Davis Leaf Wetness Sensor 
Leaf Wetness Sensor, Vantage Pro® and Vantage Pro2
Use to monitor the level of surface moisture on foliage, with range from 0 (completely dry) to 15 (saturated). Add up to 2 sensors to a single Wireless Leaf & Soil Moisture/Temperature Station. Includes 40' (12M) cable.
Compatible with both original Vantage Pro and Vantage Pro2. The Leaf Wetness Sensor must be connected via a Leaf & Soil Moisture/Temperature Station (#6345).
Quick Guide (pdf)
Owner's Manual (pdf)