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2 Choose the Location
Results depend not just on sensor accuracy and reliability but also on how
representative the site is – so choose the site carefully.
Where data is to be compared to a “standard” meteorological site, the sensors should
be exposed in a similar way to sensors at the standard sites, i.e. over a level surface of
short grass and away from trees or buildings.
These are rough guidelines. Refer to meteorological publications for further advice
.
e.g.
http://www.wmo.int/web/www/IMOP/WebPortal-AWS/Index02.html
Rules of thumb
Near a building, mount the sensors outside the
zone of influence. Horizontally this extends
roughly twice the height of the building upstream
and ten times downstream. Vertically it extends
to about twice the height of the structure.
If the requirement is to measure the true local
conditions, e.g. a field of newly planted corn,
select a relatively uniform area of the terrain. Be
aware that, as a crop grows up towards the
sensors, the measured wind speed decreases
as the canopy approaches.
Sensors are also influenced by the changing
local thermal and humidity microclimate above
the crop. There are no simple rules to follow –
but be aware of the following:
Clothes line effect
: vegetation upwind may
affect vapour gradients and heat transfer.
Oasis effect
: If an isolated source of water, e.g.
a lake or glacier, is surrounded by a relatively
arid area, then the relative humidity may be
affected if the wind direction draws air from the
water source.
Leading Edge effect
: When air moves over the
boundary between two surfaces that differ in
temperature, moisture content, roughness or
some other characteristic, it takes time for the air
to adjust. The line of discontinuity is known as
the leading edge. The boundary layer will vary in
vertical extent with distance from the leading
edge as it adjusts to the new conditions.
Thermal plume effect
: Avoid placing objects
directly under the air humidity and temperature sensor, such as a solar panel, which
can create a rising thermal plume when warmed by the sun.
Figure 2: Location
