A Strong Morning Inversion Undermines Air Quality and Messes Up the Coastal Radar

This morning, a strong low-level temperature inversion caused the air quality to decline over western Washington and created false radar echoes on local weather radar.

Reminder:  an inversion occurs when temperature increases with height, a reversal (inversion) of the normal situation in which temperature decreases with height. 

Let me show what lower atmospheric temperatures looked like this morning.

At 5 AM at Forks, on the northwest Washington coast, temperatures increased by about 10°C (roughly 18°F) in the lower few thousand feet (red line is temperature).  Above the inversion, the temperature declined with height

An inversion was also present at Seattle-Tacoma Airport at 6 AM this morning, with about a 6°F warming between 400 ft and 1800 ft (see below)

Inversions create zones of strong vertical stability, inhibiting the vertical mixing of pollutants, allowing concentrations to increase near the surface.

This was quite apparent this morning as air quality declined to the moderate level (yellow colors) in the graphic from the  EPA AirNow website.

Low-level haze was apparent on the Seattle PanoCam this morning around sunrise.

Strong-level inversions can also mess up local weather radar, acting as an atmospheric lens that bends the radar beam down.  As a result, the radar beam reflects off the surface and is reflected back as a false echo, indicating precipitation where none is occurring.

Such a false echo was clearly apparent on our local coastal radar (Langley Hill, near Hoquium) at 7:21 AM.  It was not raining offshore!

Over land, spring temperature inversions weaken rapidly during the day as the land surface warms,  as illustrated by the temperatures over SeaTac at 4 PM (below).  No more inversion as temperatures at the surface warmed into the lower 70s!

So, take a deep breath...air quality is quite good now over western Washington...and I am going to go for a few-mile run knowing that bad air quality won't be a problem.

Fog In the Wrong Season

This morning, fog was widespread over the lowlands of western Washington, with many of the river valleys in the murk (see visible image below around 7 AM)

Several local weather cams showed the fog.  Olympia had almost no visibility.

And it wasn't much better on the Chehalis River Bridge.

All this foggy action is a bit unusual in late April, which is close to the time of minimum frequency of dense fog for most locations in our region. (see below)

Why is dense fog unusual in April and May?

Because days are much longer and the sun is getting quite strong.  Thus, the surface is getting warmer. But the air is still relatively cold aloft.  

That leads to a rapid decrease in temperature with height, which results in instability and vertical mixing.

Vertical mixing is the enemy of fog, which generally forms when cold, dense air is near the surface, and warmer air is aloft.

Exactly, the situation that was occurring this morning (see the temperature above SeaTac Airport this morning below).

So why the fog this morning?  

A strong high-pressure area aloft developed to our northwest (highs of the 500 hPa pressure surface are shown below...think of pressure around 18,000 ft).  Red indicates the high pressure.

Such an offshore high-pressure aloft causes sinking, which warms the middle atmosphere.  Furthermore, high pressure causes clear skies aloft, which allows the lower layers to cool by emitting infrared radiation.

Warming above and cooling below is perfect for fog formation and the development of a low-level inversion.