower or two, even though winter still had a solid 3-4 weeks left in its reign.

Seattle finished up February as the warmest on record, on the heels of a very warm January (and record-warm December) as well, and the early spring-time weather has in tandem brought out the first signs of spring.

L.A.-area beach turned white during intense hail storm

L.A.-area beach turned white during intense hail storm »Play Video
A surfer prepares to enter the water on a hail-covered beach, Monday, March 2, 2015, in Huntington Beach, Calif. (AP Photo/Los Angeles Times, Allen J. Schaben)

You know it's been a paltry winter around here when beaches in Southern California look more the winter wonderland than some of our ski slopes.

Check out what happened in Huntington Beach when an intense hail storm moved through Monday morning.  Some of the pics from social media are truly amazing!

Here is the story from the Associated Press:

Seattle sets twin records for warmest February, winter on record

Seattle sets twin records for warmest February, winter on record
Sun sets over the Olympics on Feb. 28, 2015. (Photo: Sigma Sreedharan)

In what will go down as one of the best -- or worst -- winters on record, depending on what you want out of a Seattle winter, now there will be some meteorological trophies to go along with the memories.

Seattle has set its record for all-time warmest February since official measurements began at Sea-Tac Airport. The average temperature (high temperature plus low temperature, divided by two) was 48.8 degrees narrowly edging 1977's record at 48.7. (And I mean narrowly. Had Saturday just been one degree cooler, it would have been a tied record instead.)

WATCH: Northern Lights peek out over Western Washington

WATCH: Northern Lights peek out over Western Washington
Photo of Northern Lights on 15 second film exposure as seen from Mukilteo on Feb. 23, 2015. (Photo: Liem Bahneman)

It was a bit of a surprise considering there wasn't much solar flare activity but the Northern Lights made a faint appearance over Western Washington Monday night.

2 routine events combine for spectacular scene over Canadian skies

2 routine events combine for spectacular scene over Canadian skies
Photo of a "FallStreak" cloud spotted over Surrey, B.C. at sunrise on Feb. 22, 2015. (Photo courtesy: Zora Fernandez)

Those who were up early enough Sunday morning in Surrey, B.C. and happened to look up were treated to a spectacular scene in the heavens that looks like something straight out of the imagination of a futuristic Hollywood alien blockbuster film.

In actuality, it was the combination of two rather routine events that just happened to have impeccable timing:

A sunrise (one for the ages on its own) …and a plane descending through a solid, stable cloud layer.

Long range forecast maps: Short term gain, long term pain

Long range forecast maps: Short term gain, long term pain
Brilliant sunset on Feb. 16. (Photo credit: Mirwais Azami Photography)

It's the third week in February, and that means it's time for NOAA's monthly long range forecast update. But while skiers and snow lovers have probably trained themselves by now to just skip reading this type of entry in my blog, I bring tidings of GOOD NEWS!

Sort of.

Let's hold off the inevitable bad news for a few paragraphs to show this map in all its glory:

Central Nebraska about the only folks experiencing a normal February

Central Nebraska about the only folks experiencing a normal February
Map via WxBell showing expected temperature deviations from normal later this week, but is also essentially a snapshot of this winter's persistent pattern. (Photo courtesy: Susie Martin)

The weather pattern this winter has been stark in its dramatic differences -- temperatures at record-warm levels in the West, and a relentless march of arctic air masses pummeling the East.

The map above is a snapshot in time -- actually a forecast depicting areas of expected below and above normal temperatures for later this week, but it's been the consistent story the past several days anyway.

Seattle easily on pace for warmest winter on record

Seattle easily on pace for warmest winter on record
Photo: Brad Spiegel

As you look around to flowers budding, lawns needing mowing, and skiers frowning, signs are everywhere it's been a very mild winter. So it should come to no shock that we are indeed on pace to shatter records for warmest winter -- and autumn-winter combined -- since 1945 when Sea-Tac Airport became Seattle's official observation.

First, let's look at the overall numbers:

Mountain snowpack now totally gone in some spots

Mountain snowpack now totally gone in some spots
The Hurricane Ridge parking lot that shows a distinct lack of snow on Feb. 16, 2015. (Photo: Hurricane Ridge Park Web Camera / National Park Service)

The numbers have been ugly…and they're getting uglier by the hour.

The National Weather Service has put out its twice-monthly report on the mountain snowpack and the numbers for Feb. 15 and, well, skiers should probably stop reading here. Perhaps water managers and those who have to battle wildfires might just head on over to the sports sectio…well, maybe the offbeat news?

To those who are brave enough to stomach the results, here goes: