Saturday, July 29, 2017

Summer Humidity

As a follow-up to recent posts on summer temperatures, let's take a quick look at summer humidity trends in Fairbanks.  This topic is timely, as Rick drew attention yesterday to a very remarkable statistic: the number of hours this July with a dewpoint of 55°F or greater in Fairbanks is higher than last year, despite the month not being over yet - and last year the number was apparently far higher than any other year in recent decades.  Here's a chart from Iowa State University.

When I first saw this I really thought there had to be a mistake somewhere, but this is what the hourly observations from the airport have recorded.  There does seem to be a chance that the ASOS sensor is malfunctioning to some extent, as the Eielson ASOS data (see below) show similarly high humidity in a number of earlier years; but there's little doubt that this month has been much more humid than normal.

For a longer term look at each of the summer months, the chart below shows monthly mean surface dewpoint and column precipitable water.  The precipitable water is the total moisture in the atmosphere above a given location and is expressed as the depth of liquid that would result if all the moisture were condensed, i.e. the amount of moisture that is theoretically "precipitable".

The long-term upward linear trends in July dewpoint and precipitable water are highly statistically significant, although for precipitable water there has been little increase since about 1980.  June has also seen increases, but less pronounced, and the long-term changes in August have been quite small.

As noted in the previous post, higher humidity provides an obvious (although perhaps not sufficient) explanation for higher daily minimum temperatures during summer in Fairbanks, because water vapor is a powerful "greenhouse" gas.  See this previous post for more discussion on the topic.


  1. It's been muggy and humid all summer at the valley floor. As a result vegetation has prospered and soon Fairbanks will disappear from view behind this curtain of growing trees as viewed from north of town looking south:

    Here's an earlier pic of the same view with lower vegetation:

    We're starting to get early AM fog patches as the temp and dew points coincide. Not sure if that's a recent trend but bound to happen with the recent elevated levels of moisture.


    1. I look at that webcam fairly often, Gary - let's hope it remains a good view!

      I wonder what the forestry folks have to say about boreal growth rates in a warmer and more humid climate. Back in 2013/2014 there was talk of trouble from heat and drought, but overall as noted rainfall has increased in recent years.

    2. There are published studies of growth for some boreal species, primarily Spruce, Picea sp. Optimum temperatures are critical and exceeding their optimum + or - range can be limiting depending on underlying terrain and solar exposure. Search and read at leisure.

      I expect growth that reflects changes in climate would have to be tied to and correlated with diverse boreal species. Non-Picea foliage has certainly visibly prospered the last two years. Spruce are slow growth markers and time will tell for them.

      An earlier hypothesis for previous late Pleistocene to early Holocene boreal changes to the Alaskan Interior's humidity from Guthrie:

      Quaternary Science Reviews 20 (2001) 549-574

      This from a fish guy so take it for what that's worth.


    3. This research facility near Fairbanks (Bonanza Creek LTER) has been mentioned before. Given the recent changes noted in this Blog it may be a good source of future information regarding summer humidity and rates of boreal growth.