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Fig9.4.1-1: An example of HRES forecast fog distribution on right hand chart compared with the observed visibility on left hand chart (visibility as in coloured scale).  Forecast run data time 00UTC 23 January 2017 T+30 verifying at 06UTC 24 January 2017.  Thick fog, less than 100m visibility in places, caused problems over southern England.  Many flights were cancelled and others delayed.  In the early morning hours the fog was more widespread over England and Benelux countries.  HRES forecast visibility was too poor in some areas but provided good guidance to forecasters regarding areal extent of the fog risk. 

Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble  Ensemble Control Forecast output is equivalent to HRES output where shown in the diagrams.  At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig9.4.1-2: Fog probability in the early medium-range from an earlier forecast run, data time 00UTC 21 January 2017 T+78 verifying at 06UTC 24 January 2017.  The ensemble also highlighted England and Benelux countries as areas where fog was likely.

Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Considerations: Fog and Freezing fog

Prediction of fog depends strongly on the simultaneous and accurate prediction of several parameters in the lowest layers of the atmosphere on a spatial scale smaller than the IFS (scale of m rather than km).  Forecast values of temperature and moisture are of prime importance of course, but these in turn depend upon correct assessment and prediction of surface parameters (e.g. soil moisture and the efficiency of nocturnal radiative cooling) and the precise low-level wind and associated turbulent mixing.  In particular the detail of orography is very important in governing local winds (e.g. katabatic winds) and stagnating air in valleys. 

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• Fog can be very patchy and 10km-scale models (such as IFS) cannot be expected to handle this well. 
• Areal extent of fog is broadly captured by HRES and Ensemble Control Forecast and ensemble member forecasts but at best these are only indicative of areas at risk.  Such predictions do not preclude occurrence outside of the apparent "risk areas".
• Local skill in forecasting the occurrence of fog (e.g. for a specific site) can be very low.  Local skill in predicting very low visibilities (e.g. meeting motoring warnings threshold criteria) can also be very low.  Beware!
• Jumpiness in fog forecasts from HRES and Ensemble Control Forecast must be expected.  Results may well not show convergence towards the correct solution.  Forecasts with shorter lead times will not necessarily be more skilful that those from longer lead times.

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Fig9.4.1-5: Forecast probability <200m visibility at 30hr lead time (left) and 6hr lead time (right) with verifying observations for the same freezing fog case as in Fig9.4.1-4.  In this example the areas of low/moderate probability capture the observed 200m visibility fairly well over England and Wales, and the probability distribution sharpened up as lead time reduced (e.g. over East Anglia), though not clearly in a manner that verified better.  A chart of probability of fog with visibility <200m (or other low thresholds) should only be taken to be very roughly indicative of those areas most at risk.

Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Example of  prediction of unexpected fog

HRES and Ensemble Control Forecast can give good guidance on the development of fog and signal to users the possibility of otherwise unexpected hazardous conditions.  Sea or coastal fog is relatively rare in June in the Mediterranean but HRES was able to predict well ahead of time a significant event in June 2022.  

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Fig9.4.1-8: Example of visibility meteogram showing forecast visibility in cumulative frequency form.  Colours represent forecast visibility ranges (see scale). 

Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

(FUG Associated associated with Cy50r1)