Wednesday, March 11, 2015

Inversion Season Dates

A late-season cold spell has descended upon Alaska, bringing -30 °F temperatures to Fairbanks airport on the past two mornings.  Temperatures this low are a bit unusual for this late in the season, occurring in fewer than 3 out of 10 years after March 10.  Rapidly strengthening sunshine is making a big difference, however, as daytime temperatures would be much lower with an air mass this cold in the depths of winter.

With solar insolation rising quickly, Fairbanks will soon reach the date at which mean daily surface temperatures rise above mean daily temperatures at 850 mb.  According to the 1981-2010 normals, that date is March 19.  Based on the history of radiosonde data from several stations around the state, we can look at how the surface-850 mb temperature difference varies throughout the year; the charts below show the results.  Note that I would prefer to look at the 925 mb level as a measure of low-level inversion characteristics, but the 925 mb was not regularly reported in radiosonde data until 1992.

It's not surprising to observe that more northerly locations in Alaska generally spend a greater fraction of the year with surface temperatures lower than 850 mb temperatures, i.e. with an inversion in place on average; Anchorage sees this condition only during a mid-winter period of about 6 weeks in length.  An interesting feature of the charts is that the surface-850 mb temperature difference is nearly constant during most of the warm season at each of the locations outside the Arctic, so the low-level vertical stability of the atmosphere is little changed for a period of several months during summer.

It's also interesting to note that the Fairbanks and McGrath curves follow each other very closely indeed, despite a fairly large distance separating the two locations; this illustrates the relatively uniform nature of some aspects of the climate in interior Alaska, as opposed to the strong spatial variation near and along the coasts.


  1. Slightly off topic. During the last few days with the cold spell, snow has been condensing out of the air - maybe a good 1/4" total. This despite the lack of clouds. Since it hasn't gotten cold enough for ice fog or a hoard frost, I assume that moisture in the air is becoming snow directly due to the cold temperatures. What are the possible conditions that would create this snow in relatively clear skies?

    1. No positive answer here Eric but a good question that deserves one.

      I've seen this before typically in March. Sunny days, maybe some upper clouds here and there, and ice crystals/snow floating about in localized patches. We'd see it flying on the North Slope in Spring so it's not limited to Interior Alaska.

      I assume there's some intrusion of moisture aloft at some level that creates the precip, probably due to colder air below the moisture laden layer. NWS sometimes calls it a disturbance aloft for whatever reason.

      It's like having multiple layers of clouds or ice crystals/snow with altitude except one extends to the ground. There was some SW flow today and over the Alaska Range with visible Sundogs midday indicative of ice crystals.


    2. Eric, interesting question. I noticed the light snow reported from the ASOS in the past couple of days, combined with relatively clear skies as evident on the webcams. Yesterday the official snow total was 0.2" with 0.01" of liquid equivalent. I noticed a few other reports of light snow in the eastern Interior, but far less persistent than at Fairbanks it seems.

      This morning's sounding shows sub-saturated conditions, yet snow is still reported (by the way, the 850 mb temperature of -30C is the coldest of the winter!). I'll have to think about this and maybe ask around for possible explanations. I think Gary's idea about contrasting layers of air is probably correct - if moisture levels and temperature differ between layers (or near the ground), then mixing could create condensation; but I'm not sure of the physics that would create large crystals without cloud.

    3. These low layers of ice crystals below relatively clear skies can often be seen via FAA's Ester Dome and Nenana webcams ( and maybe others this time of year during daylight periods.

      Same for the popular Borough cam that looks south over Fairbanks:


  2. I have a request please for analysis that I can't provide. By winter date, when is the hourly minimum temperature reported for Fairbanks? It can be in GMT format or local time to standardize the information.

    The reason I ask is I have seen sunrise AM wind flow (like now with March morning insolation) driving the daily minimum cold layers over town via obs of plumes of heating exhaust...and I have seen it happen at peak darkness in mid winter.