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 sevenday 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 temperaturerelated 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.

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).
