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Site Description

Every USCRN observing site is equipped with a standard set of sensors, a data logger and a satellite communications transmitter, and at least one weighing rain gauge encircled by a wind shield. Off-the-shelf commercial equipment and sensors are selected based on performance, durability, and cost.

Highly accurate measurements and reliable reporting are critical. Deployment includes calibrating the installed sensors and maintenance will include routine replacement of aging sensors. The performance of the network is monitored on a daily basis and problems are addressed as quickly as possible, usually within days.

The USCRN instrument suite is designed to measure the following climate related parameters:

  • Air temperature
  • Precipitation
  • Solar radiation
  • Wind speed
  • Surface temperature
  • Relative humidity (2004/2005)

Currently, the installed sensors include temperature sensors enclosed in aspirated solar radiation shields, a cup anemometer to measure wind speed, a pyranometer to measure solar energy, and an infrared thermometer to measure surface temperature. A relative humidity sensor (RH) will be added to new station configurations upon completion of the evaluation period late in 2004. Existing stations will be equipped with RH sensors during the 2005 annual maintenance cycle.

The sensors are placed on a typical 3 meter (10 ft.) instrument tower at 1.5 meters (4.5 ft.) above the surface of the ground. Locations which experience high snowfall and snow depth are given special consideration.

The hourly observations and the fifteen minute precipitation data are stored in a data logger attached to the tower. A GOES satellite transmitter sends the data to the National Climatic Data Center where the data undergo a quality control check and are placed on the Web several times a day.

The instrument system is designed with the capacity for future expansion to accommodate additional sensors, such as soil moisture, soil temperature, atmospheric pressure, and wind speed/direction at the standard 10-meter height. The design allows for future additions of sensors on the tower without disrupting the physical site.

The Site Location

Many criteria are considered when selecting a location and establishing a USCRN site:

  • Regional and spatial representation: Major nodes of regional climate variability are captured while taking into account large-scale regional topographic factors.

  • Sensitivity to the measurement of climate variability and trends: Locations should be representative of the climate of the region, and not heavily influenced by unique local topographic features and mesoscale or microscale factors.

  • Long term site stability: Consideration is given to whether the area surrounding the site is likely to experience major change within 50 to 100 years. The risk of man made encroachments over time and the chance the site will close due to the sale of the land or other factors are evaluated. Federal, state, and local government land and granted or deeded land with use restrictions (such as that found at colleges) often provide a high stability factor. Population growth patterns are also considered.

  • Naturally occurring risks and variability:
    • Flood plains and locations in the vicinity of orographically induced winds like the Santa Ana and the Chinook are avoided.
    • Locations with above average tornado frequency or having persistent periods of extreme snow depths are avoided.
    • Enclosed locations that may trap air and create unusually high incidents of fog or cold air drainage are avoided.
    • Complex meteorological zones, such as those adjacent to an ocean or to other large bodies of water are avoided.

  • Proximity:
    • Locations near existing or former observing sites with long records of daily precipitation and maximum and minimum temperature are desirable.
    • Locations near similar observing systems operated and maintained by personnel with an understanding of the purpose of climate observing systems are desirable.
    • Endangered species habitats and sensitive historical locations are avoided.
    • A nearby source of power is required. AC power is desirable, but, in some cases, solar panels may be an alternative.

  • Access: Relatively easy year round access by vehicle for installation and periodic maintenance is desirable.

A density study has been conducted to determine the number of general geographic locations that would provide the best possible coverage in the different U.S. climate regions. Digital topographic maps and a climatological profile of the area are examined, and aerial photographs are used when available. The need for unchanging physical surroundings, particularly encroachment by man-made structures, means there is some flexibility in adjusting the optimal geographic location to take advantage of a more stable location in a region.