Monday, December 3, 2012

Does it REALLY rain all the time in Seattle?

You have heard the complaint--in fact, you may have made it yourself:
It seems like it is ALWAYS raining around here during the winter.  Endless precipitation.

The funny thing, it really isn't true.  Let me prove it to you.
Let's start with a plot of the probability of getting at least a trace of precipitation (.01 inch) anytime during the day for Seattle Tacoma Airport:

There is roughly a 50-70% chance from early November through April 1.  Sothe  odds are, if you stand out there all day, you will get wet. Duh!

But you don't do you?  You are only outside for a short errand or walk or run or whatever.
So let's ask a more interesting and important question.  What is the chance you will get wet during a particular hour?

Ready to be shocked and amazed?  According to a study by Professor Phil Church and Mark Albright (published in Weatherwise Magazine, December 1974) only 18% of hours had measurable precipitation in November, 19% in January, and 15% in February. 

You heard it right. Even during our rainiest months, more than 80% of the hours are dry.

You don't believe me, do you?   Well, lets look at a plot of cumulative rain at my department over the last 72 hr...a very wet period indeed.  (the rain is the blue line in the upper panel, the lower one is solar radiation).  During rainy periods the blue line moves upwards, during dry periods it is level.  You can see there are plenty of  level (dry) periods (at least 60% of the time), and heavy precipitation intervals (when the line moves up quickly) are relatively rare (perhaps10-15% of the time).

Now most Northwest natives understand this--you can find a few dry hours on almost any day to have fun or do your chores (like remove the leaves from the street drains!).  Few days are complete wash outs.  If you use the weather radar effectively, you can see the dry periods and time them for maximum advantage.  Or befriend a meteorologist!

Talking about wash outs, there was a relatively rare, very intense, rainfall that hit north Seattle around 2 AM last night.  Several of you have sent me emails about it.  Streets turned to rivers and the rain was deafening.  Some of you got .3-.5 inches in less than an hour.  Take a look at the radar image..you can see the intense band (red line).


 Enjoy the rain...there will be a lot more tonight and tomorrow morning...


21 comments:

Jonathan Warner said...

As a non-native this was one of the more frustrating pieces of cognitive dissonance to deal with. Everyone complaining about it always raining when observations told otherwise was maddening.

Thanks for keepin' it real, Cliff.

eldan said...

I made another kind of visualisation, that shows both the probability of rain and the likely amount in a day, so all the days on which it often rains but only a trace show up. It's a pretty interesting comparison with other cities:

http://eldan.co.uk/2012/10/rain-redux/

Alex said...

Would love to see a similar analysis of sunshine. I think most people are more bothered by the grey weather than rain, and when they complain of rain, they really mean it just looks like it should be raining and the ground is wet.

Raymond Forbes said...

It's true, it doesn't really rain all the time. However, as you pointed out, it is very common to have some rain every day for long stretches of time. Take that with constant cloud cover and short days and it just feels "drizzly"

Unknown said...

Perhaps the better question to ask would be how many cloudy days per year? Many would say it's not the rain they mind, it's the absence of sun!

db said...

Hi Cliff,

I agree. Despite my complaining this time of year about the 'rain' it is usually the persisitant cloudiness that really gets to me. Not so much the water. Normally I find it is tolerable through the winter holidays. But by mid-January, I'm looking forward to some clearer skies.


It has been an extraordinary wet autumn after a beautiful start. It appears it is approaching the total wetness of 2006, which also had a beautiful Indan Summer effect into October, as I recall. I seem to remember many hikes in the North Cascades that autumn

Here are a couple of graphics I pulled together while practicing and playing with some data visualization techniques (boundary lines and small multiples). I'm no climatologist - nor statistician - so my 95% boundary lines may be debatable. Nevertheless, I think these two charts so how wet it's been pretty clearly. Maybe extraordinarily wet for this brief period.

2006 vs. 2012 Comparison (Oct-Dec)
http://www.flickr.com/photos/96201513@N00/8239277483/in/photostream

12 Water Year Starts (2001-2012)
http://www.flickr.com/photos/96201513@N00/8234008668/in/photostream

The two graphs are probably best viewed in the 'Original' size of which there should be alink to on each respective page.

smirgol said...

Those of us living in Hilo, HI laugh at Seattle and their pretend rainy-ness. Living here is like living in the Hoh Rainforest, literally: the annual rainfall totals are very similar!

Christina Wilsdon said...

