SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

Wednesday, 21 September 2016

MONITORING DEGREE DAY UNITS


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 2016-2017 has affected our pocket books when it comes to space heating. For comparison purposes, we may try to remember how this upcoming 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. A map showing the nine regions of the contiguous United States defined by NOAA's National Climatic Data Center and used regularly in the CPC's temperature and degree-day tabulations is available.

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 large sections of the country experiencing average to above average summer temperatures in 2016, many regions designated by the Climate Analysis Center had lower than average accumulated heating degree-day totals in July and August 2016. Overall, the contiguous United States had slightly fewer heating degree day units in 2016 than average. In addition, the Mountain and Pacific had slightly more heating degree-day units accumulated this summer as compared with the first two months of the previous 2015-2016 heating season.

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 for 2016 running through August indicate that all areas of the country experienced more cooling degree-day units than the 30-year climatological "normals" to date. As a result, the cumulative departure for the first eight months of the year across the "Lower 48" was 22 percent above normal. When compared with the 2015 cooling season, the 2016 cooling season across most of the nation had approximately 7 percent more accumulated cooling degree-day units because of the higher temperatures in January-August 2016.

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@aos.wisc.edu
© Copyright, 2016, The American Meteorological Society.