A good measure of the amount of moisture in the air is dew point or more properly dew point temperature, the temperature to which air must be cooled at constant pressure to get saturation (100% RH). If you have a lot of moisture in the air, you don't have to cool it much to get saturation. Thus, high dew points imply a lot of moisture in the air.
Summer dew points over the eastern half of the U.S. often get into the 60sF and sometimes into the 70s. Dew points in the 60s is relatively rare here in the Northwest.
Here is a plot of the dew points at Seattle Tacoma Airport for the last 12 weeks. Holy humidity! Values are relatively high and were higher yesterday when a lot of locations west of the Cascades had dew points in the 60s. The only thing close was a single day during the heat wave in May. So things have been a bit on the sticky side.
What situations produce high dew points in western Washington? Ironically, it is NOT when air is coming off the Pacific Ocean. Why? Because the ocean is cold (about 50F) and the amount of moisture that air can contain is dependent on temperature. Cold water can't put a lot of moisture into the air. Strange but true.
And the air can't be coming from eastern Washington. The air is dry there, with low dew points.
It turns out that high dew points are generally associated with warm air moving southward after having contact with the vegetated surface of southern British Columbia. And the effect is particularly substantial if it had rained during the past few days...and it has.
Guess where the air was coming from on Monday? From the north. To illustrate, here is a 12-h trajectory ending over Seattle at 500 m at 2 PM on Monday using the NOAA Hysplit software. The air was coming from the north...the high dew point direction.
Global Warming, the Media, and Coal Trains
I will be giving a talk in Friday Harbor and Eastsound, sponsored by the San Juan Island and Orcas Is. libraries.
I will be discussing the serious threat of global warming, how the media is generally doing a poor job in educating about this issue, and how mankind is really not taking it seriously (e.g., the coal trains).
Friday Harbor: July 22nd, 6:30 PM, The Mullis Community Center, 589 Nash St.
Orcas Island: July 23rd, 5:30 PM, Orcas Center