Monday, July 11, 2016

High Humidity

It has been humid recently in Fairbanks - much more so than usual.  In the first 10 days of the month, the average dewpoint measured at the airport was 55.2°F, which is the highest since at least 1950 for this period of the year.  The dewpoint exceeded 60°F on each of the first 4 days of the month, and it appears Fairbanks has only seen 4 such consecutive days on a handful of occasions in the past (1962, 1990, 1994, 2004), and never earlier than the end of July.  Peak humidity in Fairbanks (in terms of dewpoint) is normally at the end of July.

The humid conditions at the surface are consistent with the presence of a very moist air mass aloft too; the Fairbanks soundings show that the July 1-10 dewpoint was the 3rd highest on record at both 850mb and 700mb.  I think the depth of the moist anomaly suggests that the moisture has been imported to interior Alaska, and the high humidity doesn't just reflect (for example) local evaporation from moist ground.

Is there a long-term trend towards higher humidity in Fairbanks in summer?  Yes, but only slightly.  The charts below show monthly mean temperature and dewpoint since 1950 for June, July, and August.

The 1950-2015 rates of change according to linear trend lines are as follows (although the changes are not linear):


In each month the temperature has risen more than the dewpoint, which means that the relative humidity has fallen as the climate has warmed.  To maintain the same evapotranspiration rates from vegetation, the dewpoint would have to increase faster than the temperature, so despite the slowly rising dewpoint, moisture demands have become greater for vegetation (at least at the Fairbanks airport site).


  1. We're forecast in Fairbanks for warmth this might be interesting to follow the forecast heights and other factors and the resulting factors above as they respond?


    1. This is what I fumbled to say...from the local NWS forecast discussion:

      .FIRE WEATHER...
      As the upper level ridge builds in for the remainder of the is likely that this week could be an active week as RH
      values will plummet with the strong subsidence. RH values will
      fall to some of the lowest values that have been seen in weeks
      across the Interior. Thunderstorm potential will be confined
      mostly to the Eastern Interior...east of Delta Junction today then
      spreading from the Alcan Border to Tanana to McGrath on Wednesday.


    2. It certainly looks warm, Gary. I think I'll put up a brief post on it.