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FOCUS


Modelling applications


Meteorologists have agreed standards of sensor exposure that defi ne the height of the sensors and the acceptable distance to obstacles of various sizes. These are used for measurements for weather forecasting, and similar rules are used internationally. The standards are often difficult to achieve, especially in an industrial environment. For example, a wind sensor should ideally


be exposed at ten metres above open terrain. This is a widespread standard, and the wind at ten metres is the wind usually given in forecasts or model data. This quality of sensor exposure is something of a luxury for most users, and even windsocks are affected by distortions caused by buildings or complex terrain. A windsock low down at a site entrance,


Figure 3: Sonic anemometers are rapidly taking over. More robust than mechanical designs, they can measure very light winds and atmospheric turbulence.


a plume out as it travels downwind (see Figure 3 above).


Location is key


To obtain good quality data, the quality of the installation and its location is even more important than the sensors themselves. A mediocre instrument intelligently used will often give better results than a state of the art sensor in a poor location. This is an important point, as most users of wind monitoring equipment are not meteorologists themselves and may not appreciate the scale of local variation or possible errors. A wind sensor incorrectly mounted on a building could indicate a wind direction literally 180° in error.


surrounded by tall obstacles and in disturbed wind flow, will be just as misleading as any other sensor. Unfortunately, no set standards for wind instruments at high risk sites such as COMAH locations are identifi ed as being a potential toxic plume risk. Site operators are essentially able to decide


for themselves, and this creates a serious risk of substandard information being used. However, there are companies installing meteorological equipment and consultants who can advise.


Data display


The traditional format for displaying wind measurement is dials, sometimes supplemented by a chart recorder. Alternatively, wind data can be displayed as numbers on a digital display. There is one major pitfall however – it is easy to forget that wind direction is the direction from, not the direction to. Passing on, for example, a wind direction


of 270°, or ‘westerly’, could potentially lead someone who is unfamiliar with the terminology to think the hazardous area resulting from a release is actually east of


Figure 4: Wind displays take various forms. They may be simple like the unit on the right, or more sophisticated like the unit on the left, which gives numerical data and has some inbuilt smoothing.


18 DECEMBER 2018/JANUARY 2019 www.frmjournal.com


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