 While weather stations today can average data to an hour, a minutes or even seconds, historical data used to calculate growing degree-days has been done mostly on daily or monthly averages (the table above was done using monthly climatological normals).Shorter term averaging to minutes, or more commonly hourly, arguably better reflects the true thermal effects on the crops, but will result in growing degree-day values that are lower than both daily and monthly.Monthly averaged data can be very problematic as it can underestimate heat accumulation during the first and last months of the growing season.