About the MRCC Chilling Hours Product
Many plants become dormant during the cool season. If the cool season is not cool enough for a sufficient time 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, accumulated chilling hours offer a way to track length of exposure to optimum dormancy temperatures. 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.
Accumulated Chilling Hours
For the seasonal MRCC Accumulated Chilling Hours product, hourly observational data from approximately 1600 stations across the conterminous US are examined starting October 1 of the current dormant season. At each station, if the temperature at the top of the hour is greater than or equal to 35°F and less than or equal to 45°F (35°F <= T <= 45°F), then the station’s season’s chilling hour1 accumulation is increased by 1 unit. Accumulations continue until the start of the next dormant season (October 1). However, note that no accumulations occur if there are no hourly temperature values for the station within the required range (e.g., throughout summer for most places). The September 1 start date products are provided for the convenience of northern states.
Chilling Hours Departure
For the seasonal MRCC Chilling Hours Departure product, seasonal accumulated chilling hour averages have been derived from roughly 1050 stations across the conterminous US. There are fewer stations for the seasonal average calculations since some stations had too many hours missing during the 1998/1999 through 2012/2013 averaging period to be viable. The averaging period, while shorter than what is climatologically desired, was chosen to maximize the number of non-missing values per stations that are utilizing the standards of the Automated Station Observing System (ASOS), which were first implemented in the mid-1990s. Subtracting the seasonal averages from the current season’s accumulated values per station derives departures.
If the Chilling Hours Departure values are below average, this could mean that the observed temperatures were either too cold or too hot. There is no way to know the cause without looking at the actual temperatures throughout the season.
Both the accumulation and departure products are presented in graphical format by applying an interpolation scheme to the non-evenly spaced station observation values.
References & Further Reading
Chill requirements for fruit types and estimated chill hour map, at Raintree Nursery website.
Chilling Hours/First Frost from Texas A&M AgriLife.
Chilling Requirements of Selected Peach Varieties, Leaflet No. 327 of North Carolina Cooperative Extension Services.
 Top of the hour values are for the minute closest to the top of the hour. It may not always coincide with :00 minutes.