About Custom Chilling Hours Tool
Description:
The Custom Chilling Hours Tool is a product developed by the Midwestern Regional Climate Center (MRCC) in collaboration with the USDA Midwest Climate Hub that lets users track chilling hour accumulations for locations across the United States using customized temperature thresholds and time periods. This tool can be used to answer such questions as:
- How many chilling hours accumulated from September 1, 2022, to March 31, 2023, at a selected location based on temperature bounds of 32°F and 45°F? How does that compare to previous seasons?
- How does the number of chilling hours accumulated at a selected station compare to other stations in the United States?
Definitions:
- Lower Temperature Bound: The lower-bound temperature used in the calculation of chilling hour accumulation. The bound is inclusive.
- Upper Temperature Bound: The upper-bound temperature used in the calculation of chilling hour accumulation. The bound is inclusive.
- Accumulation Start Date: The start date of the chilling hour accumulation period.
- Accumulation End Date: The end date of the chilling hour accumulation period.
- Chill Hour Accumulation: The number of hours when the observed temperature was between the lower temperature bound and the upper temperature bound during the period between the accumulation start date and the accumulation end date.
How to Use the Tool:
Each dot on the Chilling Hour Accumulation map corresponds to a station with a high-quality record that reports hourly temperature observations. The dots are colored based on the chilling hour accumulation for the user-selected temperature bounds and accumulation dates. Hovering over a dot shows a tool-tip that displays the station name, chilling hour accumulation, selected temperature range, and selected date range.
Date Ranges: Chilling hours are primarily accumulated during fall, winter, and spring. Therefore, the chilling hour accumulation period uses a July 1 to June 30 calendar scheme, as opposed to a standard calendar year. Selected dates must be between July 1 of a selected year and June 30 of the following year, and the range cannot exceed a one year period. For example, you CAN set the accumulation range from October 1, 2021 to February 20, 2022, but you CANNOT select the range from February 20, 2021 to October 1, 2021.
The map will not display data if date or temperature selection rules are not followed. When adjusting the temperature and date thresholds, please note the following requirements:
- The lower temperature bound must be less than the upper temperature bound.
- The accumulation start date selected must be before the accumulation end date selected.
- The accumulation end date should be no more than one year after the accumulation start date.
- The accumulation range must follow a July 1 to June 30 calendar year where the date range does not span across calendar year periods.
Data:
Hourly temperature values from stations in the ASOS (https://www.weather.gov/asos/asostech) and AWOS (https://skybrary.aero/articles/automated-weather-observing-system-awos) networks in the United States are obtained from the ACIS hourly data tool (https://www.rcc-acis.org/docs_webservices.html) for data as far back as 1944. These hourly air temperature values are then used to calculate chilling hour accumulations based on user inputs as explained in the ‘Definitions’ and ‘How to Use the Tool’ sections of this description.
Stations that have too many missing hourly temperature values are handled using the following criteria:
- For a given station, if on a given day more than three hourly observations (approximately 10% of 24 observations) are missing, that day is not included in the accumulation. A day refers to the 24-hour period from midnight to midnight in the local time zone of that station.
- If more than 10% of days in a station’s period of record are missing as a consequence of the first criterion, the station is not displayed on the Map of Chilling Hour Accumulation.
Excessive Missing Data from Mid-1960s - Early-1980s: Users may notice excessive amounts of missing data for time periods spanning the mid-1960 to the early-1980s across the entire United States. During this period, while most stations collected observations every hour, historical observations were only digitized (transferred from hand-written observations to a searchable computer database) for every 3 hours, which limits data applicability for monitoring chilling hour accumulations. Since data reported on a 3-hour basis results in too much missing data for use in this tool, data for those time periods are unavailable. This data gap resulted from a lack of funding to support digitization. If/when these data are converted into a digital format, they will be added to this tool.
Why Do People Care About Chilling Hours?
Many plants become dormant during the cool season. If the cool season is not cool enough for a long enough period, many fruit-producing plants have a difficult time knowing when to overcome dormancy. Like growing degree-day units that are used to track temperature differences over a certain threshold over a period of time, chilling hours offer a way to track length of exposure to optimum dormancy temperatures.
Chilling Hour Requirements
Each type of fruit plant requires a unique range of accumulated chilling hours to be the most productive in the following growing season. Even ranges of ideal temperatures differ depending upon the type of fruit.
Chilling Hour requirements by type of fruit (courtesy RaintreeNursery.com):
Further Reading
Chill requirements for fruit crops, at Mississippi State University Extension
Chilling hours/first frost from Texas A&M AgriLife
Winter chill brings summer fruit, from Climate Central