Models
All models require hourly recording of temperatures. The simplest model assigns one chilling unit for every full hour at temperatures below 45 °F (7 °C). A slightly more sophisticated model excludes freezing temperatures, which do not contribute to proper dormancy cycle, and counts only hours with temperatures between 32 °F (0 °C) and 45 °F (7 °C).
The Utah model assigns different weight to different temperature bands; a full unit per hour is assigned only to temperatures between 37 °F (3 °C) and 48 °F (9 °C). Maximum effect is achieved at 7 °C (45 °F). Temperatures between 55 °F (13 °C) and 60 °F (16 °C) (the threshold between chilling and warm weather) have zero weight, and higher temperature have negative weights: they reduce the beneficial effects of an already accumulated chilling hours.
Southwick et al. wrote that neither of these models is accurate enough to account for application of rest-breaking agents widely used in modern farming. They advocated the use of a dynamic model taylored to the two-stage explanation of dormancy.
Read more about this topic: Chilling Requirement
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