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Fruit & Nut Research and Information Center
Fruit & Nut Research and Information Center
Fruit & Nut Research and Information Center
University of California
Fruit & Nut Research and Information Center

Chilling Accumulation Models:
Their Calculation, Explanation, & Comparison

update Jan.11, 2015

Regarding use of the Parlier 039 station for Chill Portion values, subtracting 12 Chill Portions from given value is suggested. For Chilling Hours < 45, using the Fresno State station is suggested. The reason for this: CIMIS station temperature data was low previous to a re-calibration on Dec. 24.

Using Various Models

  • Calculations or Current and Historic Chilling Accumulation at California CIMIS Sites

Cumulative Chilling Hours:
Cumulative Chill Hours and
Modified Chill Hours
Hours below 45° F
Hours between 32 F and 45° F
November 1 through February 28/29

Cumulative Chilling Portions:
Dynamic Model
September 1 through August 31

Chilling Hour & Unit Accumulations:
Historic Accumulation of
Chill Hours, Modified Chill Hours,
& Chill Units (Utah Model)

Hours below 45° F
Hours between 32° F and 45° F
September 1 through August 31

Dynamic Model & Chill Portions Accumulation Guide

  • A how-to guide (pdf) by Kitren Glozer, retired Project Scientist, Dept. of Plant Sciences, for calculating chill portions (including converting 10 and 30 minute data to hourly data for the model), using weather data from a data logger in your orchard, and the Dynamic Model (Excel file). Both available for download.

Chilling Accumulation Models: Their Explanation and Comparison

  • Chill Hour, Chill Unit & Chill Portion: Model Comparisons
  • Fruit & Nut Crop Chill Portion Requirements: Chill Requirements in Chill Portions, calculated using the Dynamic Model. Because the Dynamic Model is still unfamiliar to many, to provide a frame of reference, a list of chilling requirements for many temperate tree crops has been compiled from the literature. (compiled by Katherine Jarvis-Shean, Dept. of Plant Sciences, UC Davis)
Page Last Updated: January 12, 2015
Webmaster Email: fruitsandnuts@ucdavis.edu