Measurements

  1. Program Overview
  2. Why A USCRN is Needed
  3. Who Can Benefit
  4. Site Selection Criteria
  5. What is Measured
  6. Station Instruments
  7. Site Photos

The primary purpose of the USCRN network is to monitor air temperature, precipitation, and soil moisture/soil temperature. In addition to these parameters, each station measures ground surface (IR) temperature, solar radiation, wind speed, relative humidity, wetness from precipitation, and several values that monitor the operating condition of the equipment. Some of the secondary parameters contribute to improving the confidence in the observational measurements, and provide insight into the reliability and performance of the primary sensors.

Highly accurate measurements and reliable reporting are critical. Station instruments are calibrated annually and maintenance includes routine replacement of aging sensors. The performance of each station's measurements is monitored on a daily basis and problems are addressed as quickly as possible, typically within days. Each station transmits data hourly to a geostationary satellite. Within minutes of transmission, raw data and computed summary statistics are made available on the USCRN web site. This page describes the details of the data stream.

  • Air Temperature

    USCRN stations are equipped with three independent thermometers which measure air temperature in degrees Celsius. The station's datalogger computes independent 5-minute averages using two-second readings from each thermometer. These multiple measurements are then used to derive the station's official hourly temperature value.

  • Precipitation

    Each station has a weighing precipitation gauge which is equipped with three load cell sensors to provide three independent measurements of depth change (in millimeters) at 5-minute intervals. The three series of 5-minute values are then used in an algorithm to derive the station's official 5-minute and hourly precipitation value.

  • Soil Moisture and Soil Temperature

    USCRN stations are equipped with three soil probes measuring temperature and moisture at 5, 10, 20, 50, and 100 cm depths (when possible). Each sensor records measurements at 5-minute intervals but only the observations at the 5-cm depth are saved to the datalogger. However, hourly observations (which are derived from 5-minute observations) are saved for all depths. Soil temperature is measured in degrees Celsius, and dielectric measurements are converted to fractional volumetric water content measured in cubic meter of water per cubic meter of soil (m3/m3).

  • Surface (Skin) Temperature

    Each USCRN station measures surface temperature in degrees Celsius using an infrared sensor pointed at the ground. Every five minutes, the datalogger averages two-second thermocouple-measured temperature readings to obtain 5-minute values. Beginning in January 2013, a correction was applied to surface temperatures at or above 15 degrees C. Please refer to documentation on the instruments page for details.

  • Solar Radiation

    USCRN stations measure global solar radiation from the entire hemisphere (direct plus diffuse) using a pyranometer. Every five minutes, the datalogger converts the sensor's two-second voltage output values into watts per meter squared (W/m2) and averages them into 5-minute values.

  • Wind Speed

    Each station uses an anemometer to measure wind speed at a height of approximately 1.5 meters above the surface. Every five minutes, the datalogger averages two-second pulse counts to derive 5-minute average wind speed in meters per second (m/s).

  • Relative Humidity

    USCRN stations have a single relative humidity sensor placed in the first of the three air temperature instrument shields alongside the primary thermometer. The relative humidity is measured as a percent of atmospheric capacity, from slightly above 0 percent to 100 percent. These measurements are made by a thin-film capacitive humidity sensor and reported as 5-minute averages.

  • Wetness

    The presence of precipitation is detected through the use of a wetness sensor, or disdrometer. When a water droplet falls on the detector pad, a circuit is completed and resistance in one channel of the instrument drops to very small numbers, indicating the presence of precipitation. This information is used in interpreting the rain gauge depth changes and assigning them to actual precipitation when warranted.

Details on Measured Parameters

Primary Measurements:
Surface Air Temperature
Precipitation Data Stream Summary
Soil Moisture and Soil Temperature
Secondary Measurements:
IR Ground Surface Temperature
Solar Radiation
Wind Speed
Relative Humidity
Wetness
Miscellaneous

Top Surface Air Temperature

Each USCRN station has three thermometers which report independent temperature measurements each hour. These three observed temperature value are used to derive a single official USCRN temperature value for the hour. This single value is sometimes a median and sometimes an average of various combinations of the three observed values, depending on information about which instruments agree in a pairwise comparison within 0.3°C. Each station transmits the three independent observed values; the computation of the official USCRN temperature value is done after these values arrive at NCEI. The discussion below describes the details of the three observed values.

