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HOME > Monitoring and Data > U.S. Climate Data & Maps > Degree Day Monitoring & Data > Degree Days Explanation

Explantion of the weekly and monthly degree day data summaries.

1. Station data

Degree day is a quantitative index demonstrated to reflect demand for energy to heat or cool houses and businesses. This index is derived from daily temperature observations at nearly 200 major weather stations in the contiguous United States. The "heating year" during which heating degree days are accumulated extends from July 1st to June 30th and the "cooling year" during which cooling degree data are accumulated extends from January 1st to December 31st. A mean daily temperature (average of the daily maximum and minimum temperatures) of 65°F is the base for both heating and cooling degree day computations. Heating degree days are summations of negative differences between the mean daily temperature and the 65°F base; cooling degree days are summations of positive differences from the same base. For example, cooling degree days for a station with daily mean temperatures during a seven-day period of 67,65,70,74,78,65 and 68, are 2,0,5,9,13,0,and 3, for a total for the week of 32 cooling degree days.

2. State, Regional, and National Data

Degree days are estimated for State climate divisions and then population weighted to more accurately reflect temperature-related energy consumption at the State, regional, and national levels. Temperature data, however, are not readily available on a daily basis for climate divisions. Therefore, equations which relate the historical temperature of State climate divisions to nearby weather stations (which have temperature data accessible daily) are used to estimate climate division temperatures. There are 198 weather stations nationwide used in the equations. Between one and four weather stations are used for each equation. There are 12 monthly equations for each of the 344 State climate divisions in 48 contiguous States. For example, the equation for estimating the average daily temperature (Fahrenheit) in the Northern Climatic Division (NJCD1) in New Jersey during December uses the average daily temperature at the LaGuardia Airport, New York weather station (LGA) for a particular day. The relationship is expressed as follows:

NJCD1 = -6.4 + 1.0 (LGA)

Once the daily temperature for each climate division has been estimated for the period of interest, heating/cooling degree days are calculated. The climate division heating/cooling degree days are then weighted according to their proportion of the State's population (2000 Census) they contain.

New Jersey Climate Division Population Weight
1) Northern .5677
2) Southern .4053
3) Coastal .0270

Using the State weights determined in this manner, regional degree days are calculated in proportion to populations in the respective States.

Mid-Atlantic Division Population Weights
New Jersey .21210
New York .47833
Pennsylvania .30957

Regional divisions are weighted by the same procedure in the computation of national degree days. The previously described estimation and weighting procedure is used in computing current degree days and last year's degree days. Normal degree days for climatic divisions are modified slightly and are derived from normal degree days for stations. The comparisons against the previous year are all estimated and population weighted using this procedure.

The gas, oil, and electric home heating customer weighted degree days are computed on a regional and national basis by a similar procedure by weighting the states in regions and the regions in the country by the number of residential customers heating with gas, oil, or electricity (2000 Census).

The following is a listing of the parameters in the files and their meaning for the weekly/monthly summaries.
WEEK (or MONTH)TOTAL Summation of the daily values for the given period.
WEEK (or MONTH) DEV FROM NORM The difference of the period total from the normal (1971-2000) period total. Negative values indicate less degree days than normal.
WEEK (or MONTH) DEV FROM L YR The difference of the period total from the same period the previous year.
CUM TOTAL The accumulated degree days from the beginning of the season.
CUM DEV FROM NORM The difference of the accumulated total from the normal accumulated total.
CUM DEV FROM L YR The difference of the accumulated total from the accumulated total of the same period the previous year.
CUM DEV FROM NORM PRCT The ratio of the current degree days to the normal degree days expressed in percent.
CUM DEV FROM L YR PRCT The ratio of the current degree days to last years degree days expressed in percent.

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NOAA Center for Weather and Climate Prediction
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Page Author: Climate Prediction Center Internet Team
Page last modified: January 24, 2005
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