Understand your weather
Chill Hours
A way to track accumulated exposure to winter cold.
Chill Hours measure time spent within a defined cold-temperature range. They help describe winter chilling in the context of dormancy and seasonal development of many temperate perennial plants.
A cold moment
How cold is it right now?
Accumulated exposure
How long has it been in the cold range?
At a steady 5°C, one hour contributes 1 Chill Hour; twenty hours contribute 20 Chill Hours under Corydal’s default model. The temperature is the same, but the exposure is very different. Equal hours in this simple model do not imply equal biological effects for every plant.
What temperature counts?
Corydal’s current default range for the simple Chill Hours model is 0°C to 7.2°C, inclusive. Time at either boundary counts. Temperatures below or above the range add no Chill Hours.
10°C
Above range · no added time
7.2°C
0°C
Upper boundary included
Time in this band counts
Lower boundary included
−2°C
Below range · no added time
More time in the band → more Chill Hours
This range is a calculation setting, not a universal biological definition of chilling. Other methods and applications may use different limits. Always read a total alongside its model, temperature range and period.
How Corydal calculates Chill Hours
Corydal adds real elapsed time between observations. It assumes each reading’s temperature stays constant until the next reading. If that starting temperature is in the configured range, the interval contributes its duration in hours.
02:00
5.1°C
02:05
5.0°C
5 minutes of estimated chilling
≈ 0.083 Chill Hours
5 ÷ 60 hours, using the 5.1°C starting reading.
The 02:05 reading closes this interval.
Two observations do not mean two Chill Hours. Likewise, thirteen readings spaced five minutes apart span one hour and contribute 1 Chill Hour when every interval starts in range. The final reading adds no further time on its own.
This is an estimate between measurements. If the next reading were 10°C instead, those first five minutes would still count: the calculation uses 5.1°C until that next observation, without estimating the temperature’s path between them.
Why winter chilling matters
Many temperate perennial plants enter winter dormancy. Sufficient chilling helps release that dormant state so buds can resume normal seasonal development when conditions allow. Needs vary by species and variety; there is no single target for all fruit trees.
Dormancy and seasonal development
Chilling records provide context for dormancy progression, leaf emergence and flowering. A total alone cannot tell you exactly when a bud will open.
Orchards and fruit production
Insufficient chilling can be associated with irregular bud development and flowering. Interpretation needs the right model and locally appropriate information about the variety.
Winter comparisons
Compare equivalent seasonal periods using the same model and settings, while checking coverage. A recent seven-day result does not represent a whole winter.
One part of the picture
Subsequent warmth and other growing conditions also shape plant development. Combine chilling estimates with observations of the plants themselves.
Chill Hours and Chill Portions are different measures
Winter chilling can be estimated in several ways. Models make different assumptions about which temperatures are effective and how cold and warm periods interact, so the same weather can produce different results.
- Simple Chill Hours · used by Corydal
- Adds time within a defined temperature range. Every hour in range contributes equally; hours outside it add nothing and do not subtract previously counted hours. The result is expressed in hours.
- Utah Model · another established approach
- Assigns different weights to temperature bands. Some temperatures add more chilling than others, and warm temperatures can subtract chill units. Its result is expressed in Utah Chill Units.
- Dynamic Model · Chill Portions
- Represents chilling as a two-step process. Temperatures affect an intermediate accumulation that can be lost in warmth; once enough builds up, it becomes a stable Chill Portion. The sequence of temperatures matters, not just time below a threshold.
500 Chill Hours and 50 Chill Portions are not interchangeable totals.
These illustrative numbers use different models and units, not a conversion rule or a plant requirement. Match a published chilling requirement to the model used to estimate it.
Corydal currently implements simple Chill Hours only. Utah Model, Dynamic Model and Chill Portions are explained here for context; they are not available Corydal features.
Temperature history matters
A missing block of readings can hide warming or cooling and make the total uncertain. Corydal’s duration model holds the earlier temperature until the next observation, even across a long gap. That can overestimate or underestimate chilling and make coverage look more complete than the sampling really is.
- Complete
- The full requested duration is covered under the held-temperature assumption. This does not guarantee continuous sampling or sensor accuracy.
- Provisional
- Some, but not all, of the requested duration is covered. The total reflects only the duration analyzed.
- Missing data
- Without enough readings to measure a valid interval, Insights shows “Insufficient data”. A missing estimate does not establish zero chilling.
Corydal’s methodology
Corydal uses saved weather-station temperature observations within the requested period, in time order. It sums the elapsed hours between consecutive readings whose starting temperature is valid and inside the configured inclusive range, currently 0–7.2°C by default.
Insights normally shows the previous seven completed days in the station’s local calendar and timezone. If a valid station timezone is unavailable, Chill Hours uses a rolling seven-day period displayed in UTC. Duration always uses actual elapsed time, including across daylight-saving changes.
No time is estimated before the first observation or after the last. A missing or invalid temperature leaves the interval starting at that observation uncovered; the preceding interval still uses its own starting temperature. A lone reading cannot establish a duration.
Coverage is the duration of intervals starting with a valid temperature, whether inside or outside the chill range, divided by the requested duration. The total is not scaled to compensate for uncovered time. No extra readings are fabricated and no linear interpolation is applied, but there is no maximum gap limit: the earlier temperature is held until the next observation.
References & learn more
Background on dormancy, chilling models and their interpretation. External methods may use different ranges, seasonal periods and units from Corydal.
- UC Davis Fruit & Nut Research & Information Center — About Chilling Hours, Units, and Portions
- UC Davis — Evaluation of chill models from historical rest-breaking spray experiments on ‘Bing’ Sweet Cherry (PDF)
For the warmth side of seasonal development, learn about Growing Degree Days.
From understanding to observation
See winter cold through weather-station data
Corydal turns saved weather-station observations into accumulated Chill Hours and other environmental Insights. Explore the current station’s results, analyzed period and data coverage.
Explore station Insights →