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.