A CLIMATOLOGY OF HEAT UNITS FOR THE NORTHEASTERN USA AND SOUTHEASTERNCANADA

Information concerning the frequency of seasonal and monthly heat unit accumulations is essential in a variety of applications ranging from the selection of crop varieties to the scheduling of crop management p ractices. More importantly, differences in heat unit calculation metho ds and temperature observations practices between the USA and Canada h ave impeded the transfer of knowledge regarding variety trials across the border. Recently, a regionwide climatology of heat unit statistics has been developed for the northeastern USA and southeastern Canada. Using a high-quality set of climatic data, this climatology depicts th e regional distribution of seasonal heat units for five percentile lev els (10th, 25th, 50th, 75th, and 90th) and three calculation methods ( base 50, 50-86 method, and corn heat units [CHU]). In addition, median monthly heat unit accumulations and median days until corn (Zea mays L.) emergence and the five-leaf stage in corn are also presented. This study details the procedures used in developing the climatology. In p articular, efforts to assure the comparability of heat unit accumulati ons derived from different observation times and practices are discuss ed. The resulting heat unit maps across the USA indicate a general nor theast-southwest alignment of the heat unit isopleths. This pattern mi rrors the orientation of the Atlantic Coast and Appalachian Mountains. In Canada, a similar isopleth pattern is indicated in the interlake r egion of Ontario. Comparing heat unit accumulations across the U.S.-Ca nadian border, suggests that crop varieties (particularly corn and soy bean [Glycine max (L.) Merr.]) bred to meet maturity requirements in s outhern Ontario, can supplement varieties currently grown across much of New York, northwestern Pennsylvania, and parts of central New Engla nd.