Air with lots of water vapor makes us miserable in two ways: first, we can't evaporate water off our skin (sweat) as effectively...and thus feel warmer. Second, water vapor acts as an atmospheric blanket--keeping temperatures up at night.
Many of you have heard of relative humidity--with air have 100% relative humidity when completely saturated. But relative humidity varies with temperature and drops as the day warms. It is NOT an absolute measure of the amount of water vapor and thus meteorologists prefer to use another measure, dew point. (air with 100% RH at 45F has far less water than air with 100% RH at 85F)
Dew point is the temperature that air must be cooled to to become saturated (100% RH). The more water vapor in the air, the less you have to cool it down to get saturated (remember, warm air holds more water vapor than cold air). So dew point is actually a measure of the amount of water vapor in the air.
Still with me? In Seattle, the typical summer dew point is in the lower 50s. In DC it is closer to 70F (a very good reason to avoid living in DC and to appreciate our congressional/senatorial delegation that has to live there).
Take a look at the plot of dew point at Sea Tac for the last two weeks above. The dew point starts in the 50s and rises well into the 60s in the day leading up to the heat wave...that is why it felt so terrible. (some local stations had dew point rising near 70F) Then on the day of the heat, as the dry, easterly flow developed, the dew point dropped . During the past few days the dew point dropped in to the fifties again...and boy does it feel better. Today was spectacular and comfortable...with dew points dropping into the upper 40s in some locations.
And why are our dew points generally low and our summer air much drier than that of the East Coast? The Pacific Ocean. How can that be? Generally our air has traveled thousands of miles of the Pacific...but because the Pacific and the surface air is relatively cold...the air can't pick up much moisture. In contrast, those poor people of the East and Central U.S. have air coming off the Gulf of Mexico...which is well above 80F in the summer. If you want to read more about dew point, there is plenty more about it in my book.
So that's the unsung issue that made last week a terror...unusually high dew points.
PS: There was a question of why it was so humid. The air trajectory was not off the ocean (air would be too dry because the water is cold) and not from eastern WA (too dry because of the surface). Rather, it came from the north--passing over vegetated areas of southern BC....cliff
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