Weather and Climate Data
Precipitation, Temperature, and Degree Day Comparison for Five Selected Years
The 2004 growing season had an extremely early bud break, warm, dry weather and an early harvest with lighter yields (the latter probably due to cool weather through bloom the previous year). In 2005, bud break was only a bit early; and was followed by significant May rainfall. Mild temperatures continued throughout the season and a cold September delayed ripening a larger crop.
In the 2009-2011 growing seasons mild temperatures predominated along with year-specific memorable weather events. Yields were average or slightly above average in 2009 and lower in each of the following two years.
Lower than average temperatures in 2009 affected crop levels the following year. In 2010 low spring temperatures prevailed for most of the growing season with the exception of a few hot, dry days in August that caused total crop loss in some vineyards. In 2011 unusually cool temperatures dramatically reduced fruit set in several regions yet in spite of the lighter crop, harvest got off to a very late start.
Each of these years were “late” thus exposing more tons of fruit to October rainfall and bunch rot - especially in 2010 and 2011.
These were complicated vintages.
Sources of Data for Graphs
Daily maximum and minimum temperatures were obtained for Healdsburg and Graton “climate stations” through 2010 from the weather database available on the UC Integrated Pest Management (UC IPM) website.
Climate stations provide long-term records of daily maximum and minimum temperature and precipitation and are maintained by the U.S. Department of Commerce/National Oceanic and Atmospheric Administration (NOAA). The stations also provide current weather data for dates prior to the last three to six months.
The graphs provided below utilize data from climate stations located at the Healdsburg Fire Department and the Hallberg Butterfly Gardens in Graton.
For 2011, temperature, precipitation, and degree day data were obtained from “automatic stations,” which are part of the CIMIS network operated by California Department of Water Resources (DWR). Automatic stations supply current daily values of several agriculturally important weather variables. Data are collected daily by DWR, which then provides them to UC IPM.
The automatic stations used to obtain 2011 data for the Healdsburg and Graton graphs are the “Windsor.A” and “Santa Rosa.A” stations respectively.
Accumulated degree days are provided as another comparison of growing conditions. A degree day (DD) is the amount of heat that accumulates over a 24-hour period when the temperature is between the lower and upper development threshold for an organism. (From Glossary of Grape Terms; eViticulture)
In general, the lower the temperature, the slower the rate of development. In grapevines, the lower development threshold is 50 °F (10 °C) which is considered the lower temperature at which development stops. There is no upper development threshold for vines, presumably because it does not increase the accuracy of the prediction for growth and development of the vine. Traditionally, DD are accumulated between April 1 and October 31; however viticulturists often choose to vary the accumulation period to compare targeted periods across years.
Various methods exist to calculate DD; a common method, and one used in the figures and tables below, is the single sine method. The accumulated DD in the charts were calculated using the weather utility on the UC IPM website.
Weather and Climate Data
UC IPM Weather Database / UC IPM Model and Degree Day Calculator
University of California maintains a database of historical weather data from both CIMIS stations, and from NOAA climate stations. Evapotranspiration data is only available from stations ending in .A (CIMIS stations); stations ending in .C provide only temperature and precipitation data.
Degree day models are used to predict biological development phases for both food crops and the pests which feed on crops. This information has many implications including site evaluation and insecticide timing. The UC Model and Degree Day Calculator uses CIMIS and NOAA weather data to provide customized reports for a variety of insect pests, and optional user-specified model thresholds. Degree day data may also be used to track crop development.
California Irrigation Management Information System (CIMIS)
CIMIS is operated by the California Department of Water Resources and provides daily reference evapotranspiration (ETo) from over 120 automatic stations throughout California.
ETo data are utilized in the Vineyard Irrigation page on this website.