Chill units

From Glossary of Meteorology
Redirect page

Redirect to:

chill unit

An index calculated from air temperature to estimate fulfillment of plant dormancy requirements and the ability to start springtime growth, especially tree fruits.

Chill units are calculated in hourly time steps and summed over a time period. Cold temperatures add to chill units as the index ranges from 0 at temperatures less than 1°C, to 1 near 7°C, and 0 at 14°C. Temperatures above 14°C reduce chill unit accumulation as the index falls to -1 at 21°C. A minimum total number of chill units dependent upon species and variety must be exceeded before normal springtime growth can begin. Chill hour calculations frequently require hourly data from late evening to early morning.

Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.