It's hardly a day to be contemplating residential and commercial heating demand. However, as we reach the end of June, we mark the end of the heating season, typically considered to be the period between July 1 and June 30. The primary climatological measure of heating demand is an index called "heating degree days" (HDD). Heating degree days are derived using mean daily temperature. If the mean daily temperature falls below 65 degrees F, the difference between the mean temperature and 65 degrees is said to be the number of "heating degree days" for the date. For example, if the mean daily temperature is 10 degrees on a January day, the heating degree day total for the date is 65 - 10 = 55 HDD. The colder the temperature, the larger the heating degree days for the date. The daily values are summed throughout the July 1 through June 30 time period and offer a quantitative measure of heating demand for the season. The lower the total ... the warmer the season.
As Mark Seeley points out, the Twin Cities heating degree day summation for the 2011-2012 heating season is the lowest on record. See table below. This season's total of 5852 is 23% percent less than the modern normal of 7581. Assuming a constant cost of energy, this season's heating bills were roughly one-quarter less than average.
I offer an apology to Dr. Seeley. When I provided him with the data used in his narrative, I had yet to look at data from the 1870s and 1880s. This season's total breaks the all-time low record set during the 1877-1878 season.
We would also like to add thanks to Dan Baseman who pointed out this anomaly to our office.
Twin Cities Seasonal Heating Degree Days (base 65 F), 1872 - 2012
five lowest heating degree seasonal totals * season 1 2011-2012 5852 2 1877-1878 6177 3 2005-2006 6611 4 1986-1987 6621 5 1999-2000 6660 1981-2010 normal 7581 1872-2012 average 7896 five highest heating degree seasonal totals * 1 1874-1875 9807 2 1887-1888 9614 3 1873-1874 9327 4 1903-1904 9082 5 1916-1917 9062
* Please recall the significant impact that the urban heat island has on these totals. The preponderance of higher values early in the record and lower values later in the record are almost certainly due to the combined influence of a growing urban footprint and a warming planet.