Fog and Freezing Fog in the Inland Empire: Intense Inversion Aloft

The Columbia River basin is well known for sun and warmth for much of the year, compared to the sodden, cloudy western side of Washington State.  

During the cool season from mid-November through mid-February, the reality is often different, with cool, cloudy air trapped within the basin, often producing fog and FREEZING fog.

The last few days are great examples of this chilly, fog-laden situation.

Consider the visible satellite image around noon today (Friday). Clear over most of western Washington and Oregon, with the Columbia Basin full of stratus and fog.

Checking on the Washington Department of Transportation webcams, on the eastern side of the Cascades, Kittitas was in fog:

And Yakima was not better. Much of this fog was of the freezing variety, a great danger for ground transportation.

Not surprisingly, my colleagues at the National Weather Service had dense fog and freezing fog advisories out last night (see below).

To appreciate the cold and unpleasant nature of the old air trapped within the Columbia Basin, below is a plot of the maximum temperatures today around the state.

Yikes.  Much of the Columbia Basin never got out of the mid-30s.    Some locations never rose above freezing.  In contrast, lots of 50s in Western Washington and some very warm conditions in the mountains (Cascades and Olympics). 66F in the Olympic.

Why so warm in the mountains? 

Because a strong inversion (temperature increasing with height) was created by the high-pressure system aloft (see the upper level map for 4 PM Thursday below).

High pressure aloft causes sinking and warming aloft, with descending air warmed by compression (like when you pump up a tire!).  The sinking decreases as one approaches the surface (air can't move downward through the surface).  Thus, temperatures remain cool at low levels, which is further chilled by the emission of infrared radiation to space.

There is a world-class inversion above us today.    To show this, below are temperatures above SeaTac Airport at 8 PM, measured by aircraft landing and taking off.

Unbelievable!  Low 40s near the surface and nearly 59F at 1400 ft.    Take a hike tomorrow morning.  You will not believe how temperatures warm with height.

Consider the situation on the slopes of Tiger Mountain in Issaquah.  At the Tiger Mountain Family Nudist Park, the temperature was 40F, but up on the slopes below Poo Poo Point (about 1500 ft), it was 57F!   

My pro-tip for the nudists is to take a hike up the hill before removing their clothing!

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.