A method is shown for chlculating long-term kelvin-day values of accumulated temperature departure by first ordering the temperature data and then adding up kelvin-day values from the low-temperature end of the data such that one summation gives a complete table from which values below any base can be read or interpolated. An alternate form using two summations is also given. A partial table for Fort Collins is given as an example. The tables are generated before the base is chosen. The user selects a base according to need after all the processing is done. Freezing kelvin-day values can be read from the heating table without further processing. The method is applicable to cooling kelvin-days as well. Growing and thawing kelvin-day values can be read from the cooling table.