HEATING AND COOLING DEGREE-DAYS
AND WIND-CHILL

Objectives:

Weather, by definition, refers to the state of the atmosphere mainly in terms of its effect upon life and human activities. Perhaps the most noticeable aspect of weather to individuals is air temperature. Outside air temperatures can be such that energy usage is necessary to make interior living spaces comfortable. The chilling effect of temperature combined with wind on exposed flesh is another aspect of weather that people in colder climates must guard against.

After completing this investigation, you should be able to:

• Calculate the number of heating or cooling degree-days accumulated on a given day, and demonstrate the use of current data to determine the number of heating or cooling degree-days in selected locations.
• Describe the pattern of average annual heating-degree totals over the coterminous United States.
• Determine the wind-chill temperature based on temperature and wind observations.

Introduction:

1. During cool or cold weather episodes, fuel is consumed to make buildings comfortable living spaces. A useful indicator of fuel consumption for heating purposes is the determination of heating degree-days. They are calculated by accumulating one heating degree-day (HDD) unit for each Fahrenheit degree the daily mean (average) temperature is below the base value of 65 °F (18 °C). For example, a day with a maximum temperature of 70 °F and a minimum of 50 °F has a mean temperature of 60 °F. Subtracting 60 from 65 yields 5 heating degree-day units for that day. Hence, a day with a high temperature of 50 °F and a low of 30 °F produces [(10)(25)(30)(65)] heating degree-days (HDD).

The map in Figure 1 displays the average annual total number of heating degree-days accumulated at various locations around the country based on normals from 1981-2010. Assume that each location is at the center of the number plotted on the map. Determine the pattern of degree-days accumulated yearly by drawing on the map contour lines representing 2000, 4000, 6000, 8000, and 10,000 heating degree-days. Be sure to label each contour.

Figure 1. Average total annual heating-degree days.

2. Compare your map analysis to the map appearing in Figure 2, which is based on data from many additional locations. According to the Figure 2 map, Southern [(California)(Texas)(Florida)] has the lowest annual heating degree-day totals among the lower-48 states.

Figure 2. Average annual heating-degree day totals over the lower 48 states using a base of 65 °F.

3. Latitude, elevation, and nearness to large bodies of water are factors that influence the annual heating degree-day pattern appearing on the map you analyzed. Of these three factors, [(latitude)(elevation)(nearness to large water bodies)] is generally the most important for the vast majority of the coterminous U.S.

4. Examine Figure 3. Figure 3 shows the annual temperature curves for (A) San Francisco, CA, a maritime station and (B) St. Louis, MO, a continental location. They both have nearly the same annual mean temperature as represented by the dashed line. Draw a horizontal line across the graph representing the 65 F° isotherm. Although both locations are at about the same latitude, the city that accumulates heating degree-days during more months of the year is [(St. Louis)(San Francisco)].

Figure 3. Variation of average monthly temperature for (A) San Francisco and (B) St. Louis.

5. The city that records more heating degree-days annually is [(San Francisco)(St. Louis)]. (We suggest you locate San Francisco and St. Louis on the Figure 1 map to confirm your response. This result is typical for West Coast maritime locations compared to continental locations at the same latitude. Even though heating degree days may be accumulated at continental locations over a shorter part of the year, much lower temperatures during the cold part of the year cause rapid accumulation of heating degree-days.)

6. When the weather is too hot for human comfort, there is a need to cool air in living spaces. Cooling degree-days are calculated to estimate fuel needs for air conditioning when the day’s mean temperature rises above 65 °F. The calculation is made by subtracting 65 °F from the day’s mean temperature. On a day when the maximum temperature is 90 °F and the minimum temperature is 70 °F, the number of cooling degree-days (CDD) produced is [(15)(65)(80)(90)].

7. It takes only a short time outdoors in cool or cold weather for a person to realize that temperature and wind both play major roles in the rate at which the human body loses heat to the environment. The National Weather Service Windchill Chart (Figure 4) enables us to determine the wind-chill temperature index or wind-chill equivalent temperature (WET) based on combined effects of air temperature and wind speed. The WET is an attempt to approximate the rate of sensible heat loss from exposed skin caused by the combined effect of low air temperature and wind. However, when the air is calm and there is no additional heat-loss effect from wind, one would expect the WET to be [(higher than)(the same as)(lower than)] the existing air temperature. Actually, wind-chill temperatures are calculated only when winds are above 3 miles per hour.

Figure 4. NWS Windchill Chart.

8. According to the Windchill Chart, the WET is [(0)(–7)(–16)(–22)] °F when the temperature is 0 °F and the wind speed is 20 miles per hour (mph).

