The ‘why’ of Anchorage weather

A July 18 letter to the editor titled, “Wrong on records,” contained information on the recent record temperatures and climate location for Anchorage that included several misrepresentations. The author was rightly concerned that temperature measurements in Anchorage — and everywhere, for that matter — are made in such a way that comparisons across space and time are an apples-to-apples assessment.

Climate data in Anchorage has been collected since Feb. 1, 1916. Between 1916 and 1922, the official station was located along Ship Creek. Observers noted this station location was not representative of the rest of the developed area, and it was moved to downtown. Between 1922 and 1943, the station was located in the western part of downtown at various locations -- including City Hall and the Federal Building.

As was common at the time, thermometers were frequently located on top of buildings. This resulted in nighttime low temperature readings that were 5°F to 10°F too warm, which was specifically noted by an observer in the 1940s. On Feb. 3, 1943, the Weather Bureau Anchorage climate station was moved to Merrill Field. Interestingly, a total solar eclipse was observed in Anchorage the very next day under a cloudless sky.

Merrill Field was the official climate site for Anchorage until 1953 (10 years). Even though Merrill Field is only two miles east of the previous climate locations, its climate is more representative of inland locations. Downtown and West Anchorage, on the other hand, are more affected by the waters of Cook Inlet.

Observations at the “new” international airport were collected beginning in January 1952, and a continuous climate record exists since April 1, 1952 (67 years). On Nov. 2, 1953, the U.S. Weather Bureau moved all operations to the international airport and the Merrill Field station was shuttered. In November 1997, automated observations were resumed at Merrill Field (after 45 years without observations). Between 1998 and 2014, temperature data were collected at both the international airport and the National Weather Service office on Sand Lake Road. During that time period, the Sand Lake Road data were considered official for Anchorage. On Sept. 1, 2014, that decision was reversed and all temperature data collected at the airport during those 17 years were entered back into the climate record. Therefore, there is a continuous stream of data from the International Airport since April 1, 1952.

As many readers know, Merrill Field has wider extremes than the international airport. For example, in the last 20 years, Merrill Field averages twice the number of days with a high temperature of 75°F and 10 more sub-0°F days per year. This makes comparisons of extremes between the two stations difficult at best. Interestingly, both Merrill Field and the international airport reached exactly 90°F on July 4, 2019. At the international airport, this exceeded the all-time record by 5°F and at Merrill Field, it exceeded the all-time record by 3°F.

This is a good time to note that climate stations at most cities are located at the largest airport in town. Pilots require the best, most timely weather information for the safety of aircraft operations. This is a good thing. Airport observations are not designed to be representative of the city as a whole, but we take what we can get. Given the temperature recording problems prior to 1943, the marine-inland differences, the 45-year data gap at Merrill Field, and the 67 years of continuous data at the current airport site, there is no other practical location to consider official for Anchorage besides the international airport. Therefore, the climate of Anchorage is formally measured at the international airport and the period-of-record begins on April 1, 1952. Older data remains for historical purposes, but not for comparison purposes.

Brian Brettschneider is an Anchorage-based environmental planner and climatologist who writes an Alaska weather blog.

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