Utility companies in the United States use the term “cooling degree days” to refer to the number of days when it is likely that air conditioners will be turned on, and “heating degree days” to determine when furnaces are likely to run. Because it would be impractical to visit every home and business to see whether the air conditioner or furnace was running, utility companies assume that customers are most comfortable when the temperature is 65°F (18.3°C) and that they will run their air systems accordingly. A “degree day” is not actually a 24-hour period. Rather, it is a measurement of the difference between the optimum of 65°F (Fahrenheit is used as the standard because it is a U.S. system) and the mean (average) temperature of a particular day. So, for example, if the mean temperature in Dallas, Texas, on a warm July day is 85°F, the utility companies will count that as 20 cooling degree days (85°F – 65°F = 20 cooling days). The states with the most annual cooling days on average are Florida, Texas, Arizona, California, and other southern states that, on average, rack up about 4,000 cooling days annually. Heating degree days use the same concept to calculate furnace use. As one might imagine, northern states log more heating degree days than southern states.