Evaporative Cooling Potential for FAWN Stations

Using a Critical Temperature of 34°F

Set new critical temperature: °F

StationIrrigation
Risk
Tair
(F)
WetBulb
(F)
Windmax
(mph)
AlachuaN/A*77678
ApopkaN/A*766920
ArcadiaN/A*777014
AvalonN/A*766913
BalmN/A*796915
Belle GladeN/A*767016
BronsonN/A*80689
CarrabelleN/A*70672
CitraN/A*786812
ClewistonN/A*777016
Dade CityN/A*796810
DeFuniak SpringsN/A*726814
DoverN/A*81719
Fort LauderdaleN/A*767215
FrostproofN/A*787022
HastingsN/A*736911
HomesteadN/A*74728
ImmokaleeN/A*786917
Indian RiverN/A*767013
JayN/A*746916
JoshuaN/A*74698
KenansvilleN/A*76709
Lake AlfredN/A*787011
LecantoN/A*79678
Live OakN/A*79679
MacclennyN/A*75677
MariannaN/A*766712
MayoN/A*78665
MonticelloN/A*75667
North PortN/A*777110
OcklawahaN/A*786912
OkahumpkaN/A*786912
OkeechobeeN/A*767011
OnaN/A*766923
PalmdaleN/A*76719
PiersonN/A*74687
Putnam HallN/A*766813
QuincyN/A*75669
SebringN/A*75706
St. Lucie WestN/A*757113
UmatillaN/A*776912
WellingtonN/A*757015

*Note: 'N/A' for Irrigation Risk indicates that Tair was at least 10 degrees F above your critical temperature, or was at least 6 degrees above Twetbulb with little wind. In either case you should always use your best judgement to determine whether to irrigate.

Evaporative Cooling Potential

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.