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Lighting Seattle since 1905 Jorge Carrasco, Superintendent
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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.

 

MONTHLY DEGREE DAY RECORD

Seattle-Tacoma Airport Station

  

2009 Monthly Degree Day Record
Month Normal HDD % of Annual HDD % of Normal
JAN 747 16% 797 107%
FEB 613 13% 649 106%
MAR 582 12% 714 123%
APR 447 9% 465 104%
MAY 291 6% 273 94%
JUN 150 3% 70 47%
JUL 55 1% 29 53%
AUG 45 1% 36 80%
SEP 138 3% 99 72%
OCT 383 8% 385 100%
NOV        
DEC        
         
TOTALS        
Normals are based on 1971-2000 year records which were established for Seattle-Tacoma and became available March 2002.
Historic Records:
1998-2001
2002-2006
2007-present
in Adobe Acrobat
(*.pdf) format
http://www.adobe.com/products/acrobat/readstep2.html
 
Water Conditions
 

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