9. It can be seen from the Windchill Chart that above 35 mph, changes in wind speed have relatively little effect on WET values. At 10 °F, an increase in wind speed from 5 to 10 mph causes a 5-degree drop in the WET value. At the same temperature, an increase in wind speed from 40 to 45 mph reduces the WET by [(1)(3)(5)(6)] Fahrenheit degree(s).

For more information about windchill and to use a Windchill Calculator, go to NOAA’s National Weather Service Office of Climate, Water, and Weather Services Windchill website at: http://www.nws.noaa.gov/om/windchill/ ().

Heating and Cooling Degree Days and wind chill values are calculated from observational data collected at weather stations. Annual totals of Heating Degree Days are accumulated from 1 July through 30 June of the next year. Annual totals of Cooling Degree Days begin with 1 January and run through 30 December.

Figure 5 is a NWS map of maximum and minimum temperatures, in degrees Fahrenheit, at selected stations across the U.S. for 28 September 2013. (This was the same date that included the map of air mass advections that we investigated in the Applications section of Investigation 4A.) In color, the red upper number is the day’s maximum temperature and the blue lower number is the day’s minimum temperature at the station denoted by a green dot to the right of the minimum value. For the contiguous U.S., in general, temperatures in the southern and southeastern portion of the country were warm, while temperatures in the northwest interior of the country reflected the continental polar air mass that moved into that region.

Figure 5. Map of maximum and minimum temperatures at selected stations for the 24-hour period ending at 12Z on Saturday, 28 September 2013.

10. The highest maximum temperature on the map was at San Antonio and Corpus Christi, Texas. The maximum temperature there was [(89)(95)(99)] °F.

11. For the contiguous U.S., the reported lowest maximum temperature for this date was 38 °F. This maximum temperature occurred at [(Casper, WY)(Caribou, ME)(Duluth, MN)]. The minimum temperature there was 32 °F.

12. At the other extreme, the lowest minimum temperature in the contiguous U.S. was shown at Ely, Nevada and Cedar City, Utah. The temperatures were [(18)(24)(31)] °F. And, the highest reported minimum temperature for the period occurred at Brownsville and Corpus Christi, Texas, where it was 81 °F.

13. Based on Des Moines, Iowa’s maximum and minimum (87 and 67 °F, respectively) temperatures, the mean temperature was [(69)(71)(77)] °F.

14. The mean temperature derived from the maximum and minimum temperatures at Des Moines indicates [(heating)(cooling)] degree day units were accumulated.

15. The number of degree day units accumulated in Des Moines for this day was [(9)(12)(25)]. Recall from Figure 3 of Investigation 4A, Des Moines was located ahead of the approaching frontal system.

16. Casper, Wyoming, experienced a mean temperature of [(29)(35)(47)] °F. Casper was located in the cool air mass following the frontal system.

17. The mean temperature derived from the maximum and minimum temperatures at Casper indicates [(heating)(cooling)] degree day units were accumulated.

18. Therefore, Casper accumulated [(8)(16)(27)(30)] units on this day.

19. Philadelphia, in eastern Pennsylvania, with a high of 73 °F and a low of 56 °F, experienced a mean temperature of [(61)(65)(67)] °F. (Note: Following NWS practice for calculating HDD or CDD, if necessary, the mean daily temperature is rounded up to the nearest whole degree for reporting purposes.)

20. Philadelphia therefore [(did)(did not)] accumulate heating or cooling degree day units.

Consider the use of abbreviated August CDD data for the following item. Note the headings labeling the different columns in the list. For example, the first numerical column shows that Birmingham, AL accumulated 423 CDD units in August 2013 while the total for the season was 1525.

