Chill Accumulation Trends in Florida: Are We Heading Toward Warmer Winters?

According to the World Meteorological Organization (WMO) the global average temperature in 2019 (January to October) was about 2.0 degrees Fahrenheit (1.1 degrees Celsius) above the pre-industrial period. The year 2019 concludes a decade of exceptional global heat, retreating ice and record sea levels driven by greenhouse gases from human activities. Average temperatures for the five-year (2015-2019) and ten-year (2010-2019) periods are almost certain to be the highest on record. The National Oceanic and Atmospheric Administration (NOAA) also reports a positive trend for the contiguous U.S. annual average temperature (+0.15°F per decade) from 1895 to 2018.

Annual average temperature for the contiguous U.S. showing a positive trend of 0.15°F per decade.

Given these global and continental temperature trends we decided to investigate potential implications in the accumulation of chill hours in Florida. All economically important fruit and nut tree species that originated from temperate and cool subtropical regions have chilling requirements that have to be fulfilled each winter to ensure homogeneous flowering, adequate fruit set, and economically sufficient yields. The development of dormancy and cold hardiness is a gradual process which begins in late fall or early winter in Florida. Dormancy enables plants to tolerate freezing temperatures and prevents growth during mid-winter warm spells. Once dormant, plants require accumulated exposure to cool temperatures for budbreak and the resumption of normal growth in the spring. The amount of accumulated cool temperature exposure required for normal growth varies by species and cultivar and is referred to as chilling requirement. Insufficient chilling may result in one or more of the following physiological symptoms: a) delayed foliation, b) reduced fruit set and increased buttoning, and c) reduced fruit quality.

Hourly temperature data for three Florida Automated Weather Network (FAWN) stations (Gainesville, Quincy, and Sebring) were used to estimate the number of hours with air temperature between 32˚F and 45˚F accumulated from October 1st to February 28th.

FAWN Station 

Number of years

(start year – end year)

Chill Hours accumulation (Oct-Feb)






Gainesville Alachua County

20 years

(1999 – 2018)







Gadsden County

17 years

(2002 – 2018)







Highlands County

16 years

(2003 – 2018)






*Year refers to the year that the winter started. Year 2016 for example corresponds to the period between October of 2016 and February of 2017.

The results revealed expected differences of chill accumulation between the counties with Highlands County (Sebring) accumulating on average 108 chill hours during the period with available data (2003-2018), Alachua County (Gainesville) accumulating on average 503 chill hours (1999-2018) and Gadsden County (Quincy) accumulating on average 611 chill hours annually (2002-2018). 

Regarding the years with minimum chill accumulation, 2016 was the warmest winter in all stations with chill accumulation of only 30, 270 and 318 hours in Sebring, Gainesville, and Quincy, respectively. The high accumulation years were 2009 with 299 chill hours accumulated in Sebring and 818 in Quincy, and 2002 when the maximum chill accumulation (745 hours) was observed in Gainesville.

A screenshot of a cell phone

Description automatically generated

Chill hours accumulation variability observed in the counties of Alachua (Gainesville station), Gadsden (Quincy station), and Highlands (Sebring station)

Variability in chill accumulation from year to year is expected as climate patterns during the winter vary and are frequently affected by annual cycles such as the El Niño Southern Oscillation (ENSO). However, all stations showed a decreasing trend in chill accumulation (red lines in the figures) indicating a potential trend to warmer winters that may require blueberry and other temperate fruit growers as well as plant breeders to develop adaptation strategies for a low chill future. The number of years evaluated (16-20) is not long enough from a climate perspective to draw firm conclusions but giving the fact that longer time series of temperature records also indicate warmer trends we can expect that the accumulation of chill hours will follow similar patterns leading to a low chill accumulation future.

A close up of a sign

Description automatically generated

A close up of a sign

Description automatically generated

A picture containing text

Description automatically generated

Florida and Georgia growers can keep track of chill hours accumulation during the season using the Chill Hours calculator in the AgroClimate platform ( The information available includes the monitoring and forecasting of chill accumulation from October 1st to April 30 in all weather stations that belong to the Florida Automated Weather Network (FAWN) in the State of Florida and to the Georgia Automated Environmental Monitoring Network (GAEMN) in Georgia.

The Chill Hours Calculator tool, when loaded, shows chill accumulation up to the current date observed at the selected weather station. The default model is in number of hours with air temperature below 45˚F but the user can select the number of hours between 45˚F and 32˚F. The projection cone beyond the last observation represents the possible range of chill accumulation predicted for the current climate El Niño Southern Oscillation (ENSO) phase. The color of the projection cone indicates if the forecast is for El Niño (red), La Niña (blue), or neutral (green) phases. 

Chill accumulation is presented in two different graphs: i) the cumulative chill in the current winter season since October 1st and ii) the by-weekly accumulation during the same time period. The user can also check the options that are available above the top graph to visualize the historical average chill accumulation observed at the selected county and the accumulation observed during the past winter season.

Chill accumulation observed in the Citra station (Marion County) this winter since October 1st through December 11 (84 hours) is similar to last year when 98 hours were accumulated at this time (Source: AgroClimate Chill hours calculator:

by CLYDE W. FRAISSE, professor, & MAURICIO KARREI, research scholar, Agricultural & Biological Engineering, University of Florida IFAS

Share this post:

Comments on "Chill Accumulation Trends in Florida: Are We Heading Toward Warmer Winters?"

Comments 0-5 of 0

Please login to comment