Research indicates that leaf wetness (LW) is one of the factors concerning the potential for disease to occur in crops. Yet, measurements of the occurrance and duration of LW are not routinely taken by cooperative weather stations or National Weather Service Offices in Kentucky or the U.S.
In an attempt to estimate hours of leaf in Kentucky, the UK Ag. Weather Center has developed a Leaf Wetness Model as a simple estimater of the occurrance and duration of LW for available weather stations in Kentucky. Two weather variables are used; relative humidity and hours when weather observations indicated precipitation (eg. rain, drizzle, fog, thunderstorm, thundershower, showers, etc.) An attempt will also be made to include wind speed to this model since moderate winds can influence the formation of dew.
As Relative Humidity (RH) approaches 100%, condensation occurs... depositing moisture on leaves of plants. It is important to note that condensation (RH=100%) will usually occur first in a crop canopy with a large LEAF AREA than in a weather shelter five (5) feet above the ground. Therefore, RH of less than 100% may be more appropriate for a given user's operation and location. Also, it may be important to count hours where rainfall occurred but RH at the surface was less than 100%. Hence, the value of this model depends on the user's particular crop status, distance from the weather stations and lay-of-the-land (aspect of cold air dranage).
To use this model, the user can select those days of interest to estimate LW by entering a start and stop date, a relative humidity level important to the user's operation of interest and the dates of occurrence or the counts by day (with monthly and final summary provided). The column listed as Precip provides those hours where the weather observation indicated some type of precipitation that could cause leaf . The column marked as RH provides the number of hits or hours where leaf may have occurred due to high RH. PLEASE NOTE: the leaf column is NOT the sum total of the Precip column and the RH column. The leaf column is the total hours in a given day that leaves were wet based on the number of hours of data available for that given day. For an hour where RH was high and precip occurred; that counts as one (1) hour of leaf .
One final note: Studies have yet to determine the value of LW measurements based on the above assumption with respect to distance from a weather station.Tom Priddy