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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 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.

The West North Central division had the largest number of heating degree days in 2019.

Heating degree days by census division in 2019 West Midwest Northeast South Heating degree days by census division in 2019 FL GA SC NC VA WV DE MD South Atlantic2,383 NH ME MA CT RI New England6,531 VT PA NJ NY Middle Atlantic5,754 TN AL East South Central3,180 KY MS IL MI OH IN WI East North Central6,428 OK AR LA TX West South Central2,152 KS NE SD MN IA MO ND West North Central7,073 NV AZ UT ID CO WY MT NM Mountain5,308 CA OR WA Pacific3,534 Note: Population-weighted degree days. Pacific division includes Alaska and Hawaii. Source: U.S. Energy Information Administration, Monthly Energy Review, Table 1.9, July 2020

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.

The West South Central division had the largest number of cooling degree days in 2019.

Cooling degree days by census division in 2019 West Midwest Northeast South Cooling degree days by census division in 2019 FL GA SC NC VA WV DE MD South Atlantic2,516 NH ME MA CT RI New England536 VT PA NJ NY Middle Atlantic784 TN AL East South Central1,879 KY MS IL MI OH IN WI East North Central832 OK AR LA TX West South Central2,750 KS NE SD MN IA MO ND West North Central954 NV AZ UT ID CO WY MT NM Mountain1,397 CA OR WA Pacific846 Note: Population-weighted degree days. Pacific division includes Alaska and Hawaii. Source: U.S. Energy Information Administration, Monthly Energy Review, Table 1.10, July 2020

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 divisions. Learn more about EIA’s degree day modeling and forecasting methodology.

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.

Last updated: August 17, 2020