In March, 1989, the College of Agriculture and the California Irrigation Management Information System (CIMIS) established a weather station for monitoring the variables needed to manage the efficient use of irrigation water. We are Station 78 in a statewide network of 139 active stations. In the summer of 2011, we began retrieving data from the station for use on the Cal Poly Pomona website.

# Weather At Cal Poly Pomona

Air Temperature 69 ° F (21 ° C)
Wind Direction NW (337 ° )
Wind Speed 2.5 mph ( 0.9 m/s)
Relative Humidity 58 %
Soil Temperature 61 ° F (16 ° C)
Vapor Pressure 14.47 mBars (1.45 kPa)
Sunlight 1006.2 W/m2
Is it raining? No

## Interpreting the data

• Air Temperature: Temperature in the shade. Because of the agricultural setting, the temperature may differ from measurements in nearby developed areas.
• Wind Direction: The compass direction ("SW" means that the wind is blowing from the southwest) and the azimuth (0° = north, 90° = east, 180° = south, 270° = west)
• Wind Speed: How fast the air is moving.
• Relative Humidity: The amount of water vapor in the air, expressed as the percent of the maximum amount of water vapor that the air can hold at its current temperature.
• Soil Temperature: The temperature 15 cm (about 6 inches) below the soil surface. Because soil is a thermal insulator, the soil temperature changes slowly.
• Vapor Pressure: The component of air pressure that is caused by water vapor.
• Sunlight: Technically "insolation", the amount of energy from sunlight that is currently hitting the surface of the earth.
• Precipitation: The amount of rainfall during 60 seconds at the last reading, expressed as whether it is raining or not.

There are three things most people want to know about precipitation: What form is it in, how hard is it coming down, and how much of it has there been lately. For Cal Poly Pomona, the answer to the first question is almost always "rain"; we have had snow and hail, but they are both exceedingly rare.

The weather station records the second item directly: every minute, it records the amount of precipitation during the previous sixty seconds. We currently express that as "yes" or "no" in answer to the question, "Is it raining?" Eventually we will be able to tell you how hard it's raining, by comparing the value with some benchmarks.

Because we take a reading every five minutes, but the reading covers only one minute of rainfall, we can't add up the numbers to get cumulative precipitation. The weather station accumulates the amount, and that information is retrieved by CIMIS, but not currently by us. Please see the next section for information on how to retrieve it.

## Credits

The Cal Poly Pomona Weather Station is a cooperative venture of the College of Agriculture (and especially the Plant Science Department), the Division of Instructional & Information Technology, the Office of Public Affairs, and the California Irrigation Management Information System.