Overview

Wind, air temperature, and water vapor pressure measurements are affected by surface type and roughness, soil moisture, regional topography, and obstructions. Sites selected for their applicability to a broader area should be free from obstructions such as buildings, trees, and steep slopes. Ten meter towers are often used to raise measurement heights above low-lying obstructions. The following table lists the suggested measurement heights and exposure (distance to an obstruction) for each type of sensor.

Suggested Measurement Heights and Exposure

Sensor Type Measurement Height or Depth Exposure Considerations
Wind
  • 3 m ±0.1 m recommended (AASC)
  • 2 m ±0.1 m, 10 m ±0.5 m, optional (AASC)
  • 10 m (WMO & EPA)
 No closer than ten times the obstruction's height
Air Temperature & Relative Humidity
  • 1.5 m ± 1 m (AASC)
  • 1.25 to 2.0 m (WMO)
  • 2.0 m for temperature only (EPA)
  • 2 m & 10 m for temperature difference (EPA)
 The sensor must be housed in a ventilated radiation shield to protect the sensor from thermal radiation. The EPA recommends the sensor be no closer than four times the obstruction's height and at least 30 m from large paved areas.
Solar Radiation Height should be consistent with the exposure standard (AASC, WMO, EPA). To facilitate leveling/cleaning, Campbell Scientific recommends installing at a height of 3 m or less. The sky should not be blocked by any surrounding object. However, objects <10° above the horizontal plane of the sensor are allowed.
Precipitation
  • 1.0 m ±0.2 m (AASC)
  • 30 cm minimum (WMO)
 AASC & EPA suggest the sensor be no closer than four times the obstruction's height. The orifice of the gage must be in a horizontal plane, open to the sky, and above the level of in-splashing and snow accumulation.
Soil Temperature
  • 10 cm ±1.0 cm (AASC)
  • 5 cm, 10 cm, 20 cm, 50 cm, 100 cm (WMO)
 Measurement site should be 1 m² and typical of the surface of interest. The ground surface should be level with respect to the immediate (10 m radius) area.

References

The State Climatologist (1985) Publication of the American Association of State Climatologists: Heights and
Exposure Standards for Sensors on Automated Weather Stations, v. 9, No. 4 October, 1985.

EPA (1987). On-Site Meteorological Program Guidance for Regulatory Modeling Applications, EPA-450/4-87-013.
Office of Air Quality Planning and Standards, Research Triangle Parks, North Carolina 27711.

WMO (1983). Guide to Meteorological Instruments and Methods of Observation. World Meteorological Organization
No. 8, 5th edition, Geneva Switzerland.

Tanner, Bertrand D. (1990). Automated Weather Stations, Remote Sensing Reviews, 1990, Vol. 5 (1), p.73-98.