Monday, December 2, 2013

Extreme Inversion

[Update 5 pm AKST: this afternoon's sounding from Fairbanks showed an inversion of 31.7 °C or 57 °F as warming continued aloft.  This has only been matched on 7 previous occasions since 1948.]

This morning's balloon sounding from Fairbanks airport revealed a temperature difference of more than 30 °C (55 °F) between the surface temperature and the temperature at 5300 feet above ground level.  As shown in the chart below, deep warming has occurred in the past few days, and at 3 am this morning the temperature was only just below freezing at around 4000-5500 feet.  Computer models suggest that the temperature is probably now above freezing at the same levels, despite valley surface temperatures below -20 °F.

An inversion of 55 °F or greater is quite unusual in Fairbanks; the last time it was observed was in January 2005.  In the history of upper-air observations since 1948, the strongest inversion was 60 °F on December 4, 1956.  The chart below shows the maximum inversion strength by winter since 1948-49, with the red line indicating the 10-year trailing average.

An even stronger inversion was observed in McGrath this morning (31.9 °C or 57 °F), but this is slightly less unusual there, as very warm air aloft can reach McGrath more easily.  The strongest inversion observed in McGrath was 40.0 °C (72 °F) on January 13, 1966.


  1. Very nice Richard. Are you using constant pressure levels to define the inversion parameters; e.g., surface and 500mb? Also, we've had a decent inversion here in Anchorage too. Any idea where it stacks up?

    1. Sorry, I meant 850 mb, not 500 mb.

    2. Brian,

      I'm looking at the maximum temperature inversion between the surface and any height aloft. Of course the warmest air aloft may not be reported by the rawinsonde, but nevertheless I was most interested in the maximum observed inversion strength rather than specific levels.

      The inversion peaked at PANC yesterday morning at 17.9 C difference between the surface and 907 mb. This is impressive, but the record is 25.9 C (Dec 23, 1968), and 20+ C inversions have been observed before in every month from November through February.

      Of course it's possible that changes in the Anchorage sounding location over the years might have made large differences in the inversion strength climatology.

    3. Thank you for digging up the ANC information Richard.