Air temperature
This sensor measures temperature from -35 *C to +50 * C and is mounted in a Plate Gill Radiation Shield allowing air to pass freely through the shields, keeping the probe at or near ambient temperature

Relative Humidity
This sensor is mounted in a Plate Gill Radiation Shield at a predetermined height above irrigated cut grass. Relative Humidity (RH) is the ratio of the amount of water in the air at a given temperature to the maximum amount it could hold at that temperature -- a key factor in determining the level of evaporation and plant transpiration.

Precipitation
Rainfall is measured using a tipping bucket mechanism to measure actual and effective rainfall. We measure effective rainfall and use it to determine the watering schedule. Not all rain is effective rain - rain that falls too quickly will run off before reaching the root zone. Simple Rain Switches only use a rain shut-off technology - simply shutting off watering when it rains.

Solar Radiation
Solar radiation, or pyranometer, measures the energy imparted by the sun -- a key factor in determining the level of evaporation

Wind Speed
Wind Speed is measured using 3-cup anemometers and can measure wind speeds of up to 175 km/h. Wind increases the rate of evaporation -- more wind means faster drying.

Wind Direction
Wind Direction is measured by a potentiameter. The direction does not factor into the calculation of ET.

* American Society of Civil Engineers