AES Guidlines for Co-operative Climatological Autostations, Version 2.0, Climate Information Branch,
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Canadian Climate Service, Atmospheric Environment Service, Downsview, Canada, M3H 5T4, 1992. 85
pages.
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Figure 3.1. Diagram indicating appropriate distances from an obstruction meteorological
instrumentation (from AES Guidlines for Co-operative Climatological Autostations, Version
2.0).
radiation are separate, the diffuse and infrared (if shaded) measurement should be the furthest
poleward and slightly elevated, while the direct instrument should be closest to the equator and at
the lowest height. The global instrument should be centred between these two instruments and
higher than the direct instrument. The global, diffuse, and infrared instruments should be at the
same height, with only the shade portion of the diffuse apparatus extending above the height of
the thermopile of the global instrument. In the case where the direct and diffuse instruments are
set on the same tracking platform, the direct beam instrument(s) should not interfere with the
horizon of the diffuse instruments.
When locating instrum ents that measure upwelling fluxes it is important to be able to service these
instruments from their poleward direction to reduce ground disturbance that may affect direct
reflectance into the sensor from the sun. For example, significant differences in snow albedos can
be observed because of the effects associated with the depressions caused by footsteps near a
downfacing instrument on its solar side.
For meteorological instrumentation, distant horizon problems are minimal but interference
between instruments is significant. For the measurements of temperature and pressure, the
Stevenson screen (or equivalent) should be at least twice the distance apart from the height of all
significant objects. These objects should be located poleward of the measurement site so that
shading will not interfere with the instruments within the screen. Instruments used to measure
precipitation should be located no closer to the nearest obstruction than four times the height of
the obstruction. An instrument for the measurement of wind must be at least 10 times the height of
an object distant from that object. For example, the 10-metre mast in Figure 3.1 is located 200 m
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away from the 20-metre tall tree. If the instrumentation cannot be located the prescribed distance
from the obstruction, then the instrument should be located in a location where the obstruction
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