What are heating degree days and cooling degree days?

Heating degree days are indicators of household energy consumption for space heating. It was found that for an average outdoor temperature of 65 degrees Fahrenheit, most buildings require heat to maintain a 70 degree temperature inside. Similarly, for an average outdoor temperature of 65 degrees or more, most buildings require air-conditioning to maintain a 70 degree temperature inside.

How heating and cooling degree days are computed

Take the high and low temperature for the day, and average them. If this number is greater than 65 F, then we have (Average temperature - 65) cooling degree days. If the average temperature is less than 65 degrees, then we have (65 - Average temperature) heating degree days. Running totals are kept for these units over a time period of a year so fuel distributors and power companies can assess average demands.

Growing Degree Days
Courtesy of Michigan State University Extension

Growing degree day accumulations involve the amount of accumulated heat required for insects and their host plants to flourish. Growing degree days are those days necessary for these organisms to complete their growth and development. Insects are unable to control their body temperature and are dependent upon the temperature of their surroundings for warmth, thus the measure of temperature can allude to the existence of insects.

The use of growing degree day (GDD) is becoming a more popular way to determine when to control insect pests that attack ornamental trees and shrub. It is hoped that accumulated growing degrees will become a guide in timing the application of a pesticide to control a specific pest more accurately. This information can also be very useful in monitoring landscapes for plant problems.

The determination of growing degree days is very similar to that of the heating degree days used by a company selling home heating fuel to determine the schedule of the next shipment of fuel oil. GDD takes into account the average daily temperature accumulations, which influence insect development; providing an estimate of the insect's development based on temperature measurements. For each day that the average temperature is one degree above the base temperature, one degree day has accumulated. Due to temperature differences, insect development may vary from year to year and among locations in any given year; basing pesticide applications by a particular week on a calendar cannot take these variations into consideration.

The formulation of GDD calculations is developed from a general base of knowledge inherent to the environment. The temperature at which growth starts for woody plants in the north-eastern United States is approximately 45 F t0 55F; to standardize the calculations used in determining a growing degree day, the base temperature has been arbitrarily set at 50 F. With this information, the calculations of the growing degree day for a 24-hour period require the following formula: Max. temp + Min. temp./2 -Base temp.(50)=GDD. For example: If on March 3 the maximum temperature is 60 and the minimum temperature is 50 the GDD for March 3 is 60+50/2 = 110/2 = 55F and 55F - 50F = 5 GDD. If the average temperature is equal to or less than the base temperature, no degree days are accumulated.

For this system to work, the maximum and minimum temperatures need to be taken every day from March 1 to September 30. Early in the season the growing degree days will accumulate slowly; however, as temperatures rise they accumulate faster.

Growing Degree Information courtesy of Michigan State University Extension.