Climate Change Indicators in the United States Heating and Cooling Degree Days


Climate Change Indicators in the United States: Heating and Cooling Degree Days
www.epa.gov/climate-indicators - Updated August 2016

Heating and Cooling Degree Days

This indicator examines changing temperatures from the perspective of heating and cooling needs for
buildings.

Background

Outdoor temperatures can affect daily life in many ways. In particular, temperature affects our comfort
level and our demand for heating and air conditioning. Collectively, heating and cooling the spaces in
which we live accounts for 48 percent of the energy that American households use every year.1 As
climate change contributes to an increase in average temperatures, an increase in unusually hot days,
and a decrease in unusually cold days (see the U.S. and Global Temperature and High and Low
Temperatures indicators), the overall demand for heating is expected to decline and the demand for
cooling is expected to increase.

One way to measure the influence of temperature change on energy demand is using heating and
cooling degree days, which measure the difference between outdoor temperatures and a temperature
that people generally find comfortable indoors. These measurements suggest how much energy people
might need to use to heat and cool their homes and workplaces, thus providing a sense of how climate
change could affect people's daily lives and finances.

About the Indicator

This indicator uses daily temperature data from thousands of weather stations across the United States
to calculate heating and cooling degree days. A "degree day" is determined by comparing the daily
average outdoor temperature with a defined baseline temperature for indoor comfort (in this case,
65°F). For example, if the average temperature on a particular day is 78°F, then that day counts as 13
cooling degree days, as a building's interior would need to be cooled by 13°F to reach 65°F. Conversely,
if the average outdoor temperature is 34°F, then that day counts as 31 heating degree days, as a
building's interior would need to be warmed by 31°F to reach 65°F. This does not mean that all people
will actually heat or cool buildings to 65°F; it is just a number to allow for consistent comparisons over
time and across the country. For reference, New York City experiences far more heating degree days
than cooling degree days per year—a reflection of the relatively cool climate in the Northeast—while
Houston, Texas, has far more cooling degree days than heating degree days—a reflection of the much
warmer climate in the South.2

Figure 1 shows each year's average heating and cooling degree days across the contiguous 48 states.
Figures 2 and 3 show how heating and cooling degree days have changed by state, based on a
comparison of the first 60 years of available data (1895-1954) with the most recent 61 years (1955-
2015). State and national averages were calculated by finding the total number of heating and cooling
degree days per year at each weather station, averaging the results from all stations within regions
called climate divisions (each state within the contiguous 48 has up to 10 climate divisions), then
calculating state and national averages weighted by the population of each climate division. With this

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Climate Change Indicators in the United States: Heating and Cooling Degree Days

www.epa.gov/climate-indicators - Updated August 2016

population-weighting approach, average state and national heating and cooling degree days more
closely reflect the conditions that the average resident would experience.

Key Points

• Heating degree days have declined in the contiguous United States, particularly in recent years,
as the climate has warmed (see Figure 1). This change suggests that heating needs have
decreased overall.

• Overall, cooling degree days have increased over the past 100 years. The increase is most
noticeable over the past few decades, suggesting that air conditioning energy demand has also
been increasing recently (see Figure 1).

• Heating degree days have generally decreased and cooling degree days have generally increased
throughout the North and West. The Southeast, with the exception of Florida, has seen the
opposite: more heating degree days and fewer cooling degree days (see Figures 2 and 3).

Figure 1. Heating and Cooling Degree Days in the Contiguous 48 States, 1895-2015

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This figure shows the average number of heating and cooling degree days per year across the contiguous
48 states.

Data source: NOAA, 20163

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Heating degree days (colderthan 65°F)

Cooling degree days (warmer than 65°F)

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Climate Change Indicators in the United States: Heating and Cooling Degree Days
www.epa.gov/climate-indicators - Updated August 2016

Figure 2. Change in Annual Heating Degree Days by State, 1955-2015 Versus 1895-1954



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This map shows how the average number of heating degree days per year has changed in each state
over time. The map was created by comparing the first 60 years of available data (1895-1954) with the
most recent 61 years (1955-2015). "Warmer" colors indicate an increase in temperatures between the
two periods, leading to less of a need to turn on the heat—that is, fewer heating degree days. "Cooler"
colors indicate a decrease in temperatures, leading to more of a need to turn on the heat—that is, more
heating degree days. Visit this indicator online at: www.epa.gov/climate-indicators for an interactive
version of this map.

Data source: NOAA, 2016

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Climate Change Indicators in the United States: Heating and Cooling Degree Days
www.epa.gov/climate-indicators - Updated August 2016

Figure 3. Change in Annual Cooling Degree Days by State, 1955-2015 Versus 1895-1954

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150

250

350 450

This map shows how the average number of cooling degree days per year has changed in each state over
time. The map was created by comparing the first 60 years of available data (1895-1954) with the most
recent 61 years (1955-2015). "Warmer" colors indicate an increase in temperatures between the two
periods, leading to more demand for air conditioning—that is, more cooling degree days. "Cooler" colors
indicate a decrease in temperatures, leading to less demand for air conditioning—that is, fewer cooling
degree days. Visit this indicator online at: www.epa.gov/climate-indicators for an interactive version of
this map.

Data source: NOAA, 2016s

Indicator Notes

Heating and cooling degree days suggest how temperature changes affect energy demand, but they do
not necessarily reflect actual energy use. Many other factors have influenced energy demand over time,
such as more energy-efficient heating systems, the introduction and increasingly widespread use of
cooling technologies, larger but better-insulated homes, behavior change, and population shifts (such as

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Climate Change Indicators in the United States: Heating and Cooling Degree Days
www.epa.gov/climate-indicators - Updated August 2016

more people moving to warmer regions). All of the population-weighting in this indicator is based on the
population distribution according to the 2010 U.S. Census, so any changes in heating and cooling degree
days over time in this indicator reflect actual changes in the climate, not the influence of shifting
populations. A nationally applied baseline—in this case, 65°F—has certain limitations considering the
various climate regimes across the United States.

Data Sources

The data for this indicator were provided by the National Oceanic and Atmospheric Administration's
National Centers for Environmental Information, which maintains a large collection of climate data
online at: www.ncei.noaa.gov.

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EIA (Energy Information Administration). 2015. 2009 Residential energy consumption survey. Accessed May
2015. www.eia.gov/consumption/residential/index.cfm.

NOAA (National Oceanic and Atmospheric Administration). 2015. 1981-2010 U.S. climate normals. Accessed
May 2015. www.ncdc.noaa.gov/data-access/land-based-station-data/land-based-datasets/climate-
normals/1981-2010-normals-data.

NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental
Information. Accessed January 2016. www.ncei.noaa.gov.

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