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Units and calculators explained

Degree days

What is a degree day?

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 energy use for space heating or cooling.

Heating degree days (HDDs) 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 HDDs. Two such cold days in a row have 50 HDDs for the two-day period.

The West North Central division had the largest number of heating degree days in 2022.

Heating degree days by census division in 2022 West Midwest Northeast South Heating degree days by census division in 2022 FL GA SC NC VA WV DE MD South Atlantic2,521 NH ME MA CT RI New England6,013 VT PA NJ NY Middle Atlantic5,632 TN AL East South Central3,439 KY MS IL MI OH IN WI East North Central6,348 OK AR LA TX West South Central2,193 KS NE SD MN IA MO ND West North Central6,905 NV AZ UT ID CO WY MT NM Mountain5,131 CA OR WA Pacific3,351 Data source: U.S. Energy Information Administration, Monthly Energy Review, Table 1.10, July 2023 Note: Population-weighted degree days. Pacific division includes Alaska and Hawaii.

Cooling degree days (CDDs) 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 CDDs. If the next day has a mean temperature of 83°F, it has 18 CDDs. The total CDDs for the two days is 33 CDDs.

The West South Central division had the largest number of cooling degree days in 2022.

Cooling degree days by census division in 2022 West Midwest Northeast South Cooling degree days by census division in 2022 FL GA SC NC VA WV DE MD South Atlantic2,296 NH ME MA CT RI New England647 VT PA NJ NY Middle Atlantic842 TN AL East South Central1,728 KY MS IL MI OH IN WI East North Central816 OK AR LA TX West South Central2,989 KS NE SD MN IA MO ND West North Central1,051 NV AZ UT ID CO WY MT NM Mountain1,577 CA OR WA Pacific1,089 Data source: U.S. Energy Information Administration, Monthly Energy Review, Table 1.11, July 2023 Note: Population-weighted degree days. Pacific division includes Alaska and Hawaii.

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.

What are population-weighted degree days?

Degree days 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 forecast energy consumption for the United States and for U.S. census 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 Weather Service Climate Prediction Center is a source for historical temperature and degree-day data for the United States.

Historical monthly (from 1973) and annual (from 1949) population-weighted degree-day data for the United States and U.S. census divisions are available in Tables 1.10 and 1.11 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.

Last updated: August 21, 2023.