National Weather Service United States Department of Commerce

Temperature Sensors
Temperature Shelters

Image of a Cotton Region Shelter
Cotton Region Shelter


  • Many Cooperative Observers use the Cotton Region Shelter (CRS) to record maximum and minimum temperature data.   A CRS is typically a wooden structure with louvered sides, a slotted bottom, and a solid top.   A CRS is usually made of pine, painted white, and sits atop a wooden or metal base 5 to 6 feet above the ground.   Some CRS's have an electric fan inside to allow for better circulation during light wind conditions (less than 5 mph) as thermometers often register too warm during light wind conditions.   This is especially true during the day in bright sunlight, although this may occur at night as well.

 Temperature Sensors - Liquid
  • Thermometers used in a CRS are Liquid In Glass (LIG) and are either alcohol or mercury.   Alcohol thermometers are employed in the colder climates where winter temperatures drop below -40 degrees, the freezing point of mercury.   Minimum temperature thermometers have a small bar embedded in the liquid that is pulled down the tube as the temperature falls.   As the temperature warms again and the liquid moves back up the tube, the bar remains at the minimum temperature.   This allows the observer to read the lowest temperature.   Maximum thermometers have a small break near the base of the well of liquid at the bottom of the thermometer.   As the temperature falls from the maximum, this break in the liquid keeps the liquid in place at its high point.   The maximum and minimum thermometers are mounted on a rack.   After noting the highest and lowest temperatures, the observer then tilts the rack.   This resets the thermometers by rejoining the liquid in the "maximum" thermometer and sending the bar back to the top of the liquid in the "minimum" thermometer.   The thermometers are now reset, allowing observation of the highest and lowest temperatures for the next day.

 Temperature Sensor - Electronic

Image of a Maximum Minimum Temperature System (MMTS)


  • Another and newer type of thermometer is the Maximum Minimum Temperature System (MMTS).   An MMTS is an electronic thermometer, not too different from the type one might buy at a local electronics store.   The MMTS is a thermistor.   This thermistor is housed in a shelter similar in appearance to a bee hive.   This design is similar in functionality to the CRS.   Currently, the MMTS requires a cable to connect the sensor with a display.   Future plans are for wireless displays.   This would eliminate many of the problems associated with cabled systems.

Rainfall Gages
  Rain Gauges - Hourly Precipitation

Image of a Fischer Porter rain gauge
Fischer Porter Rain Gauge

  • Fischer Porter Rain Gauge: Sites reporting hourly precipitation are most likely equipped with a Fischer Porter automatic rain gauge, a white and cone-shaped gauge resembling a rocket.   It stands about 5 feet tall and 2 feet in diameter.   This gauge collects all types of precipitation through a hole in the top.   Precipitation is continuously collected in a bucket on the inside.   As the bucket grows heavier, the weight of the bucket presses down on a scale.   Every 15 minutes, a "ticker tape" is punched with holes according to the weight of the bucket.   The reading on the tape keeps a running tally of the amount of rainfall and snowfall (in inches, tenths, and hundredths) that has occurred since the last time the bucket was emptied.

  • Weighing Rain Gauge: A few sites are equipped with a Weighing Rain Gauge.   Like the Fischer Porter gauge, there is a hole in the top for precipitation to fall through into a bucket inside.   The bucket presses down on a scale as the precipitation accummulates.   However, instead of a tape rolling through, the weight of the bucket and precipitation is recorded by pen-and-ink on a sheet of paper.   The paper is mounted on a metal drum that rotates around once every 24 hours.

  Rain Gauge - Daily Precipitation

Image of a Standard Rain Gauge
Standard Rain Guage

  • Standard Rain Gauge: A Standard Rain Gauge (SRG) is a gauge that collect precipitation in a hollow metal tube with an open top.   The opening at the top is 8 inches in diameter, the reason the SRG is sometimes also known as an "8 inch gauge".   Place in a stand, the top of the gauge is about 3 feet high.   During warmer months, a smaller tube (2 inches in diameter) is placed inside the larger 8 inch tube.   A funnel fits on top so that the rainfall channels through the larger 8 inch opening into the smaller 2 inch diameter tube.   This allows the rainfall to be measured more accurately in smaller increments.   The observer uses a normal ruler to measure the depth of the water in the small tube.   In the winter, the funnel and smaller tube are removed.   This allows snow to fall directly into the larger 8 inch tube.   The snow is melted, then poured into the smaller 2 inch diameter tube for measurement of the liquid equivalent.