What is a degree day?

As defined by the National Oceanic and Atmospheric Administration (NOAA), a degree day gauges the amount of heating or cooling needed for a building using 65 degrees as a baseline. To compute heating/cooling degree-days, take the average temperature for a day and subtract the reference temperature of 65 degrees. If the difference is positive, it is called a "Cooling Degree Days". If the difference is negative, it is called a "Heating Degree Days". The magnitude of the difference is the number of days. For example, if your average temperature were 50 degrees for a day in December, the difference of the average temperature for that day and the reference temperature of 65 degrees would yield a minus 15. Therefore, you know that you are going to have Heating Degree Days that day. Since the magnitude of the difference is 15, you know that you are going to have 15 Heating Degree Days. Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs.

For historical degree day statistics: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/

You can also contact Office of the Arizona State Climatologist: http://www.stateclimate.org