Sunday, June 22, 2014

Evaporation Calculations

As a follow-up to my recent post about potential evaporation rates in Fairbanks last summer, I decided to go one step further and include the effect of wind speed and solar radiation.  Higher wind speeds and higher solar radiation both increase the rate of evaporation from land, water, and vegetative surfaces, and so it's of interest to see whether these factors also contributed to high moisture loss last summer.  Hourly radiation, wind, temperature, and humidity data are recorded (among other variables) at the Fairbanks Climate Reference Network (CRN) site, which is located at 1140' elevation about 10 miles northeast of Fairbanks.  Based on this data, and with a requirement for no missing hourly reports in each 24-hour average, I calculated the theoretical daily evaporation rate from an evaporation pan (formula supplied on page 11-12 of this document - thanks Brian).

The chart below shows the monthly mean calculated evaporation rate during the growing season months since 2011.  Unfortunately the humidity measurements did not come online at the Fairbanks CRN until August 2010, so it was not possible to include a larger number of years.  Nevertheless, the results confirm the very high evaporation rate in June 2013, and interestingly the evaporation was higher in each month from May through August 2013 than in the same months of the previous two years.  The theoretical evaporation from June 2013 amounted to well over 8 inches for the month, which is not much less than the precipitation total for the entire year in Fairbanks.

Looking at the solar radiation and wind speed in isolation (see below), the solar radiation was higher last year in May and June than in the same months of any of the previous 10 years.  The wind speed was also a little higher than normal in June and July.  (The reported wind speed looks much too low for May and June of 2011, but I double-checked the data and this is what the supposedly high-quality CRN platform reported.  There is no evidence of lower wind speeds in these months at Fairbanks airport.)

In conclusion, the CRN data confirm that Fairbanks-area land-surface moisture loss in summer 2013 was caused not only by dry air but also by abnormally sunny conditions.  Of course we would expect these two anomalies to go together; both were quite extreme last summer, leading to highly elevated moisture removal rates.  Fortunately, fire activity was not catastrophic as in 2004 - probably because of lack of lightning.  Here's what the state 2013 fire report had to say: "2013 will be remembered as one of the shortest, but hottest summers on record... Fuel conditions reached near record dryness at some locations... Despite many record-setting hot temperatures, the stable high pressure kept thunderstorms at bay, and lightning to a minimum... With many comparisons between the hot, dry summers of 2004 and 2013, it seems that the lack of lightning, and therefore the lack of natural fire starts, is what kept the 2013 fire season from becoming catastrophic."


  1. Enlightening analysis Richard. Thanks. One point: the wind sensor on CRN sites is located at the same height above ground as the lip of the precip gauge, generally close to 1.5 meters AGL, so not directly comparable to 10 meter wind speeds from ASOS.

    1. Thanks, Rick. I noted the 1.5m height and made the correction to 0.6m which apparently is the standard for the pan evaporation calculation.