What is a degree day?
Degree days are measures of how cold or warm a location is. Degree days compare the mean (the high temperature plus the low temperature, divided by 2) outdoor temperature to a standard temperature; we use 65° Fahrenheit (F) in the United States.
How do you calculate a degree day?
The base temperature we use is 65°F. If the mean temperature is above 65°F, we subtract the base temperature (65°F) from the mean, and the result is Cooling Degree Days (CDD). If the mean temperature is below 65°F, we subtract the base temperature from the mean, and the result is Heating Degree Days (HDD).
A cooling degree day indicates a hot day and measures how much air conditioning we need to keep a building cool. A heating degree day indicates a cold day and measures how much heating we need to keep a building warm.
Example 1: The high temperature for the day was 90°F and the low temperature was 66°F. The mean temperature for that day was:
- (90°F + 66°F)
- /
- 2
- =
- 78°F
Because the result is above 65°F:
- 78°
- -
- 65°
- =
- 13 Cooling Degree Days
Example 2: The high temperature for the day was 33°F and the low temperature was 25°F. The mean temperature for that day was:
- (33°F + 25°F)
- /
- 2
- =
- 29°F
Because the result is above 65°F:
- 65°
- -
- 29°
- =
- 36 Heating Degree Days
What do people use degree day data for?
Degree days are calculated for each day of the year, and we use the daily degree days to compare months and seasons. We often count CDDs and HDDs by census regions and divisions. Generally, people study degree-day patterns to assess the climate and the heating and cooling needs for different regions of the country. For example, the West North Central Census Division generally has the most HDDs, and the West South Central Census Division generally has the most CDDs each year.
What are population-weighted degree days?
Degree days can be weighted based on the population of a region. We use population-weighted degree days to model and forecast energy consumption for the United States and for U.S. census divisions. When you combine the population data of a region with the degree day data, it helps people more closely estimate how much energy that region will consume. This information helps people like city planners to understand how much energy infrastructure they need.
Where can people find degree-day data?
Some electric and natural gas utilities publish degree-day data on their websites and some include it on 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 1.11 and 1.12 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.