Chill Portions and the Dynamic Model

Click on the link at the left "2017/18 Chill Portions - Delta Region" to see current and past chill hour data.

Explanation for using chill portions instead of chill hours:

Weather in California, like many other parts of the world where winters are mild 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.

"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. However, 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, a canceling effect can occur for some of that chill; 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.