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 2013-2014 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. (The heating degree-day unit is determined from the difference between the average daily temperature and the 65 degree Fahrenheit base; negative departures are counted as heating degree day units, with accumulated totals summed from the beginning of July and running through the end of the following June.) 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 approximately 250 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 1981-2010 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.
To help interpret the meaning of these heating degree-day unit tabulations, we may try to see how temperatures this upcoming winter would compare with those of last winter or to some long-term average. One could inspect the tables of monthly average temperature (in degrees Fahrenheit) that are furnished by the National Weather Service a few days after the end of each month for the selected U.S. cities.
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 most of the experiencing average to above average summer temperatures in 2013, many regions designated by the Climate Analysis Center had lower accumulated heating degree-day totals than average in July and August 2013. However, sections of the Midwest, the Mid-South and the Southeast had below average temperatures in July and August, leading to greater than average cumulative heating-degree day totals. In addition, most of the regions of the continental United States had more heating degree-day units accumulated this summer as compared with the first two months of the previous 2012-2013 heating season, as that summer was one of the hottest across most of the nation.
Corresponding sets of cooling degree-day units for selected cities and for population-weighted regions were also compiled and are made available. (The cooling degree-day unit is determined from the difference between the average daily temperature and the 65 degree Fahrenheit base; positive departures are counted as cooling degree day units, with accumulated totals summed from the beginning of January and running through the end of the year.) These statistics indicate that most of the country with the exception of sections in the Midwest and Southeast 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 an eight-percent increase over "normal." New England had increases that were over 40 percent above normal. When compared with the 2012 cooling season, which featured the record hot summer, the 2013 cooling season across most of the nation had fewer accumulated cooling degree-day units. Only the West had more cooling degree-day units in 2013 than in 2012. When averaged nationwide, this current cooling season to date is experiencing a 12-percent decrease from the 2012 season, but still is 8 percent above normal.
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, 2013, The American Meteorological Society.