July 22, 2011

Why the Northwest is Nearly Heat Wave Proof

 As we complain about our cool summer, we should not forget that the central and eastern portions of the U.S. has been experiencing major drought and heat waves (as noted in my previous blog there is an intimate condition between our coolness and their heat).  It turns out that when one does a careful accounting of the meteorological causes of death, heat waves far exceed tornadoes, hurricanes, lightning, and floods in causing deaths and injury.   Only one other meteorological parameter is in the same league:  roadway icing--which injuries and kills thousands of people per year around the U.S.  Consider a few examples of heat wave losses:  the U.S. heat wave in 1980 killed more than 1250, while the famous Chicago heat wave killed 700.  And of course the 2003 European heat wave brought an extraordinary toll of roughly 50,000.
     Definitions of heat waves vary, but the National Weather Service likes to use the heat index, which combines temperature and humidity.  Humidity reduces our ability evaporate water from our skin--a very potent cooling mechanism (take a look at dog or cat in hot situation--they can't sweat and suffer for it!).  Here is a table for that index:

In essence, it tells you what the air feels like, or in other words how effectively you can get rid of body heat.  Generally, the NWS puts out warnings when the heat index gets to roughly 105F. In the Northwest we rarely get such values--not only because we are generally cool, but because when we get hot, the air is very dry and so the relative humidity is low.  Today over the northeast U.S. some locations had heat indices over 120F!!  For example Warrenton, VA had a heat index of 133F at 3 PM.  That is truly dangerous.   Washington National hit 120F.

Only once in recent years did we get into serious heat index values, and that was the heat wave of July 28-29, 2009 when we got to 103F in Seattle.   We had unusually high humidity values during much of that heat wave because the air reaching western Washington originated over the moist western slopes of the British Columbia Cascades:  a relatively unusual trajectory the low-level flow during our heat waves.

Eastern Washington can get over 100F pretty easily during the summer, but that heat is relatively dry.  And dry heat has another advantage--the air temperatures often cool substantially at night.  Why?
Humidity acts as a blanket.  Water vapor is a potent greenhouse gas the is very active in the infrared part of the spectrum.  It absorbs IR radiation emitted by the surface and re-emits IR radiation back to the ground...thus contributing to warming at the surface.

Here is an example of the large daily (diurnal) range of temperature at Pasco the last few weeks:

Eastern Washington has had a delightful climate this summer, but notice the big changes in temperature..particularly when they get warm.  Highs mid-80s and lows around 50-55F.  I hope it is warm enough for the grapes, which I believe like warm days and cool nights.

Of course, the key reason we don't have many heat waves is the Pacific Ocean, which keeps us cool and dry (see sea surface temperature map below).
While cold water is found west of the Northwest, the eastern U.S. gets air moving northward off the steamy Gulf of Mexico (temps in C, 30C is 86F)

No that last word is not a typo.   The ocean temperatures only rise into the low 50s during the summer at best, and with the East Pacific high offshore, we generally have onshore flow, bringing the cooling effect of the ocean.  Cool air can not pick up much water vapor so air off the ocean is relatively dry (low dewpoints).  To get really warm, we need offshore flow, but that air is generally dry too.  And offshore flow generally doesn't last very long around here.

And there's more!  When the west side heats up the pressure generally falls over the region (warm air is less dense and thus pressure declines).  We call that low pressure the thermal trough (or more accurately, the thermally induced trough).   So we have high pressure and oool air offshore and lower pressure/warm air over us.  Well, air wants to go from high to low pressure...it is just hankering to move in in this situation!  The warmer we get, the lower the pressure, the more than cool air wants to move in.  When large scale offshore flow weakens, the cool air surges in...giving us an onshore or marine push.

The bottom line:  when the Northwest was designed, natural air conditioning and protection against heat waves was a built-in feature. Even under global warming, this natural cooling system should still keep our summers tolerable.  In fact, if the interior of the continent heats up sufficiently, producing lower inland pressure, our local AC might get stronger!

Now it doesn't seem so bad to be in the cool Northwest anymore....


  1. "when the Northwest was designed" aha a closet intelligent-design-ist, eh? ...or perhaps it was designed unintelligently? yess, perhaps some of the bits around federal-way...

