GRAPEGROWING The Limitations of the Winkler Index By Patrick L. Shabram he Winkler Index or Winkler Scale is Methodology of the index days in the month for each month from April a standard for describing regional Modern-day calculations of GDD most often to October, then summed for the entire grow- Oct climates for viticulture in the United utilize daily accumulations of degree days. That ing season [∑Apr monthly((T_mean-50)•30)]. TStates. Developed by A.J. Winkler is to say that they sum the total degrees of aver- Further, a 1998 assessment of temperatures and M.A. Amerine at the University of Cali- age daily temperatures above 50° F (10° C) for in the city of Sonoma, Calif., determined that Oct 31 fornia, Davis in the first half of the 20th every day from April 1 to Oct. 31 [∑Apr 1 daily Amerine and Winkler’s original calculations century, the index was constructed to corre- (T_mean-50,0)]. If the average temperature for may have been simplified to account for only late wine quality with climate, focusing on a given day is 75° F, then the total degree days 30 days in each month.7 Hence, GDD originally California viticulture. Wine-producing regions added to the total sum for that specific date calculated in 1944 may have had four fewer of California were broken into five climatic would be 25° F. Any average below 50° F con- days figured into the equation than what mod- regions using heat summations above 50° F, stitutes zero GDD for the given day. ern assessments typically apply. or growing degree days (GDD). Heat summa- An initial problem with some calculations The discrepancy between daily accumula- tions are a way of looking at accumulated is how “average” is determined. In most cases, tion of degree days and monthly accumulation temperatures over a given time period. De- average is calculated as the mean of the lowest of degree days is most pronounced in early- spite the common usage of the Winkler Index, recorded temperature and the highest re- season and late-season numbers, when mean the classifications offer greater uncertainty corded temperature, so a high temperature of daily temperatures may be below 50° F. As a than the system suggests. 85° F and a low temperature of 65° F would simple example, assume that the average tem- An obvious drawback to the Winkler Index produce 75° F [(Tmax-Tmin)/2]. Another method perature for each day from April 1 to April 15 is the focus on temperature alone. Winkler’s to calculate average temperatures is to take is 48° F and mean temperature for each day groundbreaking resource “General Viticulture” the mean of all temperature readings for a April 16 through April 30 is 54° F. Using the notes the influence of “rainfall, fog, humidity given day. If a weather station reports hourly daily accumulation method, each day from and duration of sunshine,” but Amerine and temperature readings, the mean of those tem- April 1 to April 15 would have a degree-day Winkler’s work and subsequent research found perature readings would constitute the average total of 0, while each day from April 16 to April that temperature plays the greatest role in the of the 24 daily temperature readings, while if 30 would have a degree-day total of 4, for an development of wine grapes. As the focus of a station reports in 15-minute intervals, the April total of 60 degree days. With an average Winkler and Amerine’s wine-grape research average temperature would be the mean of the monthly temperature of 51° F, using the was on California viticulture, their climatic 96 daily temperature readings. The simple monthly average would yield a GDD total of regions had the luxury of ignoring precipita- difference in methodology for calculating aver- 30 degree days for the month. tion, as little rain falls during the growing age temperatures can have an impact on GDD Even with consistency in methodology, season. Indeed, Winkler et al. in “General Vi- readings. A recent study of five weather sta- other factors impact the overall GDD total, and ticulture” suggest that the vinifera grape “is not tions in the Livermore Valley AVA, for example, hence the climatic region assigned to that total. suited to humid summers, owing to its suscep- found variations in 10-year average GDD as Equipment and placement of weather stations tibility to certain fungus diseases and insect high as 206 based on the methodology uti- are considered, although most government pests that flourish under humid conditions.”9 lized.6 As many weather stations have software and research weather stations, and even most Advances in viticulture have expanded the generating GDD figures for the user, the meth- range of successful commercial vinifera pro- odology deployed is not always obvious. Fur- FIGURE 1: GDD (°F) AT CAMINO, duction, while more recent research