| What does a CIMIS weather station do?
CIMIS stands for California Irrigation Management Information System. This is a state-wide system of standardized weather stations that are connected to a
central database at the University of California campus at Davis. The data and calculated ETo are accessible by modem or the web. Weather stations and
their sites are now purchased by local sponsors. However, major funding to support research and development, as well as the central database and communication
system, is provided by the California Department of Water Resources.
All CIMIS weather stations are virtually identical. All stations have their equipment and sensors mounted on a mast which is mounted on a tripod base (pictured
above). An explanation for each sensors is below. All CIMIS weather stations are serviced and maintained on a regular basis. The grass which is under each weather
station is mowed regularly to a height of 3 inches (7.5 cm). The grass is irrigated and fertilized to keep it actively growing.
Anemometer - This three cup anemometer, placed 80 inches (2 m) above the ground, is a mechanical device which measures the rate
of rotation pushed by the wind which is then a quadratic function of the wind speed. These anemometers must be perpendicular to flat ground and are not be affected
by vertical winds.
Pyranometer - This instrument, also placed 80 inches (2 m) above the ground, measures the energy content of the direct and diffuse
solar radiation. The sensing element is a silicon photocell with hot and cold areas (black and white). The greater the radiation intensity, the greater the
temperature difference between the white and black areas. Temperature difference is sensed by a differential thermopile whose output is nearly linear with solar
radiation flux density.
Lightening Rod - A simple device used to ground potential lightening strikes thus eliminating short circuit problems with
Wind Vane - Placed 80 inches (2 m) above the ground, this instrument simply measures the horizontal direction of wind
flow. While not too important in helping predict ETo numbers, this instrument does help document wind direction for climatic patterns.
Air Temperature and humidity sensors - Placed 60 inches (1.5 m) above the ground, these sensors monitor air temperature
and atmospheric relative humidity. Air temperature and humidity have a strong influence over energy gradients which directly affect ETo.
Solar Panel - Since the CIMIS weather stations are usually in remote areas, it is imperative that the station be self contained in
terms of energy. The solar panel collects solar radiation as an energy source to log hourly weather data. At night, a battery which has been trickle charged during
the day, takes over.
Datalogger - The datalogger is the brain of a CIMIS weather station. The datalogger takes input from all instruments attached to
the weather station device and stores the information. After midnight, the main computer at CIMIS headquarters interogates all CIMIS weather station dataloggers
and downloads the day's data via modem and telephone connections. In case of communication problems, CIMIS data loggers can keep more than a week's worth of
information before over data writing occurs.