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
© 2007 Delta-T Devices Ltd
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 c
local thermal and humidity microclima
above the crop. There are no simple rules
to follow – but be aware of the following:
Clothes lin
hanging
te
e effect
: vegetation upwind may
.
ater,
or glacier, is surrounded by a
rce.
er
f
lacing 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.
X
10 X
2 X
2 X
Clothes line effect
Oasis effect
Leading edge effect
Rules of thumb
affect vapour gradients and heat transfer
Oasis effect
: If an isolated source of w
e.g. a lake
relatively arid area, then the relative
humidity may be affected if the wind
direction draws air from the water sou
Leading Edge effect
: When air moves ov
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 o
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 p
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