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FOCUS


Key factor


Michael Brettle examines the importance of wind speed and direction for hazardous smoke plumes, and future developments


from fi res. It affects both the area of spread and concentrations of material within the plume. Knowledge of risk areas and equally of safe areas at the start of a major incident can make a signifi cant difference to fi nal costs. This article’s context is the supporting of the


W


immediate response within an hour or less of an incident starting, when emergency services and others will need to approach the incident safely, and when critical decisions such as closing roads or issuing public warnings are made. These can have major benefi ts later, and getting them right can have huge effects on the scale of casualties, damage or disruption. As well as describing how wind information


is obtained and the limitations of different sources, the article will also consider how wind information can be displayed to those who need it, alongside an overview of computer based dispersion modelling as used in the UK.


Wind basics


The key thing to remember about wind direction is that it is defi ned as the direction the wind


IND IS obviously the most important factor in the spread of toxic airborne plumes or smoke plumes coming


is blowing from, not the direction it is blowing to. This simple fact causes a huge amount of confusion. It is fortunate that high risk sites such as top tier COMAH (Control of Major Accident Hazards) sites and nuclear power stations carry out exercises, as confusing the direction in which the wind is blowing has led to imaginary people being evacuated from upwind of an imaginary incident and moved downwind! Wind is rarely steady, and wind speed


and direction vary widely. As a result of this, meteorologists have defined rules that are designed to process wind data. Normally, wind measurements are averaged over ten minutes, and this is an important issue, as the emergency services could be dependent on a rapid snapshot of wind speed or direction taken from a quick glance at a windsock or dial (see Figure 1 on p17).


Sources of data


A fundamental, but often neglected question is: ‘What is the actual wind speed and direction?’. A wide range of sources exists for an answer, including low or high technology on or near a site, or even computer models running hundreds of miles away.


16 DECEMBER 2018/JANUARY 2019 www.frmjournal.com


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