Sprinkling with water for cooling is frequently used in tree fruit production, particularly in arid and hot regions. The principles are similar as for frost protection. As water changes from a liquid to a gas heat is consumed. As this heat is transferred to the water the plants are cooled. The rate of cooling is a function of the water content of the air in relation to its ability to hold water.
Warm air can hold more water than cool air.
Table 1 shows the change in relative humidity at a constant air water content of 50 grains per pound of air as the temperature increases. The difference in water content between the air and the vines actually drives the cooling process. As the air temperature increases during the day the cooling capacity also increases. During the last week of July the dew points were so high that little evaporative cooling would have occurred, but since the water temperature would likely have been cooler than the air temperature some cooling would have ensued. One story related to sprinkling for evaporative cooling that I keep running into, not only from cranberry growers, but from other crops as well is that water droplets on the leaves act like little magnifying glasses that will concentrate sunlight on a small area and burn the leaves.