Dealing with scald
Mid-day cooling could be an option if/when hot days are present. Berries that are exposed (higher in the canopy) are more at risk for scald development. Running for mid-day cooling is always a balance between running long enough to take advantage of the evaporative cooling but not so long that your increase the moisture in the soil and increase your risk for fruit rot. If the heads on your sprinkler system complete their rotation quickly, that is advantageous as you get the evaporative cooling effect in a short time frame. The irrigation system can shut down sooner and minimize the increase in soil moisture. With older systems and/or those with slow rotating heads, the opposite is true, and the balance is harder to maintain. According to Croft (1992, 1995), scald conditions can include times when the relative humidity ranges from 35% to 45% (dry air), when midday dewpoints are 55 F or less within and above the plant canopy, sheltered temperatures exceed 80 F, sky conditions are clear or scattered clouds, high pressure dropping from the north. Contributing factors include low soil moisture, wind speed greater than 10 knots and no rainfall in the last 48 hours (taken from Caruso, CP-08, p.35).
The stage of fruit development is also important when considering evaporative cooling. Fruit are more susceptible to scald later in the season when color formation starts. This might possibly be due to anthocyanin being less efficient at harvesting sunlight compared to chlorophyll in greener fruit. Therefore, it is imperative to begin protecting for scald then.
Consider purchasing an infrared thermometer to see what the temperature of the fruit is. An inexpensive thermal camera is also a good alternative. This could help you decide whether you should think about running your irrigation or not. The decision to run the irrigation should be based on the temperature threshold for occurrence of scald. A study published out New Jersey has shown this to be 80 °F or higher for ambient temperature (Croft, 1995). In terms of canopy and mid-canopy temperature, this translates to 98 °F and 106 °F, respectively.
Croft, P.J. 1995. Field conditions associated with cranberry scald. HortScience 30(3):627.