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PROPER EXPOSURE OF METEOROLOGICAL INSTRUMENTS
Generally recognized guidelines follow which depict "ideal" sensor mounting locations. These guidelines or
"rules of thumb" are only suggestive in nature in an attempt to aid the user to selecting optimum representative
sampling locations for a particular sensor.
Reference was made to US Weather Bureau Installation criteria in preparing this data (See Reference 1).
WIND EQUIPMENT:
So far as available sites permit, wind sensors should be placed above the ground on a freely-exposed tower (20
feet or higher) and over terrain that is relatively level and free from obstructions to wind flow. When a
compromise must be made, sensing units should be exposed at least 12 feet above any obstruction within 100
feet and at least as high as any obstruction within 100 to 200 feet of the wind equipment. Support towers or
masts should not be of such bulk or shape as to create an appreciable obstruction to wind flow. Avoid sites
where local obstructions may create up-or-down drafts, eddy currents or jet-flow effects. When sensors are
roof-mounted, they should be installed at least 10 feet (or greater) from the roof surface depending upon the
particular installation site. Turbulence and other local effects can be reduced somewhat by mounting sensors on
the upwind and of the building (that end of the building exposed to the most common local prevailing winds).
Horizontal-mount booms which extend from existing towers should be fabricated so that sensors will extend a
distance of 5 to 10 feet from the tower assembly (dependent on tower thickness).
Wind direction sensors are oriented upon installation in reference to either true north or magnetic north. True
north is obtained by applying a local magnetic variation correction factor to a magnetic north compass
indication (magnetic variation for a particular locality is obtainable from the nearest Weather Bureau Branch
Office). Indicator readings for a true north sensor orientation will then be in terms of true geographic compass
points. All U.S. Weather Bureau surface wind data used for observational network reporting purposes and
general public use is given in reference to this true north format. Indicator readings for a magnetic north sensor
orientation will be in terms of actual readings as would be obtained from directly viewing a magnetic compass
instrument. Wind direction data at Federal Aviation Agency and other aircraft reporting facilities (for direct
control tower-to-pilot utilization) is always made in reference to this magnetic north format.
REMOTE TEMPERATURE/HUMIDITY SENSORS AND INSTRUMENT SHELTERS:
Whenever possible, instrument shelters
*
as well as remote temperature and/or humidity sensors should be
installed at a height of 4 feet (or greater) over earth or sod at least 100 feet from any concrete or other hard-
surfaced area and not closer to any other object than four times the height of the object above the instrument
shelter or remote sensors. Avoid roof installations if possible. If it is necessary to roof-mount shelters and
sensors, they should not be closer than 30 feet to any large, vertical reflecting surface (walls, etc.), exhaust fans,
or cooling towers. Electronic remote sensors when roof-mounted should be at least 9 feet (or greater) above the
roof surface. To minimize radiation effects from the roof, they can also be mounted on a horizontal boom so
that they will extend from the side of a building roof or existing tower. Horizontal booms should extend
approximately 5 to 10 feet from the side of the building roof or tower assembly.
*
Standard U.S. Weather Bureau cotton-region shelter (Spec. No. 450.0615, Rev. 8/67)