Why Such High Humidities Last Week?

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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.

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