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:
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).
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| 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....
