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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 (ex-HRES) 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 (ex-HRES) 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.

Errors in forecast near-surface data associated with cases of thick fog.

Fig9.4-8: An example of incorrect temperature and dew point forecasts in a case of predicted fog.  Imperfectly modelled mixing processes near the surface induce errors in 2m temperature and humidity.

See also potential cause of poor stratus forecast.

An example of fog prediction by HRES

HRES and Ensemble Control Forecast (ex-HRES) 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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