A "heating degree day" is a measure of weather coldness. It's defined to be the difference between the outdoor ambient temperature and 65 degrees F, but has a value of zero when the ambient temperature is above 65 degrees. This difference is averaged over time and multiplied by the number of days in the time period covered. The heating degree day is often used as a measure of the demand for domestic heating in a locale.
A data frame
HDD_Minneapolis with 1412 rows and 4 variables:
year the year
month the month
hdd the number of heating degree days for that period.
loc the location at which the temperature was measured. In the early years, this was downtown Minneapolis. Later, the site was moved to the Minneapolis/Saint-Paul International Airport.
These data report monthly heating degree days. For teaching purposes, the data give an extreme example of how a relationship (hdd vs year) can be revealed by including a covariate (month). Although interest focusses on the change in temperature over the century the data cover, there is such regular seasonal variation that no systematic trend over the years is evident unless month is taken into account.