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 2011-2012 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. Since the statistics for September are yet to be processed, those statistics for August will serve 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 1971-2000 climatological reference interval appear in the column marked "Month. Dev. from Norm." (The newer 1981-2010 normals will be available by the start of next year.) 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 was only 2 months old for the provided table and most locales have not experienced daily average temperatures significantly below 65 degrees Fahrenheit, analysis of these statistics is somewhat premature. Therefore, a more meaningful analysis could be made by revisiting this site in several months, after winter begins in earnest across many portions of the country. The preliminary results through August indicate that nationally, fewer heating degree-day units had been accumulated for the season (since 1 July) than the 30-year "normal." With many sections of the country experiencing higher than average summer temperatures in 2011, nearly all the regions designated by the Climate Analysis Center had lower accumulated totals than average in July and August 2011. In addition, most of the regions of the continental United States, with the exception of the Midwest (especially the East North Central States) had fewer heating degree-day units accumulated this summer as compared with the first two months of the previous 2010-2011 heating season.
Corresponding sets of cooling degree-day units for selected cities and for population-weighted regions were also compiled and are made available. These statistics indicate that nearly the entire country experienced more cooling degree-day units for the first eight months of the year than the 30-year climatological "normals" to date. This higher number represents a 24-percent increase over "normal," reflective of the extremely hot 2011 summer that residents of the south central US (especially the West South Central States) experienced. Only the Pacific States had fewer cooling degree-day units, reflective of below average summer temperatures along the Pacific Coast. When compared with the 2010 cooling season, the hot summer of 2011 across the southern Plains resulted in more cooling degree days. However, cool conditions in August 2011 across the northeastern quadrant of the nation resulted in fewer accumulated cooling degree days than in the 2010 season. Consequently, the nationwide average for this cooling season to date experiencing a 2-percent increase from the 2010 season.
How these cooling degree statistics translate into a change in the cost of your utility bill is not as clear-cut as the cost 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.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2011, The American Meteorological Society.