Knowledge of the temperature pattern of a place makes it possible to estimate various thermal loads. Of particular interest and in standard use are heating, freezing, and thawing loads measured in degree days per year.
The freezing index (degree days below 32°F) provides a basis on which to estimate the depth of annual ground freezing or ice thickness on bodies of water.
The thawing index (degree days above 32°F) provides a measure of ground thaw during the summer.
Heating degree day, measured below 65°F, provide the basis for developing heating fuel requirement estimates.
Degree days are a cumulative measure of the duration and magnitude of a thermal load. Note that this thermal load can be measured either above or below a given threshold. For example, a larger value for heating degree days indicates a colder region requiring more energy to heat a building to 65°F. Conversely, a larger value for thawing index indicates a warmer location, with greater cumulative above-freezing temperatures and greater summer thaw.
For design purposes it should be noted that all these indices, historical and projected, are estimates only, and can be expected to vary from year to year.