Extreme Minimum Temperature Models

The United States Department of Agriculture (USDA) has a plant hardiness map (http://www.usna.usda.gov/Hardzone/) based on average annual minimum temperature. Their map shows the lowest average temperatures that can be expected each year in the United States, Canada, and Mexico. It is based on the lowest daily temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. Zones 2-10 in the USDA map have been subdivided to represent 5 degrees F (2.8 degrees C) differences within each 10 degrees F (5.6 degrees C) zone. The light colour of each zone represents the colder section; the dark color, the warmer section. Zone 11 represents any area where the average annual minimum temperature is above 40 degrees F (4.4 degrees C). For details on the USDA hardiness map please go to the web site identified above. Please note our estimates of minimum temperature and the USDA zone may not correspond to that published on the USDA web site because different methods were used to generate the maps.

View Maps

There is interest by some in Canada's horticulture industry to compare the USDA minimum temperature zones to Canada's plant hardiness index. The Canadian index is based on 7 climate variables, not just minimum temperature. Some plants are traded between the two countries and there is sometimes a need to make such a comparison. The maps allow users to query points anywhere in Canada and get values. Users can zoom into any area and query maps. Information provided over Canada includes the equivalent USDA hardiness zone, the average annual extreme daily minimum temperature for the 1961-1990 period and the Canadian hardiness index for the 1961-1990 and 1930-1960 periods. Remember a Canadian hardiness index of 40-44 corresponds to zone 4a, 45-49 zone 4b; 50-54 zone 5a, 55-59 zone 5b and so on. For areas over the United States only the USDA zone is provided and the minimum temperature predicted from our models. We do not have North American-wide climate models for all the variables in the Canadian plant hardiness formula. In addition it would not make sense to implement that model in the United States because it was never calibrated to American conditions or plants. We also include a model for the 1974-86 period because this is the time period used on the USDA map. Users can then compare the models if desired. You can also compare observed and estimated values at particular stations.

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