So how has this winter been treating you? For most areas of the nation, a familiar gauge used to assess a winter's severity is the amount of fuel that we need to use to keep our homes warm over the winter. In other words, many of us will inquire as to how the winter of 2009-2010 has affected our pocket books when considering space heating. This question becomes more serious if fuel prices increase.
For comparison purposes, we may try to remember how temperatures this winter compare with 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 approximately 250 selected U.S. cities. January 2010 was unseasonably cold across the northeastern quadrant of the nation. Cities from the upper Mississippi Valley eastward across the Great Lakes to New England had monthly average temperatures that were as much as 8 Fahrenheit degrees below the 1971-2000 "normal" temperatures. The central Rockies and the adjacent high Plains experienced an unseasonably warm January 2010 with monthly temperatures that were as much as 8 Fahrenheit degrees above "normal" for January.
However, another way of comparison can be made through monitoring the heating degree-day units that have accumulated for these stations, which serve more quantitative indicators that gauge the amount of energy required for space heating. The heating degree-day units for each station on each day of the month are by determining the difference the station's daily average temperature (the arithmetic average of the day's maximum and minimum temperature, or the sum of these two temperature readings, which are divided by two) and a base temperature of 65 degrees Fahrenheit. If the day's average temperature were below 65 degrees Fahrenheit, the arithmetic difference in Fahrenheit degrees would represent the number of heating degree day units accumulated for that day, but if the average temperature were at or above 65 degrees, zero heating degree day units are accumulated. A running summation of the number of heating degree day units are made at each station for the entire month, or for the "heating year" extending from 1 July to 30 June of the following calendar year.
During the first several days of each new month, the Climate Prediction Center of the National Weather Service compiles heating degree data for the previous month and posts these data for approximately 360 selected cities around the country. These monthly and seasonal values are compared with the corresponding values for last year to date and for the "normals" representing the 30-year averages for the current 1971-2000 climatological reference interval.
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, which are weighted according each state's population (2000 Census), are used to show more accurately the temperature-related energy consumption on the state, regional, and national levels based upon population density.
In the monthly tables prepared by the Climate Prediction Center, 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 1971-2000 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.
Inspection of the most recent tables of the number of heating degree-day units accumulated between last July and the end of January 2010 indicates that central sections of the nation has needed more energy for space heating over the entire heating season to date as compared with the 1971-2000 normals. Some of the largest departures occurred in the South Central US (both east and west). On the other hand, the Northeast (Middle Atlantic and New England States) and the West (the Rocky Mountain and the Pacific States) had below average heating degree day units for the current heating season. Nationwide, the number of heating degree-day units accumulated from July 2009 through 31 January 2010 was only slightly above "normal." However, the number of heating degree days across the Northeast and the Great Lakes, were running below those for the same time span during the 2008-2009 season. Elsewhere, a more heating day units were accrued across the rest of the nation than during last heating season.
Corresponding sets of cooling degree-day units for selected cities and for population-weighted regions were compiled. Cooling degree-day units represent the positive departure of the daily average temperature from the base temperature of 65 degrees Fahrenheit. The arrangement of these monthly tables is similar to that for the corresponding heating degree-day units. 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.
Since the cooling season only started on 1 January and most locales have not experienced daily average temperatures above 65 degrees, analysis of these statistics would be somewhat premature. Nevertheless, the Southeast and Southwest had accumulated fewer cooling degree day units during January than normal.
Ultimately, the "bottom line" has been that for most residents across the country, the demand for energy to date has been close to the long-term average. Consequently, the pocketbook has not been severely strained in most areas - assuming that monetary inflation or other non-weather-related price increases are not taken into account.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2010, The American Meteorological Society.