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.