Chill Portions and the Dynamic Model
For weekly chill portions in 2014-15 and the last 5 years, click on "Chill Portions - Delta Region" on the left. Data are from the Lambert Road and Andrus Island weather stations (Sacramento County). In Dec. 2013 we had severe cold weather that lasted more than a week. However, we also had warm, sunny days through much of the winter, leading to low chill portions and unusual effects on flowering and fruiting. But the warm temperatures during cherry bloom in March 2014, possibly in addition to low chill, had devastating effects on fruit set.
California, like many other parts of the world where winters are warm with cycling warm and cold periods, often provides too little chilling for optimum dormancy and rest-breaking for higher chill-requiring fruit and nut crops. Research to overcome lack of chilling has focused on the use of rest-breaking agents that partially compensate for lack of chill accumulation, and how to measure chill accumulation under California’s climatic conditions.
"Chill hours" is the number of hours equal to or less than 45°F accumulated over the dormant season. Chill accumulation can be calculated using various mathematical models, the simplest of which is the chill hours model. Not all "chill" is effective, and this is one problem with the chill hour model. Also, when chill hours alternate with temperatures above 45°F, such as is common in fall and winter in the Central Valley, a canceling effect can occur for some of that chill and there is no way to measure this cancellation with the chill hour model.
The Dynamic Model is a way to compensate for the temperature fluctuations. The model calculates chilling accumulation as "chill portions" using a range of temperatures from ~35-55°F, and also accounts for chill cancellation by fluctuating warm temperatures. It begins Sept. 1 instead of Nov. 1 for chill hours.
Chilling Accumulation Models: Their Calculation, Explanation, & Comparison. This web site describes various chill models and provides both current and historic chill hours and chill portions for various locations throughout California. Developed and managed by the UC Fruit & Nut Research & Information Center.