Yesterday on my way home from UW, I stopped by the local supermarket to buy some food and some cold beer.
But I could hardly see in the refrigerators: they were completely covered by a dense layer of condensate.
I mean a lot of water (see below).
Why is there so much water vapor in the air?
Typically, the initial phases of major heatwaves are associated with runoff over the relatively dry Falls.
Then I got a few emails from people in northwest Washington complaining about the humidity.
OK, I have to figure out what’s going on.
The best measure of the absolute amount of water vapor in the air…. “stickiness” and discomfort.. is Dewpoint temperature.
This is the temperature at which you must cool the air down to saturation (100% relative humidity). For dry air, you have to cool it a lot to make it saturated.
So, dry air has a low dew point temperature. And humid air has a high dew point temperature. Here in western Washington, summer dew point temperatures are generally well below the 50s. Even lower dew points are typical in arid eastern Washington.
To find out, I plotted today’s dew point around the area and was shocked (see below)
For example, the dew point at around 11am this morning is 70F in Bellingham and around Vancouver. And to the 60s around Puget Sound.
These are very high dew points for our region. The only time they were higher in recent memory was during the great heatwave last June. Very warm temperatures can evaporate a lot of water and cause high dew points, but our temperatures today, although warm, are not even close to last summer.
The highest dew point is in the relatively cool Northwest WA/Vancouver area.
Exciting, high-resolution numerical weather models, such as the UW-WRF model, predicted high dew points over NW Washington (see below at 11 a.m. Tuesday, light green for know the higher dew point.)
So what is the origin of this moisture storm?
Firstly, this heat wave does not have strong currents from the east like many heat waves. Such runoff would dry out, with an origin east of the Cascades and lower moisture from the associated air-submerged warmth on the western slopes of that mountain range.
But then I looked at the atmospheric moisture escaping into the Pacific: a cluster of very high humidity values orbiting the high pressure offshore. This is represented by humidity and pressure/altitude close to 10,000 ft on Sunday as shown below (red and gray indicate high humidity values). You can see moisture moving around the highlands in the north.
This humidity rotates around the highlands and moves into Northwest Washington (see 10,000 ft humidity on Tuesday morning, below)
The air that reaches the surface near Bellinghan and Seattle originates northwest of those cities and exits the moist patch to the surface, warming as it descends. We can see that from the 3-D airway to Seattle this morning (see below). The top panel shows the spatial path and the bottom panel shows the altitude of the air (then on the left). The weather quickly descends in two days, July 24-25.
In short, a quantity of moisture from deep over the Pacific Ocean revolves around the highlands and then descends to the surface. And for the last few hours, it was on the wet west side of our area.
Well, after exploring this situation, I decided that I deserved some ice cream to help me cool off a bit.
I opened my freezer….and I was stunned by what I saw….something involving moist air being cooled to below freezing. Life has its disappointments….
