Friday 12 August, 20:31:35

Classification of weather stations

In addition to a good and accurate weather station, an ideal measurement of the various weather parameters also requires a completely open siting, without any obstacles in the vicinity. Unfortunately, the latter is not always possible. Codes indicate how a measurement in our network may be affected.

Below we explain how the weather stations in our network are assessed.

General exposure (E)
Temperature (T)
Precipitation (R)
Wind (W)

Note: hereafter you will often find a term such as 10h. This means a distance that is 10 times the height of the obstacle, 5h is 5 times the height of the relevant obstacle.

  • 'h' is the height of the obstacle minus the height of the measuring instrument
  • when writing 'obstacle' we mean any object that, due to its size/volume, can exert some influence on the parameter to be measured. For example, a metal pole of a clothesline - even if it is 2 meters high - will have little or no influence on the temperature, even if it is only 1 meter away from the thermometer screen. However, when the laundry is hung, it can exert a major influence! Obstacles that exert a major influence are: houses, garden houses, walls, fences, trees, shrubs, hedges, etc. They must therefore be avoided as much as possible.

General exposure (E)

  • 5 = very open arrangement: no obstacles within a radius of 10h or more from the temperature and precipitation instruments
  • 4 = open arrangement: most obstacles are located at a distance of 5h or more from the temperature and precipitation measuring instruments; no obstacle within a distance of 2h
  • 3 = standard setup: no significant obstructions or heated buildings within a distance of 2h (or more) for the temperature and precipitation instruments
  • 2 = limited arrangement: most obstacles or heated buildings at a distance of 2h or more for temperature and precipitation instruments, no obstacle closer than 1h
  • 1 = sheltered position: significant obstacles or heated buildings within 1h of temperature and precipitation instruments
  • 0 = very sheltered position: obstacles severely limit exposure to sunshine, wind and precipitation; this also strongly influences the measurement of temperature
  • R = roof weather station: measurement of temperature and precipitation should be avoided as much as possible
  • T = traffic weather station: weather station located along public roads (motorways), also including parking lots/driveways, etc… with large surfaces of concrete/asphalt
  • U = unknown setup


 

Terrible setup of a weather station…

Site ratings only relate to the location of the temperature and precipitation instruments, which ideally should be at ground level. Sensors for sunshine, wind speed, etc. are best exposed as freely as possible, and roof or mast mountings are usually preferred. Roof assemblies for wind measurement are again not recommended due to turbulence around the building, unless they are high above it.

Installation guidelines are based on a multiple of the height h of the obstacle above the sensor height; the norm is a minimum distance of twice the height (2h). So for a rain gauge 30cm above the ground, a 5m high building should be at least 9.4m away (5m minus 0.3m, x 2), and a 10m building should be at least 17m away of a thermometer screen (10 meters minus 1.5 meters, x2)

Setup on a roof has to be avoided!


Measurement of air temperature (T)

  • A = Barani measuring screen with an exposure rating of 5 or 4
  • B = Barani measuring screen with an exposure rating of 3
  • C = Barani measuring screen with an exposure rating of 2 or 1
  • D = Barani measuring screen with an exposure rating of 0 or installed on the roof (R) or along the road (T)
  • U = unknown setup
  • 0 = no temperature measurement at this measuring point
     
Another example of a bad weather station setup

Precipitation measurement (R)

  • A = standard Barani rain gauge MeteoRain 200cm², resolution 0.2mm/tip or better, height 50cm or lower with exposure classification 3, 4 or 5
  • B = standard Barani rain gauge MeteoRain 200cm², resolution 0.2mm/tip or better, height >50cm and ≤ 150cm with exposure classification 3, 4 or 5
  • C = standard Barani rain gauge MeteoRain 200cm², resolution 0.2mm/tip or better, height ≤150cm with exposure classification 2
  • D = standard Barani rain gauge MeteoRain 200cm², resolution 0.2mm/tip or better, height ≤150cm with exposure classification 0 or 1 or R or T
  • E = other rain gauge, or resolution or installation height more than 150cm
  • U = unknown
  • 0 = no precipitation measurement at this measuring point

Wind measurement (W)

  • A = wind sensor MeteoWind, mounted 10 meters above the ground on a mast or pole, with no obstacles within a radius of 100 meters
  • B = wind sensor MeteoWind, mounted on a mast or pole >6 meters boven the ground and no obstacles within a radius of 50 meters
  • C = wind sensor MeteoWind, mounted on a building (roof) or along a wall
  • U = unknown
  • 0 = no wind measurement

Overall rating

  • 5* = general arrangement E=5, temperature T=A and precipitation R=A
  • 4* = general arrangement E=3 or 4, temperature T=A and precipitation R=A
  • 3* = general arrangement E 3, temperature T = [A, B], precipitation R = [A, B, C]
  • 2* = general arrangement E ≥ 2, temperature T = all options, precipitation R = all options
  • 1* = general arrangement E = [0, 1, R, U], temperature T = all options, precipitation R = all options

Note that wind is not included in the overall rating.

Image
A large solar panel underneath the screen will influence the temperature measured, especially when there is little or no wind
(image: Vlinder-project University of Ghent)

Last update: 08 July 2022 - 09:00

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