High temperature was not the only issue in August; the vapor pressure deficit reached abnormally high values on one or more dates in August. Vapor pressure deficit (VPD) is defined as the difference between how much water vapor is in the air and the amount of water vapor the air can hold at saturation at a given temperature.  VPD is a function of relative humidity and temperature and is measured in kilopascals (kPa). A high VPD occurs when high temperature and low relative humidity occur at the same time. Figures 1-6 show hourly temperature and relative humidity data recorded at two WWG stations and VPD calculated from those data.

Daily maximum VPD values calculated from the most recent 20 years of data recorded by the CIMIS weather station in Windsor indicate a VPD greater than 6 kPa occurred on just 16 dates. That calculation assumed the maximum temperature occurred at the same time as the lowest relative humidity on each date. Since that was not likely to occur on all 16 dates, it is safe to say that a VPD over 6 kPa is very rare in Windsor.  In the Central San Joaquin Valley, daily maximum VPD values can reach 7 kPa but that is also rare.

Weekly VPD measurements we took in 2009 between 1 and 3 pm from late July through October in an Alexander Valley vineyard ranged from 1.94 to 4.36 kPa. We didn’t make those measurements in 2010; however, the new WWG station in Alexander Valley, just a few miles from our 2009 vineyard site, records data every 15 minutes and calculates hourly VPD. VPD on the same dates and hours in 2010 as we measured the previous year ranged from 0.92 and 6.47 kPa. On August 24, 2010, the maximum VPD soared to 7.69 kPa (Figure 2). That day in Graton, maximum VPD was 6.34 kPa (Figure 5). As a point of reference, that day in Parlier (about 20 miles SE of Fresno) maximum VPD was 6.59 kPa (calculated from CIMIS weather station data in the manner previously described).