Requirements and standards for National Weather Service (NWS) meteorological climate observations: instrument siting, exposure, performance, and output data for temperature and precipitation.
– Note: do not site instruments on rooftops –
TEMPERATURE: Measure the temperature of the free air conditions surrounding the station at a height between 4 feet to 6 feet above ground level. The air should be freely exposed to sunshine and wind, and not close to or shielded by trees, buildings, or other obstructions.
site the instrument:
- 4 to 6 feet over level terrain
- at least 100 feet from any extensive concrete or paved surface.
- the instrument should be no closer than four times the estimated height of any nearby building, tree, fence, or similar obstruction.
- position the instrument so it opens to the north to protect it from direct sun exposure.
- areas where rough terrain or air drainage result in non- representative temperature data,
- areas where water or drifting snow tend to collect.
shield the instrument from the following elements:
- direct and reflected sunshine, and
- direct and reflected thermal energy (i.e., infrared).
Thermometers should be shielded with a thermoscreen or radiation shelter just large enough to protect against the elements stated, and slotted sufficiently to allow air to convect naturally into and out of the thermoscreen during calm air conditions.
RAIN: A four-inch plastic gauge is suitable. Its measuring tube holds up to one inch of precipitation and can capture up to 10 inches at one time. Read the height of the collected water directly from the scale on the side of the tube. If less than 0.01 inch is present, report a “T” for a trace of precipitation. If the bottom of the tube appears dry, then report “0.” Empty the tube immediately after recording the observation.
An ideal exposure eliminates turbulence and eddy currents near the gauge that tend to carry away the precipitation. Loss of precipitation tends to increase with wind speed and opening height.
- The orifice of the gauge should be horizontal and approximately 3 feet above the ground for standard rain gauges (i.e., daily observation) and approximately 6 feet above the ground for recording gauges (i.e., monthly observation).
- The gauge should have protection in all directions of uniform height. Their height above the gauge opening should not exceed twice their distance from the gauge.
- In open areas, the heights of obstructions above the opening should not exceed twice their distance from the gauge.
WIND: Air Movement as measured by a totalizing anemometer or by an anemometer with logger with ability to produce a 24-hour average wind speed. The anemometer should be mounted to the pan support with the cups positioned between 6 to 8 inches above the lip of the pan. Air movement measurements are accurate to ±33% of the actual air movement in miles at the end of 24-hours. The measurement of one mile of wind should represent one ‘count’ unit as appears on the totalizing anemometer’s display. If a recording anemometer is used, then multiply the average 24-hour wind speed by 24 to calculate the miles of wind.