A familiar gauge used to assess a winter's severity is the amount of heating fuel that we need to use to keep our homes warm over the winter. In other words, during the next several months many of us will inquire as to how the winter of 2000-2001 has affected our pocket books when it comes to space heating. For comparison purposes, we may try to remember how this winter compares with last winter or to some long-term average. We can monitor this situation by regularly returning to the tabulations of heating degree day units.
The heating degree day unit has been a useful indicator that gauges the amount of energy required for space heating. During the first several days of each new month, the Climate Analysis Center of the National Weather Service compiles heating degree data for the previous month and posts these data for selected cities around the country. The just concluded month of September serves as an example. The number of heating degree day units accumulated for the month appears in the column marked "Monthly Total". Adjoining columns display the comparisons between this year and the "normals", as well as with last year. Specifically, the arithmetic differences between the month totals from this year and the "normals" representing the 30 year averages for the current 1961-1990 climatological reference interval appear in the column marked "Month. Dev. from Norm." The difference between this year and the same month last year appear in the column "Month. Dev. from L. YR." Similar columns show the comparisons between the total number of heating degree day units accumulated over the current heating season that started on 1 July and the corresponding values for normals to date and last season to date. Percentage differences are also presented.
The climatologists at the Climate Analysis Center have also prepared a corresponding list of population weighted heating degree day units for each state. These latter statistics are used to show the temperature-related energy consumption on the state, regional, and national levels.
Since the heating season is only 3 months old and most locales have not experienced daily average temperatures significantly below 65 degrees Fahrenheit, analysis of these statistics is somewhat premature. A more meaningful analysis could be made by revisiting this site in several months as winter begins in earnest across many portions of the country. However, the preliminary results through September indicate that with the exception of the Mountain and Pacific states, much of the country has experienced a greater number of accumulated heating degree day units for the season (since July) than the previous year and the 30 year "normal".
Corresponding sets of cooling degree day units for selected cities and for population weighted regions were also compiled. These statistics indicate that the cool summer experienced by New England, the Middle Atlantic States and the East North Central States meant that the number of cooling degree units were below "normal" and much below the previous year, which was hot. Because of hot weather in the West, The Mountain states were well above average, along with the West South Central and Pacific states accumulated more cooling degree day units than "normal". As a result, a 5 percent increase was found in the cumulative number of cooling degree day units nationwide over the long term "normals" to date. However, this cooling season to date has experienced a small 2 percent decrease over the 1999 season -- when much of the nation experienced abnormally warm conditions. How these cooling degree statistics translate into an change in the cost of your utility bill is not as clear-cut as the relationship with the cumulative heating degree day units. Other factors, such as the atmospheric humidity levels, the amount of sunshine and your life style may also significantly influence your decision to run your air conditioner.