Site Ratings
Location Attributes
The following attributes are based on site grading scheme specified by the Met Office and the WMO. Owners were required to evaluate these attributes when registering a site with the Met Office Weather Observation Website (WOW) that operated from June 2011 until December 2025.
When the attributes of a site are considered as a collective a site rating index can be derived. This index provides users of the data with an understanding of the likely reliability and quality of data produced by the site.
Site Exposure
Exposure ratings relate to the site of the temperature and rainfall instruments only. The 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 rain gauge at 30 cm above ground and a nearby 5 m high building there should be a separation of at least 9.4 m (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, x 2)
5: Very open exposure: no obstructions within 10h or more of temperature or rainfall instruments.
4: Open exposure: most obstructions/heated buildings 5h or more 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 or more 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.
T: Traffic site: equipment sited adjacent to public highway.
U: Exposure unknown or not stated.
Air Temperature Measurements
The Met Office defines standard instruments in the following ratings as calibrated mercury-in-glass thermometers or calibrated electronic temperature sensors.
The temperature should be representative of the free air passing across the site at a height of between 1.25 and 2.0 m above ground level (CIMO Guide).
A: Standard instruments in Stevenson Screen, calibration within last 10 yr, site exposure minimum rating = 3.
B: Standard instruments in Stevenson Screen or manufacturer supplied AWS radiation screen, calibration within last 10 yr, site exposure = 2 or 3.
C: Standard instruments in Stevenson Screen or manufacturer supplied AWS radiation screen, site exposure 1 or less.
D: 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.
O: No air temperature measurements made at this site.
Rainfall Measurements
Standard instrument in the following ratings is defined as 10 cm diameter manually-read (Nylex RG1000 or similar) 250 mm capacity plastic rain gauge. This is different to the original Met Office definition as it now better suits the instrumentation at an unofficial site.
The rim of the rain gauge should be level and typically at a height of between 0.5 to 1.5 m above ground level (CIMO Guide). If necessary, the rain gauge should be mounted atop a free-standing post and must meet the minimum exposure requirement of being at least 'twice the height" of the obstacle away from the obstacle.
A: Standard instrument or calibrated tipping spoon rain gauge, site exposure = 4 or 5.
B: Standard instrument or calibrated tipping spoon rain gauge, site exposure = 2 or 3.
C: Standard instrument or calibrated tipping spoon rain gauge, site exposure = 0 or 1.
D: Non-standard rain gauge and/or tipping bucket/spoon rain gauge, exposure = 0 or 1.
U: Instruments unknown or not stated.
0: No rainfall measurements made at this site.
Urban Climate Zone Index (UCZ)
The following ratings are defined by the WMO to describe the urban area surrounding the site. This attribute is reported separately to the site rating index.
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 a hospital, university, airport.
7: Semi-rural development with scattered houses in natural or agricultural area, e.g. farms, estates.
U: UCZ unknown or not stated.
Site Rating Index
The matrix below combines the location attributes for site exposure plus temperature and rainfall measurements to derive the site rating index. The index is often represented by a star rating instead of its numeric value. If temperature is measured at a site, but not rainfall, the site rating will be based on the quality and exposure of the temperature data alone. If rainfall is measured at a site, but not temperature, the site rating will be based on the quality and exposure of the rainfall data alone.
| Exposure | Temperature | Rainfall | Site Rating Index | |
|---|---|---|---|---|
| 5 | A | A | 5 |  |
| 3 or 4 | A | A | 4 |  |
| 3 or 4 | A, B or C | A, B or C | 3 |  |
| 1 or 2 | Any | Any | 2 |  |
| 0, R, T or U | Any | Any | 1 |  |
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