Chemical Meteorology (CHEMET)
CHEMET can be used to track the dispersion of a chemical release. Telephone advice is available on demand which will give a simple short-range prediction of the anticipated behaviour of the plume.
In the event of an incident involving hazardous chemicals, local Fire and Police services will contact the Met Office Environment Monitoring and Response Centre (EMARC). Typical scenarios could be a chemical spillage, a fire at a chemical plant or oil refinery, or a road traffic accident in which a hazardous substance has either escaped or ignited. For small-scale events, EMARC produces meteorological guidance and a plume prediction as a chemical meteorology (CHEMET) report. For larger release events, such as the Buncefield Oil Depot fire, more-sophisticated plume modelling techniques are utilised.
Buncefield Oil Depot fire case study
Find out more about how we helped during the Buncefield Oil Depot fire in 2005.
CHEMET can be used to track the dispersion of a chemical release. Telephone advice is available on demand which will give a simple short-range prediction of the anticipated behaviour of the plume. Within 15 minutes, this is followed by meteorological and dispersion maps which provide a more detailed forecast. A map of areas at risk is sent to the emergency services. The situation is constantly monitored, and updates given until the emergency is over.
CHEmical METeorology (CHEMET) Report
A chemical meteorology (CHEMET) report is made of two parts. Form A describes the details of the incident (the input to the model) and Form B provides a forecast of the relevant meteorological parameters, together with an Ordnance Survey map showing the main area at risk.
CHEMET Form A contains the input data, which consists of the incident details including grid reference of the location, time of the event and any additional information on the chemicals involved. If available it can also include details of the current weather at the site. The Form As are completed by the emergency services and forwarded to EMARC.
CHEMET Form B contains the weather forecast information along with an area at risk map. Further details on the different sections of the Form B are given below.
Explanation of sections on Form B
Forecast period: The length of validity is left to the forecaster's discretion, based on his or her expert interpretation of the changing meteorological conditions, but will typically be about three to four hours. If an update has been requested, a renewed Form B will normally be prepared and sent half an hour before the validity of the previous form ends. The date and time is given as, for example, 15/1200, where '15' is the day of the month and '1200' is the time (local time).
Surface wind direction: This is the direction from which the wind is forecast to blow using the 8-point compass degrees true. For example, a direction of 090° is (an easterly) wind blowing from the east towards the west.
Wind speed: This is the wind speed expected at 10 metres above ground level in kilometres per hour. Any changes in wind speed or direction during the period are given in sections 5 and 6 or in the remarks section 8.
Sections 7 and 8: These provide an indication of the behaviour of the plume due to weather conditions while the chemical is assumed to have neutral buoyancy. The remarks section may be used to give any extra details, such as whether washout or any significant or sudden wind changes may occur.
Section 9: This records the total cloud cover in oktas, or eighths of the sky. For example: 8 oktas means the sky is overcast and 4 oktas means the sky is half-covered by cloud. The total cloud cover is important in many dispersion models as it has a direct bearing on the value of the incoming radiation. The height (in feet above ground level) of the lowest significant cloud layer is also given. By 'significant' we mean a layer of five oktas or more. A full (or nearly full) cover of low cloud (height below 5,000 ft) may indicate the upper limit through which the chemical will be mixed.
Section 10: This gives the temperature of the air and the relative humidity.
Section 11: This provides information on the type of precipitation, if any, and its intensity which may be important when considering possible chemical washout.
Section 12: This is the depth of the mixing layer (the depth of atmosphere through which the chemical could be mixed).
Section 13: This describes the mean wind in the mixing layer and is a measurement of the mean over time as well as the mean within the layer.
Section 14: This is an estimate of the vertical stability of the atmosphere using the Pasquill Stability Index (where A is the most unstable, G is the most stable and D is neutral). The stability of the atmosphere will affect the behaviour of the plume.
Area at Risk Map: This shows a prediction of the plume direction, with two coloured areas. The smaller (inner) area, indicated by the darker colouring, is the Area at Highest Risk, and the larger (outer) area, indicated by the lighter colouring, indicates the Total Area at Risk.
Fig 1. CHEMET Area at Risk Map (Example) showing Total Area at Risk and Area of Highest Risk
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