## What is a degree day?

Cold winter weather or sweltering summer heat can increase the cost of utility bills. People can determine how much of the rise in utility bills is a result of the weather by using a unit of measure called the *degree** day*. A degree day compares the outdoor temperature to a standard temperature of 65°F. The more extreme the temperature, the higher the number of degree days. A higher number of degree days will require more energy for space heating or cooling.

Hot days are measured in cooling degree days. On a day with a mean temperature of 80°F, for example, 15 cooling degree days would be recorded. Cold days are measured in heating degree days. For a day with a mean temperature of 40°F, 25 heating degree days would be recorded. Two such cold days would result in a total of 50 heating degree days for the two-day period.

## What is degree day data used for?

By studying degree day patterns in an area, people can assess the climate and the heating and cooling needs based on time and season.

## 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?

In some areas, degree day information is published in the local newspapers, usually in the weather section. Degree day information may also be available from the public relations department of local utilities. Some utility bills even show degree days for each billing period.

Historical monthly and annual population-weighted degree days for U.S. Census divisions from 1995 to the most recent month available, and projections out one year, are available using EIA’s *Short Term Energy Outlook* (STEO) Data Browser.

Historical monthly and annual population-weighted degree days from 1949 to 2011 are available in Tables 1.7 to 1.10 of the *Annual Energy Review 2011*.

## What is normal for each region?

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