A Climatology of Heat Units for the Northeastern USA and Southeastern Canada

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 practices. More importantly, differences in heat unit calculation methods and temperature observations practices between the USA and Canada have 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 the regional distribution of seasonal heat units for five percentile levels (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 particular, efforts to assure the comparability of heat unit accumulations derived from different observation times and practices are discussed. The resulting heat unit maps across the USA indicate a general northeast-southwest alignment of the heat unit isopleths. This pattern mirrors the orientation of the Atlantic Coast and Appalachian Mountains. In Canada, a similar isopleth pattern is indicated in the inter-lake region of Ontario. Comparing heat unit accumulations across the US.-Canadian border, suggests that crop varieties (particularly corn and soybean [Glycine max (L.) Merr.]) bred to meet maturity requirements in southern Ontario, can supplement varieties currently grown across much of New York, northwestern Pennsylvania, and parts of central New England. Research QuestionInformation 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 practices. This study was conducted to produce an extensive and previously unavailable climatology of heat unit statistics covering the northeastern USA and southeastern Canada. The climatology depicts the regional distribution of seasonal heat units for five percentile levels (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 emergence and the five-leaf stage in corn are also presented. Literature SummaryThe selection of crop varieties, strategies for pest control, and scheduling of crop management practices are often based upon the average or “normal” number of heat units received at a specific location. These normals generally represent 30-yr averages that are routinely updated with each new decade. Although maps depicting the geographic distribution of GDD normals have been compiled on a statewide basis, a comprehensive regional climatology of heat units covering both the USA and Canada is lacking. Such regional information is essential for determining a site's suitability for new crop varieties, comparing crop development between different locations, and transferring applied research results across this international boundary. Study DescriptionThis climatology is based upon temperature observations from 640 stations encompassing the northeastern USA and adjacent portions of Canada for the 30 yr from 1961 to 1990. In developing the climatology, methods were employed to assure the veracity of the climatological data used to compile these summaries. In particular, the underlying data were (i) screened using an automated quality control routine, (ii) adjusted to compensate for differences in observation time and method, and (iii) estimated in cases where missing data could potentially influence subsequent heat unit calculations. Several of the resulting maps showing the regional distribution of specific heat unit statistics are included in this report. Applied QuestionsCan comparable heat unit accumulation statistics spanning the northeastern USA and adjacent portions of Canada be obtained? Despite different observation practices in the USA and Canada, adjustments can be made to heat unit accumulations allowing these values to be mapped across the U.S.-Canadian border. In addition, adjustments can be applied to compensate for differences in observation time among U.S. stations yielding standardized heat unit values that can be mapped on a regional basis. What is the normal seasonal heat unit accumulation that I can expect in my area and how will this value vary from year to year? Although average values have conventionally been used to define GDD normals, percentiles are presented in this study. Such values give information concerning the frequency of specific heat unit accumulations that cannot be obtained from the average. For example, knowing that the average seasonal accumulation of growing degree days at a site is 3100 gives no information concerning the number of seasons in which more or less than 3100 GDD can be expected. However, given the median accumulation is 3100, then any season has a 50-50 chance of exceeding this GDD total. Such information is useful in planning, especially if a range of percentile values is known. Can the Canadian soybean variety, OAC Shire, be grown in central New York state? Seasonal CHU accumulations are from 3200 to 3000 CHU across central New York. Since OAC Shire has a maturity rating of 2925 CHU, this variety could be successfully grown as an early maturing variety. RecommendationsThe regional heat unit maps presented as a part of this study, along with a more extensive suite of maps that is available from the authors, offer excellent guidance for crop selection and a variety of temperature-dependent crop management decisions. Heat unit maps which span the Canadian border and display heat units based on the Canadian calculation method provide essential information concerning the potential introduction of crop varieties developed in Canada into areas of the northeastern USA.