Chill accumulation at the CIMIS stations serving California’s strawberry nursery districts.
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Butte Valley and the Klamath Basin.
How to read these numbers
Three chill models are in wide use for temperate fruit crops. They read the same hourly temperatures and disagree about what each hour is worth. This page charts two of them side by side because we have found no validation of any of the three against California strawberry nursery outcomes, and the comparison shows how much the answer depends on the model.
Charted · left panel
Chilling Hours
One hour, one unit
What it measures
Every hour with air temperature between 33 and 45°F counts as one. Nothing is weighted and nothing is subtracted, so the curve only climbs. It is the number Lassen Canyon publishes for Macdoel and UC ANR relays, in the same band.911
Where it came from
Deciduous-fruit rest studies of the 1940s. Bennett put the rest-breaking range at “between freezing and about 45°F”; Weinberger, working on peach, counted hours at or below 45°F, and his cultivar requirements became the reference.12
What it assumes
That every hour in the band is worth the same, and that hours outside it add nothing and take nothing away. The lower edge matters more than it looks: recomputed from the raw record, a 32°F floor runs 4 to 13 percent higher than 33°F across Macdoel’s eleven completed seasons. We use 33°F to match UC ANR and Lassen Canyon, and we say so wherever a total prints.
The 150–400 hour target shaded on the high-elevation charts is the desirable range for high-elevation transplants: below it, low plant vigor; above it, excessive vegetative growth. The source gives it as hours below 45°F without naming a lower edge, so treat it as a reference range, not a pass/fail line.78
Charted · right panel
Utah model
A running balance from the season start
What it measures
Each hour is weighted by temperature. Hours from 2.5 to 9.1°C (about 36.5 to 48.4°F) earn a full unit, the shoulders on either side earn half, and cold below 1.4°C (34.5°F) earns nothing. Warm hours debit the total: −0.5 above 15.9°C (60.6°F) and −1 above 18°C (64.4°F).
Where it came from
An empirical temperature-response table fitted to rest completion in ‘Redhaven’ and ‘Elberta’ peach in Utah. The paper does not propose a mechanism, and neither do we.3
What it assumes
Fixed exchange rates between cold and warm hours. Richardson’s own protocol starts the count the day after the fall’s most negative point, because he was predicting when peach rest completes. This page instead carries the raw balance from September 1, the start Lassen Canyon’s Macdoel ledger uses and UC ANR relays when it reports Macdoel chill hours “offset by” hours above 60°F, so the two can be read together. Neither start rule has been validated for strawberry. That choice is why the panel can sit below zero for weeks, and it is labelled on the chart.9
Not charted yet
Dynamic model
Two steps, one of them irreversible
What it measures
Cold builds up an intermediate state that warmth can destroy. Once enough of it accumulates, it converts irreversibly into a “chill portion” that no later warm spell can take back. Output is in portions, not hours, and the order of the hours matters, not just their count.
Where it came from
Peach work in Israel’s warm winters, where fixed hourly weights could not explain how the sequence and length of warm and cold cycles changed the outcome.4
What it assumes
That warm-spell reversal is real but bounded. It is the model warm-climate fruit and nut research now favors, including the California projections of declining winter chill.56
Not charted here yet because we have not found a validated portion target for nursery transplants; a Dynamic curve would be readable as weather but not yet as a decision. It is on the list.
Where the numbers come from
Hourly air temperature from the CIMIS station named on each card. Every figure on this page traces to one station and one model.12
The high-elevation clock starts September 1, the same date UC ANR and Lassen Canyon use; the frigo clock starts November 1. Lassen Canyon’s published Macdoel tally comes from an on-ranch thermometer, ours from a CIMIS station a few miles away, so the two will not match day to day.1110
Readings CIMIS flags as missing or as a sensor fault, its missing-data sentinel, and values outside −30 to 55°C are dropped, not patched. Readings flagged only as unusual are kept. Nothing is interpolated.
A season with less than 60 percent of its hourly record is left out entirely. One between 60 and 95 percent is drawn dashed and named in the card footer: its Chilling Hours total reads low, and its Utah balance can be off in either direction.
Completed seasons are held fixed during routine refreshes. The season in progress refreshes each morning, counting complete days through yesterday.