Determining Your Station's Exposure Grade
Temperature, rainfall, wind speed and many other weather parameters are highly dependent on the precise location of a weather station, the equipment being used, and times of observations. To enable readers of the bulletin to judge the quality of the data relating to specific stations, we operate a station exposure grading system.
The exposure code consists of 7 characters, representing factors affecting:
- Temperature
- Rainfall
- Sunshine
- Hour of terminal observations
- Exposure
- Urban influence
- Network of station types to which your station will belong
The characters are then written as a 'word' with no spaces in the form 'TRSHEUN'. Examples might include 'CB-A37-' or 'AA-A35R'.
To work out your TRSHEUN code follow the guidance below. Scrolling down and assessing your site exposure first would help, as this will affect other grades.
| T - Measurements of Air Temperature | |
|---|---|
| A | Standard instruments in Stevenson Screen or approved AWS radiation screen, calibration within last 5 years, site exposure minimum = 3 |
| B | Standard instruments in Stevenson Screen or approved AWS radiation screen, calibration within last 5 years, site exposure = 2 or 3 |
| C | Non-standard instruments and/or no or non-standard radiation screen and/or sheltered site, site exposure 2 or less |
| U | Instruments unknown or not stated |
| - | No air temperature measurements made at this site |
| Standard instruments in this context means – Calibrated mercury-in-glass thermometers or electronic sensors exposed in a standard pattern of Stevenson Screen, or a model of AWS screen which has been trialled alongside a Stevenson screen and been shown to provide similar or better performance than the Stevenson screen. | |
| R - Measurements of Rainfall | |
| 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 | 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 2 or less |
| U | Instruments unknown or not stated |
| - | No rainfall measurements made at this site |
| Standard instruments in this context means – Standard-pattern (Snowdon or Met Office Mk II pattern) 'five-inch' copper raingauge, with deep funnel, the rim of the gauge level and mounted at 30 cm above ground level, meeting the minimum 'twice the height' exposure requirement. | |
| S - Measurements of Sunshine | |
| 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 |
| Standard instruments in this context means–
Campbell-Stokes sunshine recorder or electronic sunshine recorder (R&D Electronics model,
or the Met Office standard Kipp& Zonen CSD sensor). The instrument should be mounted in a
location where it has the best possible view of the sky at all seasons. |
|
| T - Terminal Hours | |
| A | Morning terminal hours. Air temperature and rainfall terminal hour is between 0600 and 0900 GMT, daily temperature and rainfall values relate to standard 24 hour period morning to morning |
| B | Midnight terminal hours. Air temperature and rainfall terminal hour is 0000 GMT daily. (This is the default on most AWS.) |
| C | Other terminal hours. Air temperature and rainfall terminal hour is other than A or B above, or extremes do not relate to 24 hour periods |
| U | Terminal hours unknown or not stated |
| E - Exposure | |
| 5 | Very open exposure; no obstructions within 10 x the height, or more, of temperature or rainfall instruments |
| 4 | Open exposure; most obstructions/heated buildings 5 x the height or from temperature or rainfall instruments, none within 2 x the height |
| 3 | Standard exposure; no significant obstructions or heated buildings within 2 x the height of temperature or rainfall instruments |
| 2 | Restricted exposure; most obstructions/heated buildings 2 x the height, or more, from temperature or rainfall instruments, none within 1 x the height |
| 1 | Sheltered exposure; significant obstructions or heated buildings within 1 x the height 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 |
|
|
| U - Oke's Urban Climate zone 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 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 |
| These are taken from Oke, T.R., 2007: "Siting and exposure of meteorological instruments at urban sites". | |
| N - Network | |
| S | Synoptic site. Contributes one or more daily observations in real-time to the WMO synoptic network |
| C | Climatological site. Contributes daily climatological data to the national climatological network (Met Office/Met Eireann), normally monthly |
| R | Rainfall site. Contributes daily rainfall data to the national raingauge network (Met Office/Environment Agency/Met Eireann), normally monthly |
| - | Station data not provided to other national networks apart from COL |
Make a note of your TRSHEUN code then return to
the new station page.