    Anyway, gawd-bless our marine cool gray dankness! (oh the dank!) anytime someone gets all "oh woe! when is summer gonna start?!?" i get on my very best sackcloth, point my pear-wood staff eastwards, and say (unto) them to get thee to the midwest! delight in the triple digits (therein), and leave us to suffer in our gray cool Zen calm thankewverrymuch.

  2. The crazy thing about the late July 2009 heat wave was the dew points as cliff pointed out. July 29, 2009 - morning low temp of 71F, high dew point of 66F and high temp of 103F recorded at SeaTac! Ah!!!! Gotta love meteorological extremes.

  3. If the only two choices are cool and gray versus heat and humidity, I'll take the cool and gray every time. But fortunately it sounds like the next few days will look more like our typical summer anyway...

    When I was a kid we used to visit my dad's family every summer - back in southern Indiana / northern Kentucky. Ah, yes, not-so-fond memories of 95 degrees and 95% humidity... oh, and let's not forget the tornado warnings (which I must admit I found kind of cool, being a kid).

  4. As Cliff points out, the Heat Index combines the affects of the temperature and humidity. So typically back East you'll see air temperatures of 95 but a higher heat index of 110. But here in eastern Washington, our hot summer days are usually associated with low humidity. So while we may have a temperature of 100F, our heat index will actually be a few degrees lower, say 95.

  5. "Even under global warming, this natural cooling system should still keep our summers tolerable. In fact, if the interior of the continent heats up sufficiently, producing lower inland pressure, our local AC might get stronger!"

    Unless if by some villainy, the eastern Pacific should *warm!*

    I suppose there's always Iceland if I have to move.

  6. According to Rush Limbaugh, the heat index is bogus; it's "the government telling us how we feel." For some, this is what passes for information now.

  7. If you look at the CPC's (Climate Prediction Center) long range outlook for the next 8-14 days you see something amazing. The forecast, as of Friday, is for the entire lower 48 states to be warmer than normal with the exceptions of the extreme southern tip of Texas and of course the Pacific Northwest.

    This forecast changes a lot from day to day, but I have never seen it with as much above average coverage of the map as this.


  8. Congratulations.

    You are coming into your own.

    Since the kerfuffle with the radio stuff I'm reading your blog every time it's updated and not listening to that radio program so much at all.

    Your voice is getting stronger and clearer and they are getting more confused.

    I truly appreciate your passion and expertise and am gratified to see that you are moving ahead without "them", whoever they are.

    This is yet another great, informative, and adult post. I do appreciate your work.

  9. I just spent 10 days in New Mexico at 100+ every day, with massive forest fires, poor air quality, and too much indoor air conditioning. It didn't take too long to be hankering for the cool, gray NW.

    Continuing to love your blog!

  10. I'm wondering what would happen to this natural air conditioning if the warm water currents changed and invaded the Eastern Pacific waters off our coast?

  11. Baased on conversations I've been in, Cliff, I'd say that what is bothering most those "bothered" is cloud cover and lack of bright light, regardless of how that is measured. There's a blog for you. Address the overall difference in "dimness" this summer as opposed to average summers and summers in other regions.

    Obviously, you can't have it both ways, brigher means more direct sunlight and consquently, usually, more warmth. But I believe you understand my point.


  12. It's honestly really, really hard to think of the PNW as having been designed. Although Redmond is getting that way. ;)

  13. I was born on July 24th and don't remember it being anything but beautiful and sunny in Seattle on that day. Am Imwrong or did God design it so my birthday would always be warm and sunny.

  14. Oh it was definitely designed by a master designer. It is called Gods country after all.

    Thank you for this wonderful blog and the Northwest weather book. I read the book often.

  15. Whatever the reason, I'm loving our comfortably cool summer this year.

    I've lived in WA (Oly) for a little over 10 years, having moved here from Phoenix. I'm delighted to report that it hasn't been 110 degrees for me since leaving Phx (not even the infamous summer of '09; close but no).

    I moved to Phx for school with the plan to move to WA before my mortar board hit the ground. Sadly that whole student loan thingie kicked in and it took me another four years after graduation before I could afford a moving truck. But when I was able to move, I kissed those horrible 110-plus degree summer days good-bye and good riddence!

    PNW weather is wonderful.


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