Whew, I heard that 2 a.m. rainfall--woke us up--sounded like hail hitting the windows. Now I'm sorry I didn't drag myself out from under the cozy comforter to take a look at the streets turned to rivers.

Spaceman Spiff said...

This is great! I did a similar type of visualization to Elban's over at my blog: http://nhuntwalker.blogspot.com/2012/11/viz-rain-in-seattle-really.html

I used a different site for weather data than the NOAA, though I did look there first and spent a bit of time trying to work with what they had. I found it to be very frustrating.

I wish I had known that you both had done this type of work, or were working on it. I would've asked you so many questions!

-Nick

John Marshall said...

The only reason we don't have the mass of people here, that we don't look Phoenix or LA, is the "horrible weather".

Please, please keep the myth alive. The one that says you have to be a duck or an erstwhile vampire to enjoy living up here. Its a great story, even more so for being completely untrue.

The beauty of science is that it tells the truth as best we are able to discern it. Which is especially useful when it conflicts with our gut impressions.

Fortunately, a lot of the people we don't want living here don't "believe" in science. So keep the fairy tales about endless rain alive.

John

Bhodda's My Ba'aag said...

Anyone who think it rains all the time in Seattle needs to spend a few winters in Olympia. Moving back to Seattle, you'll swear you've crossed the Cascades.

Christopher Shotwell said...

That was a crazy rainstorm the other night! I think that's the first time rain has woken me up in Seattle without being accompanied by thunder.

Colleen said...

As others said, most people aren't necessarily bothered by the rain, (and don't claim it rains "all the time"). We're all aware that precipitation totals are higher elsewhere. It's the *kind* of rain ~ the drizzle, the dampness, the grey ~ that people are referring to. As far as your remark that we don't stand out there all day...Ha! Rest assured, Cliff, there are still people engaged in outdoor-oriented manual labor.

Gary said...

"Fortunately, a lot of the people we don't want living here don't "believe" in science." - John M.

Thank you for that understatement.

I have traveled a lot in my work. When someone asks me if it "really" rains so much in Seattle, I respond with the slight evasion:

"I remember one December when there were only 13 HOURS of sunshine during the entire month!"

(I think it was one Dec in the 90's.)

RLL said...

Another problem with December and January is the short days. If you are retired or have the 'right' job you can go out during the sun breaks. Life is not so bad! But school kids and those stuck in offices and shops 8-5 are not so lucky.

Restless_one said...

I think this article may miss out on another fact than the grey/lack of sunshine.
Most of us don't live in Seattle. As Cliff points out, the NW has very localized weather. Seattle itself is one of the driest spots of the Puget Sound. When commuting downtown to S Everett I routinely saw the effect of the rain shadow or convergence. So sure, Seattle may only see a few hours of rain a day, but every other city/town may see more.

Also I wonder about looking at the "last 72 hrs". As you pointed out, it was a very wet period. These are exactly the periods when rain is less consistent as the storm ebbs and flows. Compared to a day where it may rain less than .2" all day, but in a state of semi-constant "liguid air". To me that's the definition of NW rain vs almost anywhere else in the world(perhaps Londoners can relate though).

J said...

I agree with Restless_one. For those of us who do not live in Seattle we see a great deal more rain and fog. I can go for days not seeing my neighbors house in drippy fog or have drizzly rain all day while a few miles away it is dry. When we say "it rains all the time in Seattle" most of us are using Seattle as a broad category for this area of Washington-especially with outsiders who wouldn't know the names of our towns or neighborhoods.It is a well known fact in my area that Seattle is a oasis of dryness and light compared to where we live.

Unknown said...

Where can I get hourly rainfall data? I've poked around but it seems hard to get, say, a CSV file with timestamps and rainfall amounts.

Last semester I taught a statistics class and it would have been nice to use local weather data sets in class.

SR said...

Professor Church also wrote a paper titled, "It hardly ever rains in Seattle, hardly." We get a lot of cloudy days and a lot of drizzly days, but few real hard rain days. I was privaleged to take Dr. Churche's Climatology classe in the early 70's.

eldan said...

Sorry this is so late, but:

A couple of people asked for a sunshine analysis. I don't have the data to do that exactly, but the files I've been working on do include visibility, which is a not-completely-worthless proxy for it, so here's the updated vis:

When the sun don't shine.

I'm not totally happy with it, and I'd love feedback on how to make it better.

soggy said...

Will 2012 set a record for annual rainfall at SeaTac? I have been unable to find the current record, but we are running more than 11 inches above the average, and it looks like we may hit 50 inches this year!