COOLING DEGREE DAY DATA MONTHLY SUMMARY

CLIMATE PREDICTION CENTER-NCEP-NWS-NOAA

MONTHLY DATA FOR AUG 2013

ACCUMULATIONS ARE FROM JANUARY 1, 2013

-999 = NORMAL LESS THAN 100 OR RATIO INCALCULABLE

STATE CITY	CALL	MONTH	MON	MON	CUM	CUM	CUM	CUM	CUM
		TOTAL DEV	DEV	TOTAL	DEV	DEV	DEV	DEV
		FROM	FROM		FROM	FROM	FROM	FROM
		NORM	L YR		NORM	L YR	NORM	L YR
							PRCT	PRCT
AL BIRMINGHAM	BHM	423	-32	-2	1525	5	-387	0	-20
MT BILLINGS	BIL	315	111	22	742	206	-150	38	-17
MT BUTTE	BTM	15	-42	-12	96	-25	-25	-21	-21
MT CUT BANK	CTB	97	3	12	196	17	-16	9	-8
MT GLASGOW	GGW	252	70	28	517	52	-135	11	-21
MT GREAT FALLS	GTF	167	60	-7	377	110	-104	41	-22
MT HAVRE	HVR	185	44	5	464	107	-37	30	-7
MT HELENA	HLN	214	114	22	525	261	-1	99	0
MT KALISPELL	FCA	100	38	-447	969	830	-427	597	-31
MT LEWISTOWN	LWT	104	-17	-5	235	2	-78	1	-25
MT MILES CITY	MLS	311	27	11	738	-15	-321	-2	-30
MT MISSOULA	MSO	192	93	36	479	233	61	95	15

21. The year to date and the month of August were generally warm in the western third of the country. A city typifying this was Kalispell (MT). Kalispell accumulated 100 CDD units for August (1st numerical column), 38 above normal (2nd num. column) and 969 cumulative total (4th num. column). These values are compared to last year’s monthly and cumulative normals in columns 3 and 6, respectively. The seasonal total CDD units are also shown as a percentage of the normal for the cumulative year in column 7 (next to last num. col.). Kalispell’s CDD are greater than normal (positive) for the entire season by [(59)(280)(597)(817)] percent through August. Ironically, while several other Montana cities were also above normal for the month and year, there were cities in the state that were below normal.

Returning to the course website, under the Climate section, click on the link to “Local NWS Offices”. Then click on the yellow circle of your closest NWS Office; the webpage of that NWS Forecast Office appears. (If that office’s webpage does not respond, seek another station that does respond.) In the menu along the left side of the page, under “Climate”, click on “Local”. This provides climatic information in various forms for one or more cities in the office’s service area. Choosing (1) Product: Daily Climate Report (CLI), (2) Location, (3) Most Recent (yesterday’s), or archived choice, then (4) GO. The new page will provide the requested day’s climatic data for the station showing maximum, minimum and average temperatures as well as heating and cooling degree days for that date and other periods including the month, meteorological season and year. You can see your local data (without having to do the arithmetic!).

HDD values and their effect on homeowners’ heating bills will likely be of concern for many in the country as heating costs through the remainder of the season may stress tight budgets. You might follow these HDD values as we progress through the remainder of winter and spring.

Go to: http://www.hpc.ncep.noaa.gov/dwm/dwm.shtml (). The NOAA-NWS Weather Prediction Center provides this archived series of maps for each day, titled “Daily Weather Map”. Click on the maps to bring up the active page. The column to the left allows you to select any date back to January 1, 2002. Call up the Daily Weather Map for 28 September 2013. Each day’s map series includes the generalized surface map, color-coded maps of maximum and minimum temperatures, mid-tropospheric flow at the 500-mb level, and total precipitation. The surface and 500-mb maps are for 12Z (7 AM EST) while the temperatures and precipitation maps are for the day. Detailed maps showing the data are revealed by clicking on each generalized map. Click on either the maximum or minimum temperature map for September 28, 2013 to display a map similar to the Figure 5 map albeit with more stations for the contiguous U.S.

22. Use the “Back to the Main Page” button to see the maps for 28 September. According to the large map, much of the central portion of the country experienced precipitation from that frontal system. Click on the small 24-hr Precipitation map to reveal a detailed map of selected precipitation totals. The greatest plotted total along the frontal boundary area in the central U.S. was at Fargo, ND where [(0.56)(0.98)(1.21)] inches occurred.

23. Heating and Cooling Degree Day units reflect energy needs to attain indoor human comfort. Outdoors the cooling effect of wind along with temperature on an individual is reflected by using the windchill equivalent temperature. Assume the Ely, NV, and Cedar City, UT, area in Figure 5 had a temperature of 25 °F and was experiencing a wind speed of 15 miles per hour. Using the windchill equivalent temperature (WET) Table in Figure 4, the WET for this combination of temperature and wind speed would have been [(‒5)(0)(10)(13)] °F.

Suggestion for further activities: High energy prices, along with increasing evidence of climate change due to human activity, highlight the need for greater awareness concerning energy use. Space heating and air conditioning are major consumers of energy (and financial resources). Tracking the accumulation of HDDs and CDDs where you live can lead to more informed decision-making and actions related to energy consumption for heating and cooling.

The HDD values you determined in this investigation are reasonably consistent in pattern with the national annual HDD map of the first part of the investigation. Tables of HDD for selected cities to the end of each current month can be found at: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/mctyhddy.txt () and tables of CDD at: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/mctycddy.txt ().