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

Chilling Accumulation Models:
Their Calculation, Explanation, & Comparison

Update 1/2/2018: #138 Famoso was vandalized 1/13/16, and will not be repaired. Stations #167 Tracy and #161 Patterson are inactive. #167Tracy has been terminated and the station is scheduled to be removed. #161 Patterson is no longer active because the site is no longer suitable for measuring reference evapotranspiration. CIMIS is in the process of finding a more suitable location. Station #047 Brentwood have not registered data since 10/24/17. It has a communication issue which should be fixed soon and the missing data will be backfilled. Station #182 Delano have not registered data since 11/6/17. At #182 CIMIS have been experiencing communication problems with the station and the station cooperator has been notified. The phone line will be repaired soon by CIMIS.

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 model
Hours between 32 F and 45° F model
November 1 through February 28/29

Cumulative Chilling Portions:
Dynamic Model or Cumulative Chilling Portion 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 model
Hours between 32° F and 45° F model

Utah model

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 2, 2018
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