Friday, August 16, 2013

Climatology of Warm Summer Nights

Back in July we had some extended discussion about high daily minimum temperatures in Alaska, so I've been meaning to look at the climatology of "warm nights" in the state.  I use the phrase loosely as the highest daily minima tend to occur when "night" is fleeting or non-existent.  For the purpose of this project, I looked at daily minimum temperatures of 60 °F or above; this is well above the peak summer average minimum temperature for any Alaska location, and so these events are unusual and notable wherever they occur.  In my mind, a 60 °F overnight minimum also marks something of a threshold in terms of comfort level - anything warmer, and summer heat tends to accumulate in buildings.

The first map below shows the average percentage of days in June through August when the daily minimum is at or above 60 °F, based on the 1981-2010 data.  I'm showing only stations with nearly complete data.  Blue "0.0" markers indicate locations where no such events occurred between 1981 and 2010, while green "0.0" markers indicate at least one event but less then 0.1 % of all days.

Some notable aspects of the data include:

- the high frequency at Fairbanks airport, 3.4 %, the highest of any location
- the low frequency at the Eagle COOP station, 0.2 %.
- the relatively high frequency at Kotzebue (1.4 %), which is not only above the Arctic Circle but also adjacent to the cold northern ocean.

Looking more closely at the situation in the Tanana valley, we see that Fairbanks airport reports "warm nights" much more frequently than other Fairbanks locations:

Fairbanks airport   3.4 %
College 5NW (a few hundred feet up from the valley floor)   2.0 %
College Observatory (UAF West Ridge)   1.2 %
University Experiment Station   1.2 %

The differences in frequency mirror the differences in average minimum temperature in summer, with the airport being warmer than the other Fairbanks locations by 1.5-2 °F on average.

The long-term warming trend across Alaska is evident if we compare the above results to the same calculation for 1951-1980, see below.  Nearly all stations with data from both periods show a higher frequency of warm nights in the more recent decades, with notable increases at Fairbanks and Kotzebue.  A major exception is Kodiak, which showed a large decrease; this is interesting because the average minimum temperatures are slightly higher in the more recent period.  It seems Kodiak had an exceptional number of warm summer nights in the 1950's and again in 1979.

It's also interesting to look at the seasonality of warm nights; the map below shows the median date of occurrence, for locations with at least 10 events between 1981 and 2010.  Early July is the most likely time of year in the northern interior, but the median date is pushed back as you move west or south, with more maritime influence and a later seasonal peak in temperature.

The seasonal distribution at Fairbanks airport is shown below; the frequency ramps up quickly in late June, drops suddenly after July 10, and then tapers off gradually into August.  Over 60 percent of the events are between June 21 and July 10.

Finally, the analysis would be incomplete without a comparison of 2013 to previous years in Fairbanks, see below.  The first thing to note is that prior to 1965, 60°-plus nights were almost unheard-of in Fairbanks.  Secondly, although 2013 beats out 2004 for number of warm nights, surprisingly it is still well behind the record of 21 days in 1975.  Nearly all of the 60°-plus nights in 1975 occurred in July, which remains - by some margin - the warmest month on record in Fairbanks.  Remarkably, the monthly average high temperature in July 1975 was only 7th highest on record, but the monthly average low temperature was an extraordinary 59 °F, by far the highest on record.


  1. What wonderful analyses Richard! Things like this should be in the public news for others to share.

    One thing about Fairbanks temps is the sensor being located at the airport. There's lots of dark heat absorbing pavement, buildings, the shallow float pond, plus a lack of shading trees. All surrounded by more highways exposed to the sun. Combined they might contribute to locally delayed cooling during the night compared with other nearby locations.


    1. Thanks Gary - kind words indeed. As you can probably tell I find all of this quite fascinating, so the work is its own reward - but it's gratifying to have a small audience!

      I agree the local human-modified environment at the airport probably contributes to warmer temperatures there; but how much difference does it make? Hard to say.

    2. Your work is good and appreciated.

      I've kept a plane at FAI for 39 years. If it's not windy, recently I can go there in the late evenings and the temp gauge on my car (probably inaccurate but relative) will often show a higher temp than on the roads heading in. I've never quantified the increase, but suspect less than 5 degrees F. The temp sensor in below and in front of the radiator and engine heat sources, and is accurate only if the car is moving.

      There's lots of heat being released by the environs and the distant image distortion from the heated air is visible when looking along the ground. How long it lingers I don't know.

      If I were interested in the actual surface air temp I'd compare the airport's temps to other reporting stations in sheltered locations away from human stuff over a specified period. The problem is what's "actual"? as it'll vary with altitude, location, and what all.

      Perhaps some of the increase in minimum temps during the mid-60's is a function of both a human developed heat island, sensor siting, and measuring equipment.


  2. This reminds me of the -40 degree data for mid-winter – only in reverse. It is the temperature thresholds at the extremes that are most sensitive to changes in climate. Thanks Richard.