Monday, August 3, 2009

The Other Side of the Heat Wave: Humidity

There was another side of the heat wave of the past week--the high humidity. It really felt like the torrid east coast the day before the big record.

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

21 comments:

bob said...

Humidity? Oh, yes! In the midst of the heat, we were having overnight lows in the mid-high 60s and I noticed the dew water dripping off of my Olympia roof every morning. That was telling me that dew points were about 20 degrees warmer than the sea water about one half mile away from my home. That's a difference almost never seen!

Craig said...

Prof. Mass—
I'm interested and a bit surprised to see you using language that implies that the air is “holding” the water vapor. After spending quite a bit of time wrapping my head around the explanations at Bad Meteorology and other sites, I'm beginning to understand that the amount of water that will be in a gaseous state depends only on temperature and is therefore independent of the composition (or even presence) of the rest of the atmosphere. Do you not feel that this is an important distinction? If not, why not?

Cliff Mass Weather Blog said...

Craig,
You are absolutely right. That is sloppy terminology, but one I am using for this public venue. Air doesn't hold anything and the amount of water vapor depends on temperatures, not the composition of the atmosphere. But this is a colloquial term that is commonly used and doesn't do any harm...cliff

Cliff Mass Weather Blog said...
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Josh said...
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Joseph Ratliff said...

Cliff, thank you for the dew point lesson...that clarified a lot for me. I appreciate it.

And, I appreciate the "sloppy" terminology...it helps to make it even clearer for us "non-science" types, LOL :)

Lance said...

Thanks for pointing out the humidity during the past heat wave. One question though, why was it humid last week? Were we getting some of the monsoonal moisture that usually stays in the desert SW during the summer months?

nina said...

This is something I quickly learned living in Georgia for two years and coming back. The RH on a regular basis was typically the same value in Seattle, but oh my does that feel different.

I've been studying this stuff up at WWU, but I too appreciate the less scientific wording... Now maybe I can explain it to my mom better, without equations this time!

strix27 said...

Cliff alluded to the heat and humidity on the east coast. I grew up in New York City pre AC. July and August were frequently three shower days. I the evening, after showering I'd towel off the shower water, but then I'd have to stop because I was toweling off sweat.

VanLeer said...

Lance raises an excellent question. So it was unusually humid, but why? The flow was offshore, and the air from eastern WA surely wasn't humid. So here's a hypothesis: 1) the mountains themselves were quite warm, which means the abundant moisture from the winter and spring (our reservoirs are quite high) locked in snowpack and soil moisture was being evaporated, making the mountain air unusually humid, and thus not delivering super-dry air common when air descends orographically; 2) it was so hot in the lowlands, evaporation from our local water bodies added much water to the atmosphere; and 3) most importantly, the air aloft was super-warm. That reduced the lifting of our surface air, even at night, effectively capping the humid air near the surface.

JewelyaZ said...

VanLeer, everything you propose as a cause for the humidity during the heat wave sounds plausible. It reminded me very much of New Jersey/Philadelphia/North Carolina temps/humidity, and since I grew up there I'm used to it. Eleven years here has spoiled me, but when push comes to shove and there is no a/c, I know how to cope. My Snoqualmie-born husband, OTOH, wanted to die. Poor baby. LOL

If I had to pick one source of the humidity from the three you propose, I'd guess the hot dry air flowing rapidly over the very moist mountains is where most of the moisture got "picked up" -- because the mixing was happening there. Our lakes and the sound are really cold and the surface/air mixing is not very aggressive. I don't think the inversion (your number 3) CAUSED the humidity, but I think it held on to it very nicely instead of letting us cool off as we normally do.

And huzzah, it's a mere 58.2F here in East Bellevue right now. Lovely!

Weather Is My Life said...

It's actually not a good thing to use "sloppy" terminology, even for the public, because then they learn the wrong thing.

Weather Is My Life said...

If given the choice, I'd surely take the humidity of the East with a/c than what we had yesterday! The morning was comfortable, but once the clouds went away, what should have been comfortable turned uncomfortable when the sun went to work burning your skin.

FYI, the humidity helps lead to all the great thunderstorms the East has! :)

natchrl8r said...

The humidity may have been unaccustomed to Northwesterners but never reached the levels of discomfort I remember from a childhood on the East Coast. I slept quite well under the covers throughout the heat wave. The big difference is nighttime temperatures are so much cooler here. I despise AC unless I have my finger on the button and I'm stuck in an unventilated room or vehicle. I resent being shoved into a refrigerator when we finally have some seasonable weather. Grocery stores are notorious for refrigerating their customers when closed cases would do.

The cooler weather is a relief but I am no longer inspired to jump in the pool. Its time to work up a sweat on the hiking trails! : )

Bham_Guy said...

The upper-level low that was parked over the state for a few days prior to the heat wave was probably the main driver of the higher dew points. The blocking high over the region kept the moisture in place, even after the low was finally ejected.

Cliff Mass Weather Blog said...

All,
The humidity probably was caused by air moving over the vegetated, moist forests to our north. Trajectories indicated that is where the air came from. Also, the Strait of Georgia is much warmer than the sound and could have contributed...cliff

Big White Ball said...

That's very interesting about the forests, Prof. Mass. Is this phenomenon discussed in your book?

Fluffyblue said...

I believe Cliff is correct about the origin of the humidity. Here in southwestern BC, we had some pretty good thunderstorms and quite a bit of rain the day before the heatwave began. The following morning we actually had dense fog, with a temperature of nearly 70 F and humidity close to 100%. During the heat wave we had even higher dewpoints than Sea-Tac (low 70's in Abbotsford, for instance).

Josh said...

Well that low is ejecting out of northern cal. They are going to get hammered with lightning. not a good situation...

yellojkt said...

I am always preaching that people need to pay attention to dew point instead of relative humidity. Thanks for the very lucid primer.

garyLambda said...

The weather being a "terror?" Seriously you jest. Having grown up in the East Coast area, this area has very mild weather, including last weeks heat wave.