Friday, August 16, 2013

Super Sticky

I have heard a number of complaints the last two days about the unusually humid conditions that have settled in over the region, particularly west of the Cascade crest.  Folks around here are not used to sweating.

 A plot of dew points and winds at 9 PM Thursday tell the story.  Over the western interior and the western side of Cascades, many locations had dew points in the mid-60s.  This is quite high for our area, where dew points generally remain in the mid-40s to mid-50s.   Dew point, the temperature to which air must be cooled to become saturated (100% relative humidity) is a far better measure of moisture in the air than relative humidity, which also depends on temperature.


In fact, a plot of the dew points at Seattle Tacoma Airport reveals that the dew point Thursday night were the highest for the entire summer!  The air has a bit of that "East Coast" feel to it.
The discomfiture we are feeling now is enhanced by the unusually light winds of the past two days, generally less than than 5 miles per hour.  The reason for the light winds is that the East Pacific High, which helps produce refreshing northerly winds and onshore flow has been replaced by a weak low or trough (see graphic: a surface pressure chart for Thursday night).  The differences in pressure (see solid lines or isobars on the chart) are quite small...thus, weak winds.


Why high dew points and thus serious sweating?   You start with light rain, which evaporates into the relatively warm air...this pushes up the dew point.  The fact the surface is now relatively moist helps of course.  Onshore flow, strangely enough, is relatively dry (low dew point) because the Pacific Ocean is cold (around 50F) and cooler air can't hold as much water vapor as warm air.  So the weak onshore winds (produced by the absence of he Pacific High) allows the dew points to rise in the interior.

The moist, wet conditions we have experienced have produced a lot of low clouds, and the combination of the clouds and the high humidity does not allow the surface to cool effectively (water vapor and clouds absorb and reradiate back down the infrared radiation from the surface).   Thus, the minimum temperatures during the last few days have been high.  Check out a plot of the Sea Tac temps:
Wednesday the temperatures only dropped to about 65F!   A few locations had record high daily minima on Friday morning:

RECORD EVENT REPORT
NATIONAL WEATHER SERVICE PORTLAND OREGON
451 AM PDT FRI AUG 16 2013

...RECORD HIGHEST LOW TEMPERATURE SET AT VANCOUVER WA...

 A RECORD HIGHEST LOW TEMPERATURE OF 67 DEGREES WAS SET AT VANCOUVER WA
YESTERDAY...AUGUST 15TH 2013. THIS BREAKS THE OLD RECORD OF 65 SET
IN 1950.

...RECORD HIGHEST LOW TEMPERATURE SET AT SALEM OR...

 A RECORD HIGHEST LOW TEMPERATURE OF 65 DEGREES WAS SET AT SALEM OR
YESTERDAY...AUGUST 15TH 2013. THIS BREAKS THE OLD RECORD OF 64 SET
IN 2008.
Remember the stories about increasing summer heat waves that were in the press last month?   This is going to be another example--but I don't think anyone is going to the hospital from it. 

Anyway, our perfect weather will return this weekend, but today's humidity is a reminder of why we are lucky to be living here, rather the humid summer miasma of the eastern U.S.!

3 comments:

Offbeatmammal said...

Ha ha, you said "East Pacific High" just in time for hempfest! Hope it doesn't rain on the Doritos

Jeff Lesan said...

Cliff, there have been quite a few days this summer that have felt more humid to me than years past. Is this correct?

eprman said...

Your comment that dew point is a far better measure of moisture in the air than relative humidity sent me looking for my 46-47 year old thermo book. That proved to be a bit too much on a nice sunny afternoon so I took the modern route, the internet. I found on wikipedia a nice chart showing the relationship between temperature, relative humidity and dew point. I had thought that there was a unique relation between dew point and relative humidity but the chart clearly showed otherwise. Now I understand. Thanks for pushing me to learn (or re-learn) something new.