Weather Blog

Seattle at 41 days (and counting) since last official sunny day

Seattle at 41 days (and counting) since last official sunny day

Forgive us if our kids need convincing the sky is actually blue, not grey, because blue skies have been hard to come by around here of late.

As KOMO morning weathercaster Paul Deanno has been tracking, amazingly, it has been 41 days since Seattle last had what qualifies as an official sunny day, which is defined as 30% or less cloud cover through the day. The last day that qualified was February 25, which was during our last arctic blast that came through following the big snow event. The high that day was only 31, so it's not like it was exactly a day to hit the beach.

If you want to find the last reasonably sunny day by Seattle outdoor standards, you have to go back 47 days to February 19th when we registered crystal clear skies (the elusive 0% cloud cover) and... 43 degrees.

Now, Seattle isn't exactly known as "sunshine city". Turns out, a month or more without a sunny day is really not too rare between October and April.

But if it seems like it's been particularly gloomy of late, take solace in that this is the longest stretch of no sunny days since, oh -- it's last spring. More on that in a moment.

Going back to 1996, for which I have monthly data available, there have been 20 months that have gone without a sunny day -- 2 in October, 5 in November, 2 in December, 4 in January, just 1 February, 3 in March (including 2011), 2 in April, and -- amazingly -- one in June, in 2007.

Seattle only averages 58 sunny days a year with, on average, about 10-12 for each of the summer months so that doesn't leave much left for the other nine months. Indeed, Seattle and its 220 overcast days ranks as the cloudiest major city in the U.S. and only ranks a little behind Forks and Astoria, Ore. of major weather stations that track the data.

How about some "it could be worse" statistics?

* Just last year, we had a 61 day streak of no sunshine, from March 7 through May 6. That means all of April had some element of cloudiness to it. But this current stretch has been a lot wetter.

* The longest stretch of no sunshine that I can find since 1996 was 73 days, set Oct. 31, 2006 through Jan. 12, 2007. There was also a 62 day stretch from Oct 31, 2004 through Jan. 1, 2005.

* In the 6 year period from 2004 through 2009, there was just 1 sunny day in November -- November 22, 2007. Every other November day in that period was partly cloudy to overcast. (But you expected that anyway).

Now, admittedly, this statistic can be somewhat deceiving. There can certainly be hours-long stretches on these days that many would consider sunny or mostly sunny. But it's additive of the day, so if you have morning clouds through 11 a.m. and then crystal clear skies the rest of the day, it might only qualify as partly cloudy. So it's not like it's been gray for 984 consecutive hours.

How do they know if it's officially sunny? Do they have some guy on a porch declaring, "Yep, it's sunny" or "Nope, it's not"?

They used to measure minutes of sunshine via a sun sensor and then divide that by number of minutes of sunshine in the day from sunrise to sunset to get a percentage, and that could correspond to the "is it <30% cloud cover to get the sunny day?" measurement. But now I believe with new automated equipment, it's not as fancy.

When an observation is given, there can be four designations in cloud cover: "CLR" (Clear), "SCT" (Scattered clouds - mostly sunny), "BKN" (Broken clouds -- mostly cloudy), and "OVC" (Overcast). Many times there are multiple readings like "SCT 030 BKN 100 OVC 250" which means scattered clouds at 3,000 feet, broken clouds at 10,000 feet and an overcast layer at 25,000 feet.

I believe -- and I'm waiting for confirmation on this -- that the National Weather Service assigns a cloud cover number based on the worst listed condition: 0 for clear, 3 for SCT, 7 for BKN and 10 for OVC. So if you have a SCT and BKN, that observations gets a 7. If you have just a SCT, it's a 3. If you have a SCT and OVC, that's a 10.

It adds up each hour observation and then divides by the number to get an average score. 0-3 is sunny. 4-7 is Partly cloudy, 8-10 is overcast.

When will the streak end?

We think Friday has a chance of rating a sunny day, but with a few clouds around it's possible it'll only get a '4' or '5' rating and the streak will continue.

One other streak we are watching is days with consecutive rain. As of Thursday, we're at 15 straight days of rain. The record is 32, but 18 days would get us into the Top 10. However, it does look like Friday will remain dry through the day.