Seattle City Light DEBRA SMITH, General Manager and CEO
Global Change
Degree Day title
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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


2018 Monthly Degree Day Record

Month Normal HDD(1) % of Annual HDD % of Normal
JAN 711 15% 615 86%
FEB 605 13% 660 109%
MAR 574 12% 572 100%
APR 440 9% 406 92%
MAY 285 6% 131 46%
JUN 144 3% 102 71%
JUL 49 1% 42 86%
AUG 36 1% 39 108%
SEP 132 3% 205 155%
OCT 380 8%    
NOV 586 12%    
DEC 755 16%    
Annual 4,697 100% 2,772 92%

Current year data have not undergone quality control and are subject to revision

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

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