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.

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. More »