Saturday, September 15, 2012

Smoke From Wildfires Cools Wenatchee

Can smoke from fires cause cooling? 

Yes,  it can if the smoke is dense enough...and I think this has happened in Wenatchee yesterday (and thanks to UW's Mark Albright for pointing this out)

A major lightning-caused wildfire is burning west of Wenatchee, leaving a pall of smoke over town.  Here is a recent map of the burned areas (currently around 30,000 acres)

and take a look at a cam in Wenatchee.   Pretty hazy.

You see the smoke clearly in the visible satellite picture:

At 2 PM the temperature at Wenatchee (abbreviation KEAT) was 71.  Lets check the temps in its neighborhood using a surface weather map(temps are in red to the upper right of the circles).  Most stations have temps that are much warmer, including all the stations at similar or lower elevation.

But I have a better way to prove the cooling effect....the same way we proved the cooling effect of the 1981 Mt. Saint Helens ash plume--look at the difference between the  forecast and actual temperature.

The forecast from the National Weather Service MOS forecasting system (which is very good, but doesn't consider local fires) is shown below, with the actual temps as well, for three times: 11 AM, 2 PM, and 5 PM).

                   Predicted    Observed     Difference (OBS-PRED)
11AM              67             62               -5
2 PM               78             71               -7
3 PM               83             77               -6

Much cooler than predicted (5-7F!)

This is a huge error, particularly consider the boring and easily forecast situation we are in now.

What about Yakima, which had only a bit of thin smoke?

              Predicted    Observed     Difference (OBS-PRED)
11AM              69             68               -1

2 PM               80             81              +1
3 PM               83             84              +1

Almost a perfect forecast!   No cooling there.
So something is causing the big cool error in Wenatchee, and I am pretty sure the smoke is too blame.

Why does dense smoke cause cooling during the day?

 By preventing some of the solar radiation from reaching the surface.  Peak solar radiation  in Seattle reached 684 watts per meter while in Wenatchee the peak was only 391.

Interestingly, smoke or dense dust can do the opposite at night--acting as a blanket that prevents the infrared radiation from the surface from radiating to space. In fact, at 1 AM Saturday morning, Wenatchee was 67F, while surrounding stations were MUCH cooler, many in the 50s  (see map below):

Wenatchee Fire

1 comment:

wallawallavigneron said...

It looks to be the same in Walla Walla today, 20 Sep. Posted 2 fotos @ G+. Charles Vigneron

Thank you for explaining the Walla Walla Valley warming in winter weather. Enjoy the blog.

I was at the Seattle Expo 12 Oct 1962. Evacuated from Science Bldg. Shattered black plexiglass flew through the air; elevators stuck on Needle—wind would whip the cables and the car would rise—then drop, repeatedly. I was 8 and grokked real adult fear/ dangerous world. Driving to Bellingham soon after, dad said it reminded him of Battle of Layte.