Exposure ratings relate to the site of the temperature and rainfall instruments only, which should normally be at ground level. Sensors for sunshine, wind speed etc are best exposed as freely as possible and rooftop or mast mountings are often preferable.
Exposure guidelines are based on a multiple of the height h of the obstruction above the sensor height; the standard is a minimum distance of twice the height (2h). Thus for a raingauge at 30 cm above ground, a building 5 m high should be at least 9.4 m distant (5 m less 0.3 m, x 2), and a 10 m building should be at least 17 m from a thermometer screen (10 m less 1.5 m, x2)
If the temperature and rainfall sensors are not on the same site, code for the most limited sensor exposure.
Your entry for E:
E EXPOSURE
5
4
3
2
1
0
R
U
5
Very open exposure; no obstructions within 10h or more of temperature or rainfall instruments
4
Open exposure; most obstructions/heated buildings 5h or from temperature or rainfall instruments, none within 2h
3
Standard exposure; no significant obstructions or heated buildings within 2h of temperature or rainfall instruments
2
Restricted exposure; most obstructions/heated buildings >=2h from temperature or rainfall instruments, none within 1h
1
Sheltered exposure; significant obstructions or heated buildings within 1h of temperature or rainfall instruments
0
Very sheltered exposure; site obstructions or sensor exposure severely limit exposure to sunshine, wind, rainfall
R
Rooftop site. Rooftop sites for temperature and rainfall sensors should be avoided where possible.
U
Exposure unstated or unknown
STANDARD INSTRUMENTS in this context means -
Your entry for T:
T MEASUREMENTS OF AIR TEMPERATURE
A
B
C
C2
U
-
A
Standard instruments in Stevenson Screen or approved AWS radiation screen, calibration within last 5 yr, site exposure minimum = 3
B
Standard instruments in Stevenson Screen or approved AWS radiation screen, calibration within last 5 yr, site exposure = 2 or 3
C
Standard instruments in Stevenson Screen or approved AWS radiation screen, site exposure 1 or less
C2
Non-standard instruments and/or no or non-standard radiation screen and/or sheltered site, site exposure 1 or less
U
Instruments unknown or not stated
-
No air temperature measurements made at this site
STANDARD INSTRUMENTS in this context means -
Your entry for R:
R MEASUREMENTS OF RAINFALL
A
B
C
C2
U
-
A
Standard 'five inch' manually-read raingauge, at standard height above ground (30 cm), site exposure minimum = 3
B
Standard 'five inch' manually-read raingauge or calibrated 0.1 mm or 0.2 mm capacity tipping-bucket raingauge, the rim mounted at standard height above ground (30 cm), exposure = 2 or 3
C
Standard 'five inch' manually-read raingauge or calibrated 0.1 mm or 0.2 mm capacity tipping-bucket raingauge, the rim mounted at standard height above ground (30 cm), exposure 1 or less
C2
Non-standard raingauge and/or tipping-bucket raingauge with capacity > 0.2 mm, and/or raingauge rim mounted higher than 30 cm above ground (30 cm), and/or sheltered site, exposure 1 or less
U
Instruments unknown or not stated
-
No rainfall measurements made at this site
STANDARD INSTRUMENTS in this context means -
Your entry for S:
S MEASUREMENTS OF SUNSHINE
A
B
C
U
-
A
Standard sunshine recorder, nil or very slight exposure obstruction (average obstruction 5% or less across the year, maximum 10% at any time of year)
B
Standard sunshine recorder, partially obstructed exposure (average obstruction 10% or less across the year, maximum 20% at any time of year)
C
Non-standard sunshine recorder or sunshine estimated from pyranometer/solarimeter record and/or very obstructed exposure
U
Instruments unknown or not stated
-
No sunshine measurements made at this site
These are taken from Oke, T.R., 2007: 'Siting and exposure of meteorological instruments at urban sites'. In: Air Pollution Modeling and its Application XVII, Borrego, C. and A.-L. Norman, (eds.), Springer, 615-632. There is a copy of the paper at http://www.urban-climate.org/ > Urban Climate Resources which includes more details and graphical examples of the 'skyline' which may help you to assign the correct rating.
Your entry for U:
U OKE'S URBAN CLIMATE ZONE INDEX
1
2
3
4
5
6
7
U
1
Intensely developed urban zone with detached close-set high-rise buildings with cladding, e.g. downtown towers
2
Intensely developed high density urban with 2 - 5 storey, attached or very close-set buildings often of brick or stone, e.g. old city core
3
Highly developed, medium density urban with row or detached but close-set houses, stores & apartments e.g. urban housing
4
Highly developed, low density urban with large low buildings & paved parking, e.g. shopping mall, warehouses
5
Medium development, low density suburban with 1 or 2 storey houses, e.g. suburban housing
6
Mixed use with large buildings in open landscape, e.g. institutions such as
hospital, university, airport
7
Semi-rural development with scattered houses in natural or agricultural area, e.g. farms, estates
U
Urban Climate Zone unstated or unknown
