Season of Freezing Temperatures

Every so often I do a quick climate analysis for a blogger in Houston, Texas, and one of the things that came up earlier in the winter was the frequency of freezing temperatures down there. In a few weeks, stations in Alaska will be approaching the date at which freezing temperatures are no longer expected. Therefore, I thought it would be interesting to look at 1) the number of days per season with sub-freezing temperatures and 2) the length of time between the average first freeze and the average last freeze. Specifically, I was interested in how Fairbanks compares to other places in Alaska and the Lower 48.

Number of Freezes

As the analysis unfolded, it became apparent that the Fairbanks International Airport station's values did not appear representative of the greater Fairbanks area. For example, the number of sub-freezing days per season at the airport's official station is 222.7. However, the Fairbanks Airport #2 station records 237.1 sub-freezing days. Therefore, I used an average of five stations to generate a single Fairbanks value of 226.6 sub-freezing days.

Figures 1 and 2 show the number of days with subfreezing temperatures for Alaska and for the entire U.S. respectively. Figure 3 shows the stations used in the Analysis. All stations with a value greater than Fairbanks' 226.6 (rounded to 227) are shown in blue dots. Note that the color scale is different on the two maps. I was a little surprised at the number of stations in Alaska that have more freezing days than Fairbanks – particularly places west of Fairbanks like Galena, Kaltag, and Tanana.

Figure 1. Map of Alaska showing the annual number of days with a temperature of 32°F or lower. Stations with a larger annual value than Fairbanks are highlighted in blue.

Figure 2. Map of the U.S. showing the annual number of days with a temperature of 32°F or lower. Stations with a larger annual value than Fairbanks are highlighted in blue.

Figure 3. Stations used in the analysis of sub-freezing temperatures. To qualify for the analysis, a station must have at least 10 complete years of data beginning in 1980-1981 (maximum of 33 years). Any year with more than 15 missing observations was excluded. There were 181 stations that met those qualifications in Alaska and 1025 in the rest of the U.S. The Lower 48 analysis did not include Cooperative or RAWS stations.

Length of Freezing Season

Another way of describing the frequency of freezing temperatures is to define a season where freezing temperatures are likely to occur. This is the opposite of a growing season. In this section I did a simple subtraction of the average date of the last freeze of the season minus the average date of the first freeze of the season. For Fairbanks, the average of the five stations that are collectively referred to a 'Fairbanks Valley' on the maps is 257 days (Sept 5 - May 20). Figures 4 and 5 show the number of days with in the freezing season for Alaska and for the entire U.S. respectively. All stations with a value larger than Fairbanks' are shown as blue dots (see Figure 3 for a map of all stations).

Interestingly, the number of stations in Alaska with a longer freezing season than Fairbanks is larger than the number of stations with a larger count of freezing temperatures. This implies that Fairbanks drops quickly into a freezing temperature regime in the Fall and bounces out of the freezing temperature regime quickly in the spring. This is also evident when comparing Figure 2 with Figure 5; that is, quite a few stations in the Lower 48 have a longer freezing season than Fairbanks (or most other Alaska stations for that matter). In the case of the Lower 48 stations, they are frequently at high elevations (less atmosphere to radiate longwave energy downward) and also have 8-10 hours of darkness in the summer (unlike Fairbanks) when heat can be radiated upward.

Figure 4. Map of Alaska showing the average number of days between the first freeze of the season and the last freeze of the season. Stations with a larger annual value than Fairbanks are highlighted in blue.

Figure 5. Map of the U.S. showing the annual number of days  between the first freeze of the season and the last freeze of the season. Stations with a larger annual value than Fairbanks are highlighted in blue.

Data Set

There are many interesting values that popped out. Far too many to mention. Therefore, for anyone who is interested, here is a LINK to the entire data set as an MS Excel file. The file is 2.8 megabytes.