Statewide Temperature Record Frequency

As a follow up to Richard's excellent post regarding the frequency of daily records in Fairbanks, I thought it would be interesting to see if the experience of Fairbanks was representative of the rest of the state. Right away there exists an apples-to-oranges problem; namely, how do you compare a station with 100 years of records to a station with 50 years of records? The station with a 50-year record should be twice as likely to break a daily record as the station with a 100-year record on any given day.

To eliminate this problem, I took the daily climate data for 14 stations across the state and only used data from 1955 to the present. That way, all 14 stations contain the exact same period of record. This makes for an apples-to-apples comparison. The stations utilized are: Anchorage, Annette Island, Barrow, Bethel, Cold Bay, Fairbanks, Juneau, King Salmon, Kodiak, Kotzebue, McGrath, Nome, St. Paul, and Yakutat (see Figure 1).

For example, if Station 'A' has a record low on July 10th of 35°F set in 1935 and their second lowest temperature for July 10th was 37°F in 1965, the 1965 value is assigned as the record for that date and the 1935 value is ignored. Also in this example, 1965 is also given a tally of 1 for that daily record. If other days in 1965 achieved record temperatures, additional tallies are given to 1965. Ties are prorated to the each year that a tie was observed. For each record category (high max, low max, etc.), every station has a total tally of 365 – a value of 1 for each day of the year. In the series of figures below, each of the four temperature metrics (high max, low max, high min, and low min) are charted for each year followed by a combined chart. On the y-axis, the value is the number of records per year per station. Using an example of 3 stations in 2013, Annette Island had 6 record high maximums, Cold Bay had 19, and Juneau had 3. The average for those 3 stations is 9.3 record high maximums in 2013.

Note that 2014 is treated equally as compared to other years. Therefore, you may want to double the values assigned to 2014 to extrapolate how the end-of-year values might look.

Interpretation:

High maximums:

I expected the number of record high maximums to show more of an upward trend but that was not the case. There is a slight upward trend but the it is unlikely to pass a statistical significance test

Low maximums:

The rate of low maximum records was far more interesting. Many of the low maximum records were set prior to the PDO shift of the mid-70s. Were they cloudy, rainy summer records or cold, clear winter records? I'll have to do some more digging around to answer that question. The variability and magnitude of the values through the mid-70s is remarkable.

High minimums:

Minimum (low) temperatures are probably more sensitive to long-term climate change, urbanization, and vegetative changes. More than any other category, the number of high minimums closely mirrors the mid 70-PDO shift (see Figure 6).

Low minimums:

A gentle, steady downward trend in low minimum records is easily apparent. As stated in the previous paragraph, minimum temperatures are especially sensitive to changing conditions. Interestingly, the PDO shift only briefly interrupts (steeper slope for a few years) the trend toward reduced low minimum occurrences.

Overall:

A gradual decrease in temperature records during this 59-year period was observed. Does that mean the climate is more homogeneous? My initial reaction is that warmer temperatures generally have less variability. For example, daily standard deviations are smaller for stations with warmer temperatures (e.g., Anchorage and Fairbanks in January).

Figure 1. Stations used in the 1955-2014 records analysis.

Figure 2. Average number of record high maximum temperatures per year per station (1955-2014).

 Figure 3. Average number of record low maximum temperatures per year per station (1955-2014).

Figure 4. Average number of record high minimum temperatures per year per station (1955-2014).

Figure 5. Average number of record low minimum temperatures per year per station (1955-2014).

Figure 6. Average number of all daily record temperatures per year per station (1955-2014).

Figure 7. Average annual high and low temperature of the 14 stations used in this analysis (1955-present)