Each station has three Thermometrics platinum resistance thermometers, each of which is housed in its own Met One 076B 7308 aspirated solar shield. Each thermometer measures the temperature (in degrees Celsius) every 2 seconds. Every 5 minutes the station datalogger computes the average of these 2-second values, giving 12 5-minute averages for each thermometer. Standard deviations are also calculated for each thermometer. Finally, a moving 5-minute average displaced 10-seconds at a time is used to determine the maximum and minimum 5-minute periods ending within the hour in question.

In addition to the thermometer values, the station also measures the speed of the fan in each aspirated shield. As the shield's fan rotates, a contact closes and generates a pulse twice per rotation. The datalogger counts these pulses every two seconds. Every hour these 2-second values are averaged to obtain an average number of pulses per second for the hour. The hourly data stream from the station thus include the average pulse rate per second for each of the three sensors. The actual speed of the fan in revolutions per second is half the pulse rate.

 
Top Precipitation

Each USCRN station measures precipitation with a Geonor T-200B precipitation gauge. This gauge produces twelve independent observed depth measurements during each hour for each of three load cells. These observed values are used to derive a single official USCRN precipitation value for the hour. Each station transmits the observed values; the computation of the official USCRN precipitation value is done after these values arrive at NCEI. The discussion below describes the details of the observed values.

The Geonor T-200B uses a collection bucket which is suspended by three vibrating wire strain gauges. Each wire, when excited with 12V DC, vibrates with a frequency relative to the weight in the collection bucket. The gauge is surrounded by a small wind/snow shield, and a controlled heater device is attached to the gauge to prevent ice buildup. The station datalogger measures the frequency of each vibrating wire and converts it to a gauge depth (in mm) every five minutes.

A Hydrological Services Tipping Bucket Rain Gauge Model TB-3 is installed at most sites for comparison purposes only. Its data are not quality controlled and are not considered official USCRN precipitation readings, but can assist in cases where the Geonor unit fails during warm conditions.

 
Top Soil Moisture and Soil Temperature

For every USCRN site with deep soils, a total of 15 Stevens Water Monitoring Systems, Inc., Hydra Probe II (SDI-12) units are placed in the ground in three plots at five depths (5, 10, 20, 50, and 100 cm) to measure soil moisture and soil temperature. These probes use reflected electromagnetic radio waves at 50 MHz to determine the dielectric permittivity of the soil in which the probe is inserted, which can be converted to volumetric soil moisture units (m3m-3) by use of a calibration equation. The probe also contains a thermistor to measure ambient temperature in the face plate pressing against the soil. All 15 probes are interrogated every two minutes and their measurements averaged over 5-minute periods for output purposes.

 
Top IR Ground Surface Temperature

An Apogee Instruments infrared temperature sensor measures the infrared ground surface temperature (in degrees Celsius) at each station. The datalogger samples the sensor every two seconds. Every five minutes these two-second samples are averaged to obtain 5-minute values.

 
Top Solar Radiation

A Kipp & Zonen SP Lite Pyranometer measures solar radiation (watts per meter squared, W/m2) at each station. The datalogger samples the sensor every two seconds. Every five minutes these two-second samples are averaged to obtain 5-minute values.

 
Top Wind Speed

A Met One Model 014A anemometer measures wind speed (in meters per second) at each station. The datalogger samples the anemometer every two seconds. Every five minutes these two-second samples are averaged to obtain 5-minute values.

 
Top Relative Humidity

The Vaisala HMT 337 HUMICAP instrument is a capacitive thin-film device that measures relative humidity by the flow of electricity across two electrodes separated by a polymer film. The capacitance of the film is related to relative humidity by a calibration equation that converts the flow of electricity to relative humidity. These values are averaged over 5-minute periods to produce final data for this variable.

 
Top Wetness Sensor

In addition to the above elemental observations, the hourly data stream from each station includes measurements from a disdrometer, or wetness sensor. A Vaisala DRD11A Rain Detector produces two variables, one of which provides yes/no information with regards to the impact of hydrometeors on a slanted sensor plate, and the other indicating the intensity of the precipitation. The former is used to provide information at least two times every 5 minutes as to whether precipitation is falling.

 
Top Miscellaneous

In addition to the above elemental observations, the hourly data stream from each station includes the following values:

  1. FGBV = Battery voltage for fan and GOES transmitter battery
  2. FGBVfull = Battery voltage for fan and GOES transmitter battery under full load
  3. DLBV = Battery voltage for datalogger
  4. DLDO = Number of minutes in this hour that the datalogger door was open