AOSC 634
Air Sampling and Analysis
Lecture 6
Atmospheric Exposure and Siting
Handbook: Chapters 4 & 5
Copyright Brock et al. 1984; Dickerson 2015
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Atmospheric Exposure: General
• Representativeness
– Synoptic ~100 km
– Microscale ~10 km
• Outdoor instruments should be installed on a level piece of
ground, preferably no smaller than 25 m x 25 m.
• For climate studies, no changes in environment – was a
climate station in our parking lot but then CSS was expanded.
• Level ground.
http://www.wmo.int/pages/prog/www/IMOP/CIMOGuide.html
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Example of a
good layout
for a met site.
Northern
Hemisphere
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Atmospheric Exposure: Temperature
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Minimize radiative heating
Maximize ventilation (without compressive heating)
Air is a poor conductor
Screened shelters induce ~1 m/s wind
WMO suggests > 2.5 m/s ventilation
Radiation error
Where Kr = sensor view
K rse Ar (Tt 4 - Tr4 )
Er =
s = Stephan Boltzmann const
HAC
e = emissivity
Ar = area of radiating body
Tt = Temp of air at time t
Tr = Temp of radiatnig body
H conductive heat transf coef
Ac = area for convection
Fritschen and Gay, Environ. Ins 1979.
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Traditional Weather Shelter
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Inside Weather Shelter
WMO guidelines
call for accuracy of
+/- 0.1 °C and a 20 s
time constant.
Is this achievable
with our
-20 to 100 °C
thermometers?
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A Class 1 WMO site has
1. Flat horizontal land surrounded by open space with slope <19o.
2. Natural vegetation < 10 cm high.
3. More than 100 m standoff from heat source, building, water body, or projected
shade.
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Atmospheric Exposure: Anemometers
• From the UK Met office
Handbook
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Standard exposure = 10 m with no
obstructions within 300 m.
For small obstructions of height h, the
anemometer should be h+10 m.
If obstruction height h >12m and lies
within h in the horizontal (X ≤ h), then
the sensor should be 1.75 to 2.25 h
above ground level.
If the distance to the obstruction is
greater than 30 h then the sensor
need only be h m agl.
X
If h > 12 m and
X ≤ h then site
sensor ~2h agl.
If X ≥ 30h then
site senor at h.
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Traditional wind speed estimates
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Atmospheric Exposure: Radiation
• Obstructions
– Absorb solar radiation
– Reflect solar radiation
– Emit IR radiation
See: Latimer Radiation Measurement
For an essay on correction for obstructions to diffuse radiation.
13
Example of signal averaging to reduce
noise from Krotkov et al., JGR, 2008.
The full benefit of spatial and temporal
averaging is seldom realized.
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References
• CIMO Guide to Meteorological Instruments
and Methods of Observation, WMO-No. 8,
2008. Available on the internet www.wmo.int
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