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 2008-2009 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 2009 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 2009 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 Prediction 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.
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 2009 indicates that much of the nation has needed more energy for space heating over the entire heating season to date as compared with the 1971-2000 normals. Only sections of the western South Central States, the Rocky Mountain States and the Pacific States had below average heating degree day units for the current heating season. Nationwide, a one-percent increase in heating degree-day units above "normal" was accumulated from July 2008 through 31 January 2009. Furthermore, the number of heating degree days across much of the nation other than some of the western states were running ahead of those for the same time span during the 2007-2008 season, when unseasonably mild 2007 autumn was responsible for the relatively small number of heat degree-day units.
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. 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.
Ultimately, the "bottom line" has been that for most residents across the country, the demand for energy to date has been significantly less than 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, 2009, The American Meteorological Society.