What is a degree day?

The West North Central region normally requires more heating than other regions.
Map of heating degree days

Source: U.S. Energy Information Administration, Annual Energy Review 2011, Table 1.9, September 2012

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The West South Central region normally requires more cooling than other regions.
Map of cooling degree days

Source: U.S. Energy Information Administration, Annual Energy Review 2011, Table 1.10, September 2012

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Degree days are measures of how cold or warm a location is. A degree day compares the mean (the average of the high and low) outdoor temperatures recorded for a location to a standard temperature, usually 65° Fahrenheit (F) in the United States. The more extreme the outside temperature, the higher the number of degree days. A high number of degree days generally results in higher levels of energy use for space heating or cooling.

Heating degree days (HDD) are a measure of how cold the temperature was on a given day or during a period of days. For example, a day with a mean temperature of 40°F has 25 HDD. Two such cold days in a row have a total of 50 HDD for the two-day period.

Cooling degree days (CDD) are a measure of how hot the temperature was on a given day or during a period of days. A day with a mean temperature of 80°F has 15 CDD. If the next day has a mean temperature of 83°F, it has 18 CDD. The total CDD for the two days is 33 CDD.

What do people use degree day data for?

People study degree day patterns to assess the climate and to assess the heating and cooling needs for different regions of the country during the seasons of the year.

What are population-weighted degree days?

Degree day data can be weighted according to the population of a region to estimate energy consumption. The U.S. Energy Information Administration (EIA) uses population-weighted degree days to model and project energy consumption for the United States and for U.S. Census regions and divisions.

Where can people find degree day data?

Newspapers may publish degree day information in the weather section. Electric and natural gas utilities may publish degree day information on their websites, and some utilities include degree day data in customer utility bills. Several weather data-related websites publish daily high and low temperatures and degree days for specific locations. The National Centers for Environmental Information is a source for historical temperature data for the United States and other countries.

Historical monthly (from 1973) and annual (from 1949) population-weighted degree days are available in Tables 1.9 and 1.10 of the Monthly Energy Review.

Historical monthly and annual population-weighted degree days for U.S. Census divisions for 20 years and a forecast for one to one and a half years are available in the Short Term Energy Outlook (STEO) Data Browser.

What is normal for each region?

The degree day maps on this page show the population-weighted degree days that normally occurred in each U.S. Census region from 1971 through 2000.