Degree days are measures of how cold or hot an area is. A degree day compares the average outdoor temperatures recorded for an area to a standard temperature, usually 65° Fahrenheit in the United States. This is based on the assumption that when the outside temperature is 65°F, we don't need heating or cooling to be comfortable. The more extreme the outdoor temperature, the higher the number of degree days. A high number of degree days usually results in higher levels of energy use for space heating or cooling.
Lynx only uses Heating Degree Days (HDD) which are a measure of how cold the temperature was on a given day or during a period of days. For example, a day with an average temperature of 40°F has 25 Degree Days (65°F - 40°F). Two such cold days in a row have a total of 50 Degree Days for the two-day period. Lynx can then use the data to calculate effeciency and predict future return dates.