Super Dew

Last night, I was struck by how wet some surfaces were...without any rain.

For example, here is an area of a brick patio around 6 PM...quite wet.

The origin of this wet bounty was the very moist air over the region, with dew points rising well into the fifties.   At the same time, the weather had been relatively cool, allowing the ground surfaces to cool below the dew point.

Dew Point 101

The dew point or dew point temperature is the temperature at which water vapor starts to condense as the air cools.

So when the surface temperature declines below the dew point, water vapor condenses onto the surface, producing a wet sheen.

Yesterday, a plume of warm, moist air from the subtropics caused local dew points to climb.

Below are the dew points around noon yesterday.  Many were in the low to mid-50s.

A plot of the dew point at Boeing Field (ending yesterday evening) illustrates that the dewpoint yesterday was the highest of the winter so far,  reaching nearly 55 F.

Ground temperatures were relatively cool after a fairly cool late December, with temperatures at SeaTac shown below (the blue line shows the daily range. 

 In general, temperatures were below 50F, allowing surfaces not in direct sunshine to cool to that or cooler temperatures.  That means dew.

Soil temperatures in Seattle near the University of Washington are shown below.  Generally below 50F.

So, with a cool surface and moist air, abundant condensation occurred, something that was apparent if one looked around.

Finally, some folks have expressed concerns about air quality under high pressure.   

At this point, there are no issues, with the latest vertical sounding over SeaTac not indicating any low-level inversion (with temperature increasing with height).  Inversions trap low-level pollutants.

And air quality measures are generally quite favorable (low), as indicated by the green dots below.  You can breathe easily.