 The system was developed at the University of California, Davis by A. J. Winkler and Maynard Amerine.The system is based on both the hypothesis and observations that grapevines do not grow if the temperature is below 50 °F (10 °C).Each day during the growing season (assumed under the system to be April 1 through October 31 in the Northern Hemisphere; October 1 through April 30 in the Southern Hemisphere) are assigned growing degree-days according to the amount that the day's average temperature exceeds this threshold; one degree day per degree Fahrenheit over 50 °F (or with SI units, degrees Celsius over 10 °C is used).