Florida Automated Weather Network

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Evaporative Cooling Potential

John Jackson, Phil Cross, & Nick Faryna

There is always a risk when using water systems, micro-sprinkler or conventional sprinkler, for cold and/or frost protection. Low humidity and wind can produce evaporative cooling which can chill plant surfaces to the wet bulb temperature. Dry and windy conditions can result in wet bulb temperatures 5F to 6F degrees lower than air temperature. Therefore, wetted plant surfaces that experience evaporation would be 5F to 6F degrees cooler than air temperature. Evaporative cooling may result in plant damage when water is used for cold protection during dry windy conditions. Evaporative cooling should always be taken into consideration.

It is possible that, on nights when temperatures are close to critical levels, introduction of water could produce more damage than would result if no action was taken!

The table below provides a guideline to help evaluate the risk of using water on a given night. The values and corresponding risk level are based on experience and observations over forty winters in Florida. The purpose of this chart is to encourage those using water to protect plants to take into consideration the possibility of causing more harm to the targeted plants by running the water than would occur if the irrigation system is not utilized. It should be noted that during severe freeze events, when the forecast is 3 to 10 degrees F below the critical temperature for the targeted plant, most growers are not concerned with the Evaporative Cooling Potential for there is little chance of additional damage occuring.

To obtain the predicted wind, air temperature and wet bulb temperature use the National Weather Service pin point forecast for your location. FAWN will develop an automated method to extract this data for each site and provide the level of risk for any given night. We hope to have this program in place before the 2005/2006 winter is over.

It appears that if the risk level is LOW or MODERATE, the operation of irrigation systems will produce little damage to the plants. On the other hand if the level is HIGH or EXTREME, growers should carefully evaluate their confidence in the forecast for considerable damage is possible. It is probably a toss up if the risk is STRONG, once again confidence in the forecast is a major consideration. The most difficult decisions to operate an irrigation system on cold nights is when the forecast is within one or two degrees of the critical temperature for the targeted plant. One does not want to incur damage to the plants, whether from cold temperatures or evaporation. These are the long nights that create grey hair and heart burn!

Risk level	Considerations
Low	Maximum temperature difference between air temp and wet bulb is <1 °F. Maximum wind speed is < 5 mph.
Moderate	Maximum temperature difference between air temp and wet bulb of 1° to 2° F. Maximum wind speed is 5 to 8 mph. If wind <5 mph, move to Low Risk. If wind >8 mph, move to Strong Risk.
Strong	Maximum temperature difference between air temp and wet bulb of 3° to 4° F. Maximum wind speed is 9 to 12 mph. If wind <9 mph, move to Moderate Risk. If wind > 12 mph, move to High Risk.
High	Maximum temperature difference between air temp and wet bulb of 5° to 6° F. Maximum wind speed is 12 to 15 mph. If wind <12 mph, move to Strong Risk. If wind >15 mph, move to Extreme Risk.
Extreme	Maximum temperature difference between air temp and wet bulb is >6° F. Maximum wind speed is >15 mph.