notes the ther, many software programs will vary on CALIF. USING TWO METHODOLOGIES importance of wind on photosynthesis rates.2 GDD calculations, including but not limited to Winkler et. al. also point out the importance growing season, threshold (e.g., 50° F or some Daily Monthly of ongoing refinement of local variations, stat- other number), and how temperatures below Accumulation Accumulation ing, “It is hoped that refinements will be de- that threshold are handled. Some weather Lowest (1998) 2889 2752 veloped so as to delimit subregions within the station software programs, for example, will Highest (2017) 4034 3932 present regions, thereby ensuring the greatest not include temperatures below 50° F in the 30-Year Average 3439 3300 potential for quality when the most favorable average temperature calculation. 10-Year Average 3583 3465 climatic subregion for a given variety is planted Daily accumulation methodologies, how- 5-Year Average 3821 3639 to that variety.” ever, are not consistent with the methodology 30-year average GDD based on 2017-1987 exclud- Therefore, the Winkler Index was designed deployed by Winkler and Amerine. Rather, ing 1988 because of incomplete data in 1988. at a state scale, and not necessarily intended Winkler and Amerine used monthly means. for smaller viticultural subregions or single Specifically, the mean monthly temperature 10-year average GDD based on 2017-2008. 5-year vineyards. above 50° F was multiplied by the number of average based on 2017-2013. 108 WINES&VINES Collector’s Edition GRAPEGROWING commercial weather stations in- offers some historical context for of Camino. Diamond Springs is a growing districts become more stalled in vineyards, are assumed the Camino area in general. Thirty much newer station, installed defined at subregional levels and to be placed to limit variables that total years of data were assessed Sept. 20, 2010, so a 30-year aver- as we test new environs for com- may impact climatic data. The from 2017 to 1987, with 1988 age is not possible. The five-year mercial viticulture, local com- duration of the data (i.e., number removed from the analysis, as average at Diamond Springs parisons are valuable. of years of data), however, can data were incomplete. The results based on daily accumulations is have a significant impact on num- in Figure 1 show that daily accu- 3,997 degree days compared to Duration of high bers. Typically, meteorological mulations led to higher GDD to- 3,821 at Camino. The seven-year temperatures normals (i.e., what is considered tals than using monthly means. daily accumulation average Another consideration with Win- to be the average weather for a (2011-2017) at Diamond Springs kler climatic regions is the dura- particular location) are based on Variations in averages is 3,913 compared to 3,679 at tion of high and low temperatures. 30-year averages, updated every Both the daily accumulations Camino. Every year with the ex- GDD calculations based on mean new decade (e.g., 2010-1981, and monthly accumulations to- ception of 2017 shows higher daily temperatures treat high and 2000-1971, etc.). Finding com- tals at the Camino station are GDD at Diamond Springs than at low temperatures equally, even if plete data sets covering 30 years relatively consistent with the the Camino weather station the high temperature is reached in a given area can be difficult, so total found in Winkler et al., (2017 shows Camino 30 degree for only for a few minutes, but the GDD averages used in viticulture although the numbers are 139 days higher than Diamond low temperature persists for sev- are most commonly given in some degree days different, based on Springs). Even if you could not eral hours. Areas impacted by significantly shorter duration. methodology. If a grower had give a good long-term average afternoon marine inversions may Further complicating data sets been given a single year’s data, GDD for Diamond Springs, it be especially susceptible to briefly is what happens around a given however, and that year hap- would be safe to say that the cli- maintained high temperatures. weather station during that 30- pened to be either the year with mate at the Diamond Springs Figure 3, created for a study done year period. Urban development the lowest GDD (1998), or the station is warmer than at the in the Livermore Valley AVA, can create warmer conditions year with the highest GDD Camino station. As viticultural shows the cumulative number of that, combined with a changing (2017), very different conclu- climate, lead to migration from sions could have been reached FIGURE 2: GDD TREND APRIL 1-OCT. 31 AT CIMIS 13 one climatic classification to an- about the climate of Camino. other over the course of historical The averages for the five most CAMINO 1984-2017 data sets.