Weather Blog

Guess what? It really does rain more often in Seattle on Saturdays

Guess what? It really does rain more often in Seattle on Saturdays
FILE -- Visitors to downtown Seattle huddle under umbrellas while walking in the shopping district. (AP Photo/Elaine Thompson)

So here we are again, Seattle.

After a sunny and warm week, the clouds roll in and the showers arrive just in time for the weekend. Just like it did two weeks ago. And if rainy weekends seem like a frequent occurrence this spring, your memory does not deceive you. It's rained on the past four weekends, and five of the last six.

It's no surprise to longtime Seattleites -- our soggy weather makes Seattle the punchline of many a rain joke, including the classic: "What do you call two straight days of rain in Seattle? A weekend!"

But is it really true? Or does it just seem that way because rain on a Saturday or Sunday gets a lot more attention than, say, a soggy Tuesday?

On an offhand check, I went back and counted the rainy days back the 19 weeks to the start of this year and lo and behold, Saturday was indeed the most frequent day of the week with measurable rain -- 13 times! The next closest was a four-way tie at 10 and yes, Sunday was one of them. (Best day? Friday at 8 times.)

All right, so for 2014 at least, Saturday has been a bad day to plan an outdoor event.

But that's a pretty small sample size. So, I decided to go for the gusto and count all the way back to near the start of Seattle's records at Sea-Tac Airport. I figure 65 years worth of data would put sample size concerns to rest.

Ready for the punch line?

Saturday is still the rainiest day of the week!

Hah, the jokes are right!!!

BUT! It's not by a statistically significant amount. In fact, it's just a handful of days over the other days of the week.

Below are the results going back to Jan. 1, 1949. I broke it down by days with greater than a half inch of rain (super-soaking days), greater than a tenth of an inch of rain (moderately rainy) and days with any kind of measurable rain.

From Jan. 1, 1949 - May 15, 2014 (23,876 days) in Seattle
Days with Saturday Sunday Monday Tuesday Wednesday Thursday Friday
>0.50" rain 207 215 196 211 207 215 200
>0.10" rain 886 862 881 886 868 880 873
Any rain 1479 1458 1468 1460 1413 1443 1457
As a percentage of days per week
Days with Saturday Sunday Monday Tuesday Wednesday Thursday Friday
>0.50" rain 14.3% 14.8% 13.5% 14.5% 14.3% 14.8% 13.8%
>0.10" rain 14.4% 14.0% 14.4% 14.4% 14.1% 14.3% 14.2%
Any rain 14.5% 14.3% 14.4% 14.3% 13.9% 14.1% 14.3%

Sunday tied for the most super soaking days while Saturday was the overall rainy day winner (I knew it!), but when you're talking about 3,410 weeks in that period, 20-40 weeks that had a bit more rain on a certain day than others is pretty small amount.

And really, that is what you would expect as there isn't any reason it would rain more often on a Saturday than a Wednesday besides dumb luck. If the days were equally spread out, you would expect 14.3% of all rainy days to fall on each day of the week (a 1 in 7 chance). And these numbers come reasonably close.

So no, Mother Nature isn't out to get us, but those weekend days do get to gripe a bit they're being picked on them a little more than their counterparts.