June 06, 2010

The First Day of Summer: When Will It Be?

The big question that is on everyone's mind:

When will summer really begin?

There is a stock answer that local meteorologists give for this query:
July 12th. And it is usually pretty close to the truth.

First, lets get real. June is often a bad month in Seattle. June gloom. Lots of low clouds. ( The good thing is you can usually get out of it by crossing into eastern Washington.) Memorial Day weekend has never been dependable. I can't count how many times the media has done the same story about the unusually cloudy spring.

There is NO reason to suspect our cold weather is due to the Iceland volcano, the oil spill in the Gulf, or El Nino/La Nino. It does not have any connection with global warming or sunspots.

The July 4th weekend is also a crap shoot. Sometimes very nice, often not. But then something magical happens...it is like someone throws a switch...and by mid July we have the best weather in the U.S. --temps in the 70s, low humidity, sunny skies, not too many bugs--heaven. In fact in July we get less rain than Tucson or Phoenix.

The change in July is really quite extraordinary...one of the most rapid climatological dry-outs in the U.S. Below is a graph of the daily probability of getting a trace of rain or more for.. July is just amazing. During the first few days of the month there is roughly a 30% chance of rain in any day. By the last week..roughly 8%.

Want to get married outdoors or hold a party?...end of July or beginning of August is the driest period of the year. No doubt that is why Seafair is scheduled then.

What about the next week? The next few days will have rain on and off. But then, next weekend, the models are going for a major warm up...we will see.


  1. I'm never really a fan of people complaining, but it's funny watching people complain about this lovely weather and not being able to do anything about it. Keep bringing it on!

  2. My July 11th birthday is MY official start of summer... easier to remember that way LOL

    Still, I am ready to put some tomato plants in the ground...

  3. I'm a little suspicious. Why would one day in March over a sixty year period have a 40% chance of rain and a few days later a 70% chance? This curve seems quite ragged for a sixty year average.

  4. It seems suspicious to dismiss the Iceland volcano as having no impact on this year's weather. In each of the major northern hemisphere volcanic events in my experience, scientists like Cliff have admitted that they had a dramatic effect on world weather for over a year after the event. Looking at the size of the cloud from the Eyjafjallajokull volcano in the month after eruption, it seems highly unlikely it did not have an effect on our weather in the month following.

  5. Cliff you rock! I've been saying this for years to people and everyone keeps complaining. Everyone thinks I'm wrong and that rain in May and especially June in Seattle is an aberration -- and then they look at me like I'm so audacious to say Summer doesn't start till after 4th of July. I'm going to link to your blog forever now :)

  6. yup, that's why we got married outdoors in the mountains 28 years ago this July 31. It rained for the 9th time in 60 years, of course. Made us feel special.

  7. The first day of summer will be June 21. It is the most common date for the first day of summer. This year it begins at 4:28 am PDT.

  8. Yes, there is a lot of talk about how cold this spring is, but isn't there some evidence to actually support that complaining this year? Top 10 coldest Mays in Seattle history, most consecutive days of dry-season rain, potentially the latest day to hit 75 degrees. Shouldn't that count for something?

  9. I always tell people not from around here that Summer starts July 15th in Western WA and end September 15th....

  10. Yea Adam I agree, with everything but the "cold" part. All those in my opinion do count for something. Based on my experience, it has truly been nicer and much more comfortable than normal this spring, I guess it makes up for the very warm winter we had.

  11. Hal said "This curve seems quite ragged for a sixty year average."

    It's a 60 year average so each date is just an average of 60 days samples of a "binary" property (1=more than a trace or 0=less than a trace). That's not a huge number hence the variation in the average.

    To get a rough estimate of the variance you count on any particular date the number of days with rain so your maximum count is 60 and the probability is that count/60. So the "standard deviation" (for a "random" Poisson distributed data) would be about sqrt(60) = 7.7 or as a probablility 0.12. Each data point would be the "actual" value plus or minus 0.12.

    Statistics tells you that about 2/3rds of the results will fall in that one standard deviation band. 95% of the results will be within 2 standard deviations (i.e. plus or minus 0.24). That's on a value that goes from 0 to 1.

    You can see the averaged data is going to be noisy because you have so few samples on each date.

  12. homer simpson said: " you can prove anything with statistics, 17% of the people know that."

  13. Thanks Cliff.

    How much rain has fallen in the last month or so?

    How does that compare to average?

  14. Ok I am one of those "complainers" but I appreciate you keeping it real. When people say this isn't typically normal for May/June, I think Ok, so maybe it will be better tomorrow...next week...next month??? And it starts to make you crazy because you are anxiously hoping/wishing for something that just doesn't exist. Then I check historical data and sites like yours and realize, Nope, it is what it is. I'm new to Seattle and have lived in a few places throughout the US and I will say I have never experienced anything like this constant gray. It can really wreak havoc on your mental state after a while. But thanks for the reality check. Like John said it's out of our control and we need to accept it, not resist it. That's what I'm trying to work on but somedays (months) are more difficult than others.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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