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Vegetable IPM Updates Archive
Cooperative Extension
Yuma Leaf Wetness Network (December 13, 2017)
Figure 1. Leaf wetness equipment recently installed at Yuma area AZMET weather station sites.

Leaf wetness equipment has been installed at the four AZMET weather stations located in Yuma County (Roll, Yuma North Gila, Yuma Valley and Yuma South). The equipment consists of two leaf wetness sensors located at 1’ above the surface and a combination air temperature and relative humidity sensor, also located 1’ above the surface (Fig. 1).
Data from the leaf wetness network is updated every 15 minutes and is presented in graphical form via the internet at the following address:
Upon entering the internet address above, you will be directed to a display page that provides the outputs from the wetness sensors at each weather station for the past 24 hours (Fig. 2). Scroll up and down the display page to view the various locations. Output from the leaf wetness sensors (2 at each location) are displayed using green and red lines. The sensors are dry (no leaf wetness) when the green and red lines remain in the grey area labeled DRY. The sensors are wet when the red and green lines move up into the blue area labeled WET with higher readings indicating increased wetness (heavier dew). The warning light to the right of the graph will turn from grey to red when the sensors are wet. The black line provides the air temperature at the height of the wetness sensors (1’ above surface) and is provided for disease models that require both wetness and temperature data. Use the scale on the right axis of the graph to determine the temperature. The current temperature is also displayed in numerical format to the right of the graph. Always check on the date and time of the last update from the sensors to make sure the displayed information is current. Weather station maintenance and/or problems with local cell phone service can delay data.
Figure 2. Example of graph showing leaf wetness (green & red lines) and temperature data (black line) for the most recent 24 hours.

At the conclusion of each day the leaf wetness data are summarized into daily totals in units of hours of wetness per day. Graphs showing these daily wetness totals for the past 14 days (Fig. 3) are provided by clicking the tab at the top of the display labeled “Most Recent 2 Weeks: All Locations.” Scroll up and down the display page to view the various locations. These data sets will also be summarized in tabular form every two weeks for this newsletter (Table 1).
Figure 3. Graph showing the number of hours of leaf wetness per day for the past 2 weeks at the Yuma Valley station.

Additional, location-specific data may be examined by clicking on the tabs labeled with the station names. These display pages provide graphs of: 1) leaf wetness, 2) temperature and dew point, and 3) relative humidity and wind speed for the most recent 24 hours as measured by the leaf wetness equipment located 1’ above the surface (wind speed at 10’ above surface). Also included at the bottom of these displays is a graph showing the maximum, minimum and dew point temperatures for the past two weeks as measured by the standard AZMET station sensors located at 5’ above the surface (shelter height).
The temperature-dew point and relative humidity graphs serve as a check or backup for the leaf wetness data. Dew (leaf wetness) forms when the temperature approaches the dew point (typically within ~ 2-3°F) which equates to relative humidity values above 90%. If the temperature-dew point difference exceeds ~3°F or the relative humidity is less than 90% the wetness signal should be declining (indicating drying) or remain in the dry zone on the graphs.
Table 1. Hours of leaf wetness per day. Hours are accumulated for the 24-hour period ending at noon on the indicated date to ensure data includes entire nighttime period when wetness is more likely.

To contact Paul Brown go to:

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.

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