The annual number of heating degree days (HDDs) is a proxy measure of heating use (i.e., number of days requiring "heating"). This metric is reflective of ongoing climate change as a regular decrease in HDDs in an area indicates that the city is consistently warming. Most places in the world require some heating on some days, so the change over time is more telling in regards to climate. This value is the sum of the daily differences between the threshold temperature (15.5°C) and the mean outdoor temperature, when the mean outdoor temperature is lower than 15.5°C, so two days in a row with a temperature of 10°C would provide a HDD value of 11. This means that more weight is given to days that are much lower than the threshold temperature than days with mean temperatures barely below the threshold, and the total annual HDDs (what this metric reports) can far exceed 365.