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Jeremy Michelson

Calculating Degree Days with Wolfram|Alpha

July 2, 2010 —
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Sunday is the United States’ Independence Day, and one of the hottest days of the year in this part of the country. Many Americans will celebrate the day with outdoor activities such as barbecues, parades, and fireworks. Chances are that after all the corn on the cob and fun in the sun, they’ll be looking to celebrate with some air conditioning, too! All that cooling will require a few degree days!

What’s a degree day? A degree day quantifies the amount of heating or cooling required to heat or cool an inside space.

Suppose you want to maintain an inside temperature of 65°F. This 65°F is called the base temperature. (65°F might sound cool, but this artificially low number is used because the actual temperature in the building will be raised by bodies and other inside sources of heat.) If the weather forecast for Champaign is as hot as expected for U.S. Independence Day—definitely above 65°F—then you’ll need to cool the building. The amount of cooling required is the difference between the base temperature and the outdoor temperature, multiplied by the time over which the temperature is different. If it is cooler outside than 65°F then you’ll need to heat the building, again by an amount equal to the product of the temperature difference and the time.

To make sense out of that, just type “degree days” into Wolfram|Alpha.

Degree days formula in Wolfram|Alpha

The temperature history pod contains a plot of the temperature over the time period of the calculation—one month back by default. If you are used to using Wolfram|Alpha to check the weather this plot should look familiar, but with some differences. The horizontal red line across the plot is the base temperature. The part of the plot that is above the red line is shaded in blue. That’s because when the temperature is above the base temperature, you have to cool the building. The number of cooling degree days is the area of the blue region. Similarly, the number of heating degree days is the area of the red region, which extends from the red baseline down to temperatures below the base temperature.

In North America it is typical to use a base temperature of 65°F for both heating and cooling, but in Europe one often uses a base temperature of 15.5°C for heating and 18°C for cooling. Wolfram|Alpha lets you use any base temperatures you want.

degree days July 4, 2008 champaign, heating base temperature 65F, cooling base temperature 70F

Wolfram|Alpha also shows you how the number of degree days changes with the base temperature. The “Base temperature dependence” pod shows us that two years ago on July 4, the number of cooling degree days is more than halved by using a base temperature of 70°F instead of 65°F. This gives you an idea of how much energy you can save by raising the thermostat in the summer.

Degree days aren’t used for just heating and cooling. Degree days are also used in agriculture. Plants need warmth to grow. Cold-blooded insects and other pests that feed on crops also thrive in the heat.

For example, roughly speaking, corn requires about 2800 degree days to reach maturity, using a base temperature of 50°F. So the hotter it is, the quicker the corn will mature—but the more air conditioning we humans will want. This means that a growing degree day is the same as a cooling degree day! The results pod below shows that there have been 1470 (Fahrenheit) degree days since April 1. As that’s about half of 2800, it looks like any corn planted here in Champaign in early April is about halfway to maturity.

Growing degree days in Champaign Illinois since April 1

Think about that as you eat your corn on the cob while celebrating the Fourth of July. Assuming you’re having corn and not partaking in a hot dog eating contest.



Posted by ELGAR ESOTS July 2, 2010 at 8:25 pm

    Thank you for bringing that to our attention. The day has been updated. We’re thrilled to hear you are enjoying Wolfram|Alpha. We hope you’ll visit the site and this blog often!

    Posted by The Wolfram|Alpha Team July 2, 2010 at 9:39 pm

Do you have some estimate for this Years World Overshoot Day (, This might be influenced by temperatures as well.

Posted by Qimet888 July 3, 2010 at 2:50 am

this is my first time visit to wolfdram, i would say it is even better than google in some fields,like search info on same page,no out going links,i really love it but thhis search engine is lacking one thing..proper name .which can be remembered by people ..any way i love it

Posted by Mark July 4, 2010 at 4:42 am

What is wrong with your long term weather data?

Every time I put in a city and weather then choose all time graph it has missing data from about 1940 to 1960 and even has data point that skew the graph to be too low at the beginning and gives a false linear trend! Every city I have tried! Very frustrating

Posted by Sylvain August 13, 2011 at 2:14 pm

    @ Sylvain –
    You are correct. That particular data was not provided by our source. We continue to work on gathering all the past weather data possible and hope to have the missing years soon.

    Posted by The Wolfram|Alpha Team August 17, 2011 at 2:39 pm