Human activities, ecological niches and physical settings are affected directly by temperature on a daily basis. However the degree of influence of these phenomena are understood to greater advantage when they are studied on a monthly basis, or over a longer period such as a year. For purposes of comparing observations at a given station, as well as from station to station, inspection of mean annual values may be more expedient.

Degree Days

The concept of degree days has many applications, as follows: for business and home dwellers because of heating needs; for outdoor works such as industrial needs, construction, engineering projects, transportation, and agriculture; and for environmental and development concerns dealing with soil disturbance involving landslides, foundations and drainage. Degree days are defined as the amount that the daily mean temperature departs from some reference temperature, such that a one-degree departure from that reference is called a degree day.

Four types of degree days are illustrated: heating (H), frost (F), thaw (T) and growing (G). Heating degree days are based on the reference of 18°C, and relates to all the outside air temperatures whose daily means are below that value. These degree days are accumulated monthly and annually and, because of the importance to communities and businesses, the amount of fuel required for heating can be calculated from these totals. Frost degree days include those data in which the daily mean temperature falls below 0°C. The calculations from these 0°C measurements are important to outdoor industry mentioned above, because of the deleterious effect on humans and materials. Thaw degree days refers to those days whose mean daily temperature rises above 0°C. These degree days have important relevance to engineering projects, industrial and domestic construction because of foundation problems, transportation corridors and pipelines because of freezing and thawing, and to various practitioners involved in the study of ground disturbance due to thaw. Because all these activities take place in areas subjected to disturbance or dislocation of the active layer, which overlies the permafrost, the appreciation of frost degree days is essential in order to plan remedial measures that may assist in circumventing accidents, hazards, and perhaps disasters. Growing degree days, which are those degree days whose mean daily temperatures rise above 5°, are cited because of their relevance to agricultural practices. Applications of these calculations comprise an expedient means of determining the length of the crop season, and the likeliest times of the year for harvesting.

Compiled by: B.R. Pelletier
Geological Survey of Canada