Wednesday, August 5, 2009

Why Such High Humidities Last Week?

Several of you have asked why the humidities got so high last week. As I mentioned in the last blog, dew point is the preferred measure of how much moisture there is in the air and the dew points last week got into the mid to upper 60s--which are very high for us (the dew points today for example are in the lower 50s).

So lets play detective! One facility meteorologists have is the ability to compute trajectories of the air..the path that the air took over time--thus, we can determine where the air came from that was so sticky and humid.

I have done this...finding the air trajectories for air ending over Puget Sound at 500, 1000, and 1500 m at 5 PM on Tuesday the 28th...the evening before the warmest day and a time with high dew points (see figures). Where did the air come? From the desert southwest as some media types said? No! The humid air came from the north over southern B.C., including the forested slopes of the Cascades.

This makes complete sense if you think about it. Earlier than week, Sat through Tuesday it was quite wet up north...an upper low caused thunderstorms that rotated westward to our north. Don't believe me? Check out the radar image from the Sunday before (see below). So we had very warm air which was passing over moist forests and vegetated surfaces, picking up lots of moisture. But the fun doesn't stop there! The Strait of Georgia is warm! Well, at least a lot warmer than Puget Sound. Water temps there were in the 60s to near 70F...which mean it could provide much more water vapor to the air than the cold Pacific.

So I think we have our answer...warm air passed over moist forests and a warm water surface and picked up lots of water vapor. That made us uncomfortable and kept the temperatures up at night (remember water vapor acts like a blanket).

Now here is the interesting thing. The air really dried out on Wednesday as the temperature surged to record levels. Why? Well, the direction of the air shifted from the moist forests to the north to the dry continental surface to our east. Want proof? Below you will also see trajectories a day later...5 PM on Wednesday, July 29th. And this air was really sinking as it descending over the Sound...producing enhanced warming.



20 comments:

Big Sven said...

OK, but then why on earth is the Strait of Georgia warmer than the Puget Sound? Isn't it, you know, further north and stuff?

Must read blogs said...

i feel like its going to take me another week just to recover from the heat. it was so nice and pleasant here in chehalis today.almost early fall like.

alpenglow said...

I'm curious why the SoG is so much warmer than PS as well. It doesn't seem like it could be that different?

Big White Ball said...

I have the same question as Big Sven. Maybe the Strait of Georgia, because it is wider, has more shallow areas (despite mean depth being similar), particularly around island, and therefore can warm faster?

Larry Rice said...

HI Cliff,

I was discussing this very thing with colleagues at the radio station... and even mentioned to Sam over at the TV station that I don't recall the 1994 event being that humid. If my fading memory serves me well from 1994... I think I remember an RH in the 20% range.. but can't recall the TD being in the 60s.

In fact I would typically remind people how it usually never gets hot around here. But on the rare occasion that it does heat up into the mid to upper 90s, it is typically the product of a thermal low migrating northward that eventually induces an strong offshore adiabatic wind from the east. That airflow would typically have a desert quality to it.

I didn't have easy(FREE)access to surface obs and other data from the period around 20JUL1994 to confirm my suspicions. I suspect you already have some students working on that assignment? Side by side comparison of both events?

I look forward to the papers on this event! See you round the sound Professor Mass!

Larry

C.P.O. said...

This is why I read this blog! Freakin' awesome analysis. You could kind of tell from the way the air felt last week: maybe more humid than the usual Western WA standards, but nothing like midwest or east coast humidity. Strait of Georgia just can't compare with the Gulf of Mexico.

When they come out, sign me up for the official blog t-shirt. CMWB!

Michael said...

I am thinking "Respect the Push!" should be on the shirt

LVDLM said...

Thanks for posting this. It was really helpful. And fascinating.

The next question is... why is this pattern apparently such an anomaly? Are there precedents? How likely is a repeat?

Lance said...

Excellent post! but I think I still have the same question as everyone else, how were the water temps in the Strait of Georgia approaching 70 degrees while Puget Sound rarely gets over 55?

Lance said...
This comment has been removed by the author.
Lance said...

I might be wrong, but after a little internet research it seems that the water in the strait of georgia is warmer than puget sound due to restricted circulation and warm freshwater from the fraser river.

Big White Ball said...

Hey Lance, that sounds intriguing, and would that mean the opposite is true in winter? Which is why the Bellingham area can get colder than the Seattle area?

mainstreeter said...

Cliff, heard your forecast on KUOW. Looks like the cool weather is here for awhile.

garyLambda said...

The salt water in the Georgia strait pretty much stays in the strait even with the daily tides. The distance and that the water has to exit the strait is great enough that it doesn't fully make it to the Pacific Ocean, instead it sloshes back and forth.

In addition, the water is warmed by the tide flats which are exposed by the greater tides we have up North, than say in the Gulf of Mexico. Last week Aug 1 thru 8, we had 10ft tides every day with the lows occurring mid day. This allows the land to get heated by the sun, then when the water runs back over it, the heat is transferred.

mainstreeter said...

Also Cliff, could we be looking at near record low daily high temps the next few days?

JewelyaZ said...

It seems as though June and August swapped places this year... I am enjoying the "June gloom" though the tomatoes are probably ripening more slowly than they otherwise would have since the temps are so low and the sky is grey.

I don't remember this happening in the past eleven years, so I'm wondering how common a hot June and a cool August are?

WeatherNerd said...

Mainstreeter, I hope Cliff is wrong is his prediction of cool weather for awhile. I am getting married (hopefully outside) on Labor Day Weekend and was hoping for sunny and mid 70s for my big day. I am already ready for the warm sun to come back to us, this June gloom is depressing weather. This is August after all, what is supposed to be the warmest and driest stretch of the year.

mainstreeter said...

Yeah, I'm hoping for another month at least. August 2004 was warm and dry until mid month, then it cooled down the rest of the summer.

Tom said...

Hey all, with the weather kind of iffy, does anyone know a good spot to view the upcoming Perseid meteor shower?

Sundodger said...

Re: Larry Rice 7/20/94, According to Weather Underground, the dew points were near 60 in the morning, but plummeted to near 50 in the afternoon. The '94 100 degree day was not looking anywhere near 100 until the late afternoon when the easterly surge rocketed the temps out of nowhere -- sort of inverse of what happened on 7/29/09 with the morning surge.