Friday, January 16, 2015

Gulkana and Fairbanks Temperatures

Reader Gary inquired last week about temperatures in Glennallen and their relationship to Fairbanks temperatures.  The two locations are separated by more than 200 miles and the Alaska Range, but the climates are quite similar; both locations are strongly continental, with very large temperature swings from summer to winter.  In the case of Glennallen this is possible because of the Chugach Mountains to the south, which block Pacific moisture despite the ocean being less than 100 miles away.  In fact Glennallen is surrounded on all sides by high terrain, being located in the Copper River Basin, and this allows relatively clear, calm and cold conditions to prevail rather frequently in winter.

The chart below shows the 1981-2010 daily normal temperatures for the Gulkana airport, which is just a few miles from Glennallen, and for the Fairbanks airport.  Fairbanks sees a somewhat larger swing from summer to winter, but overall the large seasonal variations are similar.  Peak summer temperatures occur a few weeks later in Gulkana, which reflects a slightly greater maritime influence in the more southerly location.

We can get a sense of the variability in temperatures by looking at histograms of daily maximum and minimum temperature for summer and winter, see below.  Starting with winter, the charts show that the Fairbanks temperature distribution is shifted towards the cold side compared to Gulkana, but variability is similar.  Above-freezing temperatures are about twice as common in Gulkana in December through February (10.8% of days vs 5.7%) for the overlapping period of record.  Conversely, temperatures of -40° or below are nearly twice as common in Fairbanks in December through February (11.8% of days vs 6.8%).

In high summer (June and July), the differences between the two locations are more significant compared to the range of temperatures observed at that time of year.  As shown in the charts below, Gulkana is quite a bit cooler, especially for the daily minimum temperatures.  Minimum temperatures are generally in the 40s in Gulkana, but are more often above 50 °F than below 50 °F in Fairbanks.  In Gulkana, daily high temperatures most often fall in the range 60-75 °F, but Fairbanks is typically in the range 65-80 °F.

The cooler temperatures overall in Gulkana can be explained partly by the higher elevation (1560' vs 430' MSL).  Cooler nights in particular are the result of lower humidity: Gulkana's average dewpoint in June and July is 41 °F, compared to 47 °F in Fairbanks.  Accordingly, Gulkana is about 10 percent drier in terms of June-July precipitation.  Longer nights may also play a role in nighttime coolness: the sun is below the horizon for at least 4 hours every night in summer in Gulkana.

Lastly, I'll touch on the correlation of temperature variations in the two locations.  The list below shows the correlation coefficients between the two locations for daily temperature departures from normal, based on the 1981-2010 normals and calculated over the common period of record (1943-2014).  Correlations are highest in winter and spring, and are relatively low in late summer when it appears that local variations in cloudiness have a large effect compared to the underlying temperature variance.

Jan  +0.77
Feb  +0.76
Mar  +0.74
Apr  +0.74
May  +0.71
Jun  +0.66
Jul  +0.53
Aug  +0.62
Sep  +0.65
Oct  +0.68
Nov  +0.76
Dec  +0.76


  1. The few times I've been through the area it's been somewhat windy. Is it more windy there? And wouldn't that explain the more moderate temps in winter?

    1. Eric, yes - it is more windy than Fairbanks. Since 2000, the mean wind speed in Dec-Feb (from all hourly observations) is 3.4 mph in Gulkana and 2.1 mph in Fairbanks. Perhaps more usefully, sustained wind speeds of 10 mph or higher are observed 10.5% of the time in Dec-Feb in Gulkana, but only 3.5% of the time in Fairbanks. This would certainly make a difference for temperatures.

  2. Thank you Richard for fulfilling a promise I made to answer a co-worker's question in 1981. I had flown with him to Tolsona Lake Lodge (Chet and Bessie were owners), and we discussed the temps of that locale (and Gulkana) versus Fairbanks in winter.

    He had lived there in the early '60's and claimed it was (or could be) as cold as Fairbanks. I disagreed, but without reference.

    Now I see it's possible and likely a cold spot similar to the McGrath and Northway areas both at lower latitudes than Fairbanks.

    Location, elevation, drainage advection and winds, and cloud cover all play a role in determining temperature.