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What's a Degree Day? It's the difference between the average daily temperature for the day (these and other climatological data are available on the National Weather Service Homepage) and 65 degrees. For example, if the average daily temperature on January 1 was 40 degrees, this gives a reading of 25 Heating Degree Days for January 1 (65-40=25).

What Do Degree Days Tell You? Through comparison with Degree Day data from past years, Degree Days give a quantifiable answer to the question "how cold was it?". In the chart, the first column gives the month and the second, the total number of normal Heating Degree Days for that month. The third column (percent of annual) tells you which are our coldest and warmest months. The remaining columns give the actual number of Degree Days by month and percent of normal that this represents. If percent of normal is 100, the month in question is absolutely average. If it's below 100, it was warmer than normal; above 100 means it was colder.

What Else Do Degree Days Tell You? If you just completed some weatherization work and don't see the predicted savings, Degree Day data could provide the explanation.



Seattle-Tacoma Airport Station


2015 Monthly Degree Day Record

Month Normal HDD % of Annual HDD % of Normal
JAN 711 15% 612 86%
FEB 605 13% 448 74%
MAR 574 12% 441 77%
APR 440 9% 401 91%
MAY 285 6% 181 64%
JUN 144 3% 29 20%
JUL 49 1% 2 4%
AUG 36 1% 10 28%
SEP 132 3% 142 108%
OCT 380 8% 318 84%
NOV 586 12% 703 120%
DEC 755 16% 749 99%
Annual 4,697 100% 4,036 86%

NOTE: Effective January 2012, normals used are based on the 1981-2010 year records for Seattle-Tacoma, which became available January 2012.
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