Degree Day

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City of Ames

Heating and Cooling Degree Days

Degree days in Ames are based on the assumption that when the outside temperature is 65°F, we don't need heating or cooling to be comfortable. Degree days are the difference between the daily temperature mean, (high temperature plus low temperature divided by two) and 65°F. If the temperature mean is above 65°F, we cool it to 65°F by subtracting 65 from the mean.  The result is Cooling Degree Days. If the temperature mean is below 65°F, we heat it adding degrees back to reach 65.  The result is Heating Degree Days.

Example 1: The high temperature for a particular day was 90°F and the low temperature was 66°F. The temperature mean for that day was:

( 90°F + 66°F ) / 2 = 78°F

Because the result is above 65°F:

78°F - 65°F = 13 Cooling Degree Days

Example 2: The high temperature for a particular day was 33°F and the low temperature was 25°F. The temperature mean for that day was:

( 33°F + 25°F ) / 2 = 29°F

Because the result is below 65°F:

65°F - 29°F = 36 Heating Degree Days

Calculations like those shown in the two examples are performed for each day of the year and the daily degree days are accumulated so we can compare months and seasons. The degree day chart below shows monthly totals since 1997, along with the monthly and yearly average.  Averages have been recorded by Electric Services for over 30 years.   

Heating Degree Day Totals For Ames, Iowa. (65°F Base)

  Updated: 5/8/17

 

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

 2015

 2016  2017
AVERAGE

Jan.

1402

1349

948

1298

1421

1519

1536

1522

1132

1291

1553

 1279   1367
  1231

1359

Feb.

1162

883

1061

1322

1322

1023

1284

1119

984

1085

1396

 1407   1038
   807

1107

March

677

803

816

655

937

783

753

881

415

1058

1029

   762
   395
   836

837

April

309

271

265

497

498

453

230

471

327

560

496

   356
   400
   368

442

May

110

167

138

55

160

121

164

192

66

214 

164

   135
   166
 

151

June

12

0

11

0

0

9

0

11

8

31 

1

     7
     0
 

17

July

0

0

0

0

1

3

0

0

0

7

     5
     0
 

2

      Aug.
    17      0     1     0     0     17      0      0
     8     0     0     14
    10
           8

Sept.

19

35

108

68

65

33

65

166

109

42

114

 
    48
    38
 

99

Oct.

313

363

486

260

365

554

269

320

453

 414

378

   341
   258
 

389

Nov.

644

705

705

780

796

570

755

723

704

905

1040

   644
   568
 

803

Dec.

1077

1344

950

1311

1408

1354

1348

1051

1146

1456

1118

   969
  1238
 

1239

Total:

5742

5920

5489

6246

6973

6439

6404

6456

5352

7064

7296

  5967
  5478
 

6450

 

 

Cooling Degree Day Totals For Ames, Iowa. (65°F Base)

Updated: 5/8/17

 

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

  2015  2016 2017

Average

Jan.

0

0

0

0

0

0

0

0

0

0

0

0

      0
    0
 0

0

Feb.

0

0

0

0

0

0

0

0

0

0

0

0

      0     0
 0

0

March

0

2

0

0

9

0

0

3

0

20

0

0

      0
    8
 0

1

April

27

26

22

19

22

2

6

25

4

17

7

11

      3
   11
 6

13

May

30

116

43

96

115

42

44

99

79

131

85

98

     53
    61
 

69

June

191

204

308

269

251

209

210

243

217

288

226 

213

    216
   334
 

228

July

333

306

392

412

374

338

199

380

461

522

330 

201

    298
   312
 

339

Aug.

350

183

282

292

374

245

208

383

289

269

339 

283      

    208
   278
 

271

Sept.

122

184

178

43

146

84

77

86

72

87

185 

84 

    202
   158
 

110

Oct.

28

6

41

28

43

14

0

19

28

6

24 

   0
     11
     21
 

15

Nov.

0

1

0

1

0

1

0

0

0

1

0

      1
      3
 

0

Dec.

0

0

0

0

0

0

0

0

0

0

0

      0
      0
 

0

Total:

1081

1028

1266

1160

1334

935

744

1238

1150

1341

1196

890

   992
   1186
 

1047

HOW TO USE DEGREE DAYS: The most common use of degree days is for tracking energy use. Without degree days, comparing the energy used over two periods would be analogous to calculating the miles per gallon rating for your car without knowing how far you had driven. If you wanted to know if the attic insulation you added over the summer was saving energy, you would use your energy bills to determine how much "fuel" was used before and after the retrofit. Then, using the degree days, you could determine "how far you went" during those periods. Instead of calculating miles per gallon, you would determine kilowatt hours (kWh's) per degree day or therms of natural gas per degree day.  This type of analysis can give you a rough idea of the impact weather had on your energy bills.

OTHER FACTORS: When comparing energy use, you may get a better idea of actual heating and cooling costs if you account for other energy sources in your home.  Isolating heating and cooling energy can be accomplished by examining the energy used during temperate months, such as May and October, when little heating or cooling energy is used. The energy used during these periods reflect your base monthly consumption. Subtracting the base use from the total consumption during a winter month will yield an estimate of the energy used just for heating. Subtracting the base use from a summer month will provide an estimate of cooling energy.  It is also important to consider the usage period reflected in your energy bill. Your meter is probably not read on the first day of each month and therefore will not be for the same time period as the degree day totals. You can allow for this by comparing your energy bill over a longer period, such as an entire heating season or several months.

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