Thursday, April 17, 2014
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This information appears in the weather almanac of daily newspapers, among other places.
Degree-days vary greatly by location. But on a statewide basis, Maine now averages just over 7,000 degree-days during the October-through-April heating season. That's a decline of 246 average heating degree-days since 1896.
Homes weren't heated with oil furnaces in 1896. But for comparison purposes, the decline in degree-days means a household that needed the equivalent of 1,000 gallons of heating oil a century ago would need 35 fewer gallons today. At $3.50 a gallon, for instance, that adds up to a $122 savings.
To plot this gradual drop in degree-days, the National Climatic Data Center's website was used to create a set of time series graphs. The most relevant view came from a series that focused on the seven-month heating season over the century and combined Maine's three climate divisions -- coastal, inland and northern.
The initial graph displayed wide annual temperature swings. To reveal patterns over time, a scientist at the data center applied a binomial filter, a standard statistical tool to smooth out short-period fluctuations.
Once filtered, the time series revealed temperature trends that move in wave-like cycles over 115 heating seasons, in a bumpy, undulating line. It shows extreme cold around 1900, followed by warmer temperatures later in the decade, and another cold spell that stretched into the early 1930s. Above-average cold around 1940 was followed by a very warm period in 1950.
Colder winters returned from around 1960 into the early 1990s. Since then, with a few exceptions, winters have been warmer than average. That trend was seen most clearly in a second, filtered time series covering the period from 1950 to 2011, which marked Maine's second-warmest winter on record.
The binomial filter was calculated by Trevor Wallis, a physical scientist who works under contract for the data center. In interpreting the time series, Wallis cautioned that because the trend is inconsistent, it's important not to "cherry-pick" data to reach conclusions. In his view, the 3.5 percent decline in heating degree-days isn't statistically significant. The steep, record decline in the past decade is more striking, he said.
"But there's nothing to say it won't go up again," Wallis added. "We've had some exceptional weather lately, but whether (the decline in degree-days) is going to smooth from now on, I wouldn't comment."
Wallis also noted that the number and location of weather observing stations over a century can influence degree-day collection data.
That variable is seen in Portland, where the National Weather Service moved its thermometers from downtown to the jetport in 1940. But the most recent warming trend is clear in Portland, Capriola said, reflected in a falling number of heating degree-days since 1961. The latest study period, from 1981 to 2010, shows the biggest drop -- 159 fewer degree days than the 6,586 recorded in the previous 30-year period.
The coming years will tell whether this warming trend will be reinforced, but it seems unlikely that, this winter, the state will see another heating season as mild as the last one.
The blog The Weather Centre is calling for colder-than-normal temperatures in the Northeast. The Maine-based Farmer's Almanac also foresees snow and cold. AccuWeather predicts above-average snow amounts in the Northeast and southern New England, although not Maine.
In its short-term outlook released Wednesday, the U.S. Energy Information Administration said households are expected to use more heating fuel this winter compared with last year, because temperatures are expected to be near normal in much of the country.
The agency forecast reflects a much colder winter east of the Rocky Mountains, with heating degree-days in the Northeast expected to be 20 to 27 percent greater this winter, according to the National Oceanic and Atmospheric Administration.
Staff Writer Tux Turkel can be contacted at 791-6364 or at: