Thursday, September 4, 2014

Early Hard Freeze

The temperature dropped to 28 °F this morning at the Fairbanks airport, marking the first freeze of the season.  As I mentioned last week, the first freeze at the airport typically waits until the 850 mb temperature is a few degrees below freezing, especially when it's early in the autumn.  Last week the 850 mb temperature dropped to -2.7 °C, but it was still August and no freeze occurred; but yesterday the 850 mb temperature was measured at -4.7 °C above Fairbanks.

Today's frost was one week earlier than normal and the earliest at the airport since 2005; but 28 °F or lower is not typically observed until September 23 (1981-2010 median), so in that respect we're nearly three weeks ahead of schedule.

In more rural areas of the interior much cooler temperatures were observed this morning, including:

Norutak Lake RAWS in Kobuk National Preserve  14 F
Circle Hot Springs  15 F
Chicken  17 F
Eagle airport  18 F
Goldstream Creek 19 F
Bettles  21 F
Tanana  22 F


  1. Local temps can be driven by a lack of cloud cover and brief pre-dawn AM winds this time of year. The reported low at 1453Z of 28 was accompanied by high scattered to few clouds and a slight NE wind. The light wind can move colder air over the airport from low lying spots nearby. The Sun soon warmed things up nicely.

    Those morning low level winds can be both seen (via bent furnace and vehicle exhaust) and felt while out walking with the dog.


    1. Oops, bad calculation Gary. Here are the actual sky conditions at PAFA so far in 2014. The "CLR" condition seems pretty underrepresented in my opinion.

      OVC: 21%
      BKN: 46%
      SCT: 13%
      FEW: 13%
      CLR: 8%
      VV: 0%

  2. Gary, I wouldn't put too much faith in the PAFA ASOS cloud ceiling data. Here is the percentage of sky conditions so far in 2014. As you can see, it has been overcast only 1% of the time!

    BKN: 16%
    CLR: 8%
    FEW: 49%
    OVC: 1%
    SCT: 26%

    1. Good point Brian. It's unfortunate that in these days of aviation safety awareness that we may no longer be able to depend upon reliable observations of cloud ceiling data. Maybe a discussion on what drives the sensitivity of the ASOS' reports is in order?

      However, I was out a little after 8 this morning, and as is my habit, did look up for circling predators. It was pretty clear overhead and there was a slight NE breeze with at local temp around 26F.

      Radiation cools as does sub-freezing 850 mb temps and declining photoperiod.