Beginning August 23 when temperatures climbed, many exposed clusters were damaged. Affected vineyards were not limited to those with fully exposed clusters, as standard leaf removal on the morning sun side of vine rows resulted in cluster surfaces with a range of exposures from direct (full) to indirect (diffuse) light.

Generally, cluster acclimation can protect clusters against normal heat spikes; however for several hours on August 23-25, temperatures exceeded seasonal norms. Berry temperatures are generally warmer than leaf temperatures, and when ambient temperatures exceed 104°F, clusters in direct sunlight for more than two to three hours will likely experience sunburn even if vines are well irrigated. [Larry Williams, personal communication]

Several factors affect berry surface temperature, most importantly wind velocity, solar radiation at the berry surface, and cluster compactness. The surface temperature of fully exposed berries can be 18°F greater than ambient under typical midsummer conditions, and some researchers report greater differences.

Maximum air temperatures recorded August 24 by all weather stations operated by Western Weather Group (WWG) in Sonoma County ranged from 105° to 114°F. In summer, vines must remove some of the heat energy they receive from incident solar radiation. Physical processes allowing vines to remove heat are affected by vine water status and air movement inside the canopy (which affects the extremely small “boundary layer” at the leaf and berry surfaces).

Heat energy is lost from leaves by transpiration, which results in evaporative cooling. In transpiration, water passes from the leaf blade through stomates located on the underside of leaves, then through the boundary layer just prior to evaporation. Berries have far fewer stomates than leaf blades, and by veraison, stomates have lignified and are not functioning.

As maturing berries do not regularly transpire to remove heat, most of the heat absorbed by berries is removed through convection by air movement.2 Compared to leaf blades, minimal water is transpired by clusters, especially after veraison; thus in general, cluster water loss plays a relatively small role in total vine water use.

Regardless of the small effect water loss from ripening fruit has on the entire vine, water loss from clusters can have significant impact on fruit weight. Cluster transpiration is affected by both differences in vapor pressure

(water vapor concentration) and temperature between the cluster and the air.

Low relative humidity coupled with high temperatures created a vapor pressure difference between fruit and air August 23 to 25, to levels rarely experienced in California coastal winegrape-growing regions. Berries exposed to direct light would have had much higher temperatures at times during those dates, further contributing to higher than normal transpiration rates in exposed berries, causing berry and rachis desiccation.