Wednesday, March 18, 2015

The most asked weather question, answered.

I get asked a lot of weather questions.  What do you think is the most frequent?  

No, its not about the impacts of global warming.

And what the probability of precipitation really means.

The most frequent question is:

What is the difference between partly cloudy and partly sunny? 

 Really, folks ask that all the time.  Sometimes it seems like a subtle put-down about meteorologists, but sometimes it reflects real curiosity.




Well, let's give the answer based on the official National Weather Service definitions.

There are two things you have to know about this.  First, different terms are used during the day and the night.   The National Weather Service (NWS) talks about clouds at night and sun during the day.  Why? Because there is no sun at night !   Second, the definitions are based on the percent coverage of the sky.    So let's examine what the NWS uses!  

At night, the sky goes from clear, to mostly clear, to partly cloudy, to mostly cloudy, and finally to cloudy as the sky coverage increases from 0 to 100%.   During the day, the descriptions go from sunny to mostly sunny to partly sunny to mostly cloudy (since you can't see the sun then!) to cloudy.  Here is a table with the exact definitions.

NIGHTTIME       DAYTIME         PERCENT OF SKY COVER

CLEAR           SUNNY           0 TO 5
 
MOSTLY CLEAR    SUNNY           6 TO 25

PARTLY CLOUDY   MOSTLY SUNNY    26 TO 50

MOSTLY CLOUDY   PARTLY SUNNY    51 TO 69

MOSTLY CLOUDY   MOSTLY CLOUDY   70 TO 87

CLOUDY          CLOUDY          88 TO 100

It all makes sense I guess.  And Pixar actually made a short movie in which the star is a lovable cloud called partly cloudy.   


 

11 comments:

Eric Lee said...

Whatever possessed you to drop a line like "what probability of precipitation really means" and not include a link to the answer? :-)

hidden wave said...

Walter Kelly likes to use "some sun breaks"

John McBride said...

This is a great post, Cliff. You not only educate the casual reader, but you inform more serious readers for whom the post, as in my case, can serve to make one see that there are plenty of questions remaining to be asked, even about phenomenon one might take for granted an existing supposition, only to be shown they didn't know at all. Or so say I who just discovered that what I took for granted was only about 50% accurate. I think that's less than a " D ", right?
.

snapflux said...

I recommend using the hourly forecasts from NWS. It builds a more complete picture of the days ahead, and clouds are represented as percent of sky cover.

Gregg said...

serious point/question, What is the difference between rain and showers? I've lived in rainier climates that did not frequently use both words in their forecasts (most annoying was NWS pointing to showery rain)

Scott Souchock said...

Thanks for answering this. I know these terms are probably easier to say and type but would it be better to say precisely the range of cloud cover in forecasts? I know one of your passions is improving how weather forecasters and reporters communicate information, so might these percentages be better terminology? There's a part of me that think getting a grant to work with forecasters and graphics to record forecasts on video and on the various media and test new ways with the public could be really fun and interesting, if you're not already doing that.

Patrick said...

Thanks for teaching us something! So why doesn't the NWS reduce the confusion slightly and use the nighttime terms for the daytime as well?

Tim W said...

This is very helpful. I'm left with a question that is not really yours to answer, but why doesn't Nighttime progress from Clear to Mostly Clear to Partly Clear to Partly Cloudy to Mostly Cloudy to Cloudy? In other words, why have two categories covered by Mostly Cloudy at night when each category has a distinct label during the day?

Gregg said...

Cliff, Saturday am, this has today showers, Sunday rain...both with exactly same rainfall prediction....so what the heck is the diff between rain and showers?

400 AM PDT SAT MAR 21 2015

TODAY...SHOWERS LIKELY IN THE MORNING...THEN SHOWERS IN THE AFTERNOON...ESPECIALLY SEATTLE NORTHWARD. RAINFALL AMOUNTS A TENTH TO A QUARTER OF AN INCH POSSIBLE. HIGHS IN THE MID TO UPPER 50S. SOUTHWEST WIND 10 TO 20 MPH.

TONIGHT...MOSTLY CLOUDY. LOWS IN THE UPPER 30S TO MID 40S. SOUTHWEST WIND 10 TO 15 MPH BECOMING SOUTHEAST AFTER MIDNIGHT.

SUNDAY...MOSTLY CLOUDY WITH A CHANCE OF RAIN IN THE MORNING...THEN RAIN AT TIMES IN THE AFTERNOON. RAINFALL AMOUNTS A TENTH TO A QUARTER OF AN INCH POSSIBLE. HIGHS IN THE 50S. LIGHT WIND BECOMING SOUTHWEST AROUND 10 MPH IN THE AFTERNOON.

Kiwibru said...

So Cliff, here is another one:

What is the difference between a "brief" shower and a "odd" shower?
Hint...U.S. vernacular vs. Canadian!

jenna said...

Hello: I have a fairly strange question. I swear to you all this really happened. I was in a drive thru line on a partly cloudy/sunny day with my sunroof open. I look up and I see a very SMALL patch of colors start to form like a circular rainbow. The colors got darker then it slowly faded out back to normal. Colors were rainbow colors but a violet like blue was also included. People I swear this happened. Is this something that does commonly happen?