Monday, May 6, 2013

A Hundred and Ten in the Shade* – Long-Term Heating and Cooling Season Trends in New Hampshire

I am in the Southern Hemisphere this week, and as I headed from a cool New England spring to a warm South African autumn, my thoughts turned to long-term temperature trends in New Hampshire and their energy implications.

In January this year, I read an interesting press release from University of New Hampshire in which Mary Stampone, the NH state climatologist, pointed out that 2012 was the hottest year on record in NH and much of New England. The chart below was included in the press release, and the data showed the variation above and below the long-term average calculated from the 1895 to 2012 data. The largest positive variation was for 2012 in which the average annual temperature was ~10% (4oF) above the average of 43.4oF. The data also shows that we are getting more and more of these large positive variations over the past 20 years.

Having spent a considerable amount of time shoveling my driveway this past winter and seeing my air conditioning bills increase the summer before, I was interested in trying to understand if these higher annual temperatures meant hotter summers, warmer winters or both. The tack I took was to look at cumulative temperature values, known as heating and cooling degree days, that are of great use to designers of heating and cooling systems for buildings. However, instead of looking at a single value for the year, I split the year into two periods – a winter or heating period from October to March which is normally when our home heating systems kick in and a cooling or summer period from April to September which is normally when we turn on our air conditioners.
But, before I present that data, allow me to explain the concept of heating degree days. To calculate the heating degree value, we take a reference temperature - normally 65oF (when we don't really need heating or cooling) - and then we subtract the average daily temperature from the reference temperature. For example, if the average daily temperature is 30oF then the heating degree value for that day is 65 - 30 = 35. There is a direct correlation between heating degree value and the energy we use to warm our homes. The lower the outside temperature, the greater the heating degree value and therefore the more energy we need to bring our home up to that reference temperature of 65oF.
Typically building engineers that size heating systems use cumulative heating degree days, amongst other factors, to size a heating system. To get a sense of the accumulated heating degree day numbers, consider the following example. If you have a month of 30oF days in the winter, then the total heating degree day (HDD) value for that month is 30 x (65 – 30) = 1050. If similar temperatures are experienced over a six-month period then the total number of HDDs is 6 x 1050 = 6300. This is a rather rough calculation for the six-month heating season as some days are a lot colder than the 30oF temperature I used, but, of course, some are warmer. Nevertheless, the HDD value of 6300 gives us an order of magnitude understanding of data in the figure below. This chart show the six-month total of HDDs for the October to March period for each year since 1895. The six-month HDD totals are plotted in blue. Even though there is considerable variation year to year, the long-term HDD average is 6240 which is close the approximate value we just determined. To give you a sense of how this numbers varies across the country, the equivalent number for Florida is 650 because the winter months down there are so much warmer. Clearly those folks down south are not spending a lot of time worrying about home heating, and they could probably get away with a nice thick sweater and a few extra blankets in winter.

I have also placed two trend lines over the data to draw out the long-term story. The first trend line, shown in red, is the simple linear average and it clearly demonstrates how the HDD value for the heating months has declined from 6500 to 6000. I have also overlaid a 20-year moving average which snakes above and below the linear trend line, but it too demonstrates the long-term decrease in HDD values. This long-term decrease indicates that our winters are getting warmer and that, as a result, we should be using less energy to heat our homes.
Having looked at our warming winters, my immediate next thought was; what about the summers? For the summer analysis, we use the concept of cooling degree days (CDDs). To determine the cooling degree days, we calculate the difference between the average daily temperature and the reference temperature, 65oF. So if the average daily temperature is 70oF then the cooling degrees for that day are 70 – 65 = 5. A month of similar days would give 30 x 5 = 150 CDDs for the month, and six months of similar days would lead to 6 x 150 = 900 cumulative CDDs. These numbers are a lot lower than the heating degree totals because they are mean daily temperatures and thus averages of cooler nights and warmer days. The actual numbers for NH are very much lower and the long-term average (1895 to 2012) for the six-month April to September period is 306 CDDs. For comparison purposes I determined that the equivalent number for Florida is 2500 CDDs. So, compared to the Florida folks, we need a lot less air conditioning but that appears to be changing as I will show. In the chart below I have plotted the long-term data for the six month accumulation of CDDs in blue as well as some trend lines. As you can see from the red linear trend line, the CDD average has increased over the 117 years of this data set. As with the with HDD chart, I have also included, in black, the 20-year moving average and again the upward trend is apparent. We have moved from a 20-year CDD average of 300 to a recent value of 350. Compared to Florida, this is no big deal, but for us that 17% rise represents a big fat increase in our air conditioning energy usage.
So if we put this data together it is clear we are, on a long-term trend basis, looking at warmer winters and hotter summers. This presents some challenges and perhaps opportunities if you are in the air conditioning business. A particular challenge for NH is that warmer winters mean less snow, a shorter skiing season and tough times for the ski industry. Some of this has been overcome with mechanical snowmaking, which is good for vendors of snowmaking equipment, but it does increase the industry's costs as snowmaking is a highly energy and water intensive process. Warmer winters do result in lower heating bills in the winter and reduced oil consumption which many home and business owners find helpful. This warming trend has likely contributed to the observed reduction of energy consumption in NH homes and business that I referred to in my Where have all the BTU's gone? post.
Reduced oil consumption is always welcome, but it has been replaced, in part, with increased air conditioning usage. One benefit of less heating and more cooling is that we are substituting oil for heating with electricity for cooling. Even though electricity is largely driven by natural gas and coal combustion, an increase in electricity demand does, in the long term, provide more opportunities for nuclear and renewable electricity production. I admit I am stretching here trying to find the tiny bit of silver lining on the big black cloud of global warming. The real concern is that as our winters warm and our summers heat up, we will have to deal with all the other consequences of climate change, including rising sea levels, more severe weather excursions, the spread of diseases and many others.
What can you and I do in the meantime? Well, the simplest and least expensive thing we can do right away is to better insulate our buildings as this will immediately reduce energy consumption in our homes and businesses. This would reduce energy consumption in both the heating and cooling seasons. If we all did this, it might help to slow down the long-term trend of warmer winters and hotter summers and in the process it might help to avoid some of those hot, humid days when it could get to be a hundred and ten in the shade.*
Until next time, remember to turn off the lights when you leave the room.
Mike Mooiman
Franklin Pierce University

(*A Hundred and Ten in the  Shade is a tune by John Fogerty from his Blue Moon Swamp album which received a well deserved Grammy for Best Rock and Roll Album in 1997. It is a slow tune that perfectly catches the listlessness and despair of hot, humid days when you have to go out and work in the fields. Just listening to it makes me break into a sweat)


  1. Very interesting discussion and analysis between cooling degree days and heating degree days. In addition, your charts also help create an deeper understanding regarding the effects and implications of global warming and its impact on the ski, HVAC and other industries.

    My only additional observation is that human behavior in NH will more likely react to excessive cold than excessive heat. For example, because of the relative extreme of cold in NH, people will use more fuel to heat. However, in this case, because of the relative enhanced warm(which will be less extreme in our NH area), people will be less likely to consume more energy to cool their homes. We see this play out through the limited number of people in NH who own central air units in their homes.

  2. This is a very nice info you have shared here regarding heating and cooling services, appreciate this work.


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