Fog is notoriously challenging to forecast – it varies rapidly both spatially and over time, has a huge impact on airfield operations and is poorly represented by computer model output, even in spite of recent resolution improvements. This is mainly due to its formation being dependent on a delicate balance of a myriad of processes (radiation balance, dew deposition, turbulent mixing) that are not represented perfectly within the currently available computer model output.

Local knowledge and forecaster input are crucial to create an accurate and useful forecast for the customer. The Middle Wallop empirical technique was originally developed by A.H.Perry and L.J.Symons - cited in 1991 by the Forecaster's Reference Book (FRB) to be used as an additional tool to aid meteorologists forecast the fall in airmass visibility due to nocturnal cooling of the boundary layer. It was never explicitly intended to be used as a fog forecasting technique, but it was extended and adapted by Matt Woods at Wattisham to both include back trajectories (in order to source a representative airmass), and to indicate the temperature at which the technique predicted the visibility to fall to below 1 km.

An alternative to the widely used Saunders' technique (developed in the 1940s, before the clean air act) was sought due to its dependence on readily available radiosonde ascents, which have in the past decade become increasingly sparse, therefore limiting its practicality. The adapted Middle Wallop technique has therefore been adopted in some parts of the frontline, and in order to assess its credibility has recently been studied by Anne Barber at the Met Research Unit in Cardington. A paper that summarises her investigation into its veracity was published in 2017. An example of the use of this technique over East Anglia (specifically at Wattisham) is presented here:

Favourable environment for radiation fog:

For the formation of, in particular, radiation fog, the ideal conditions include: clear skies, light winds and a moist boundary layer, these are typically favoured by a stationary anticyclone - however as forecasters we know that conditions are rarely ideal! The 26th October 2016 was one such non-ideal situation with a narrow ridge of high pressure extending westwards from Denmark through to the southwest of the UK, and weakening occluded fronts moving SE through the ridge and into southern England.

A large amount of StCu associated with the decaying frontal system was trapped within the anticyclone (see imagery in Figure 1, right), though this was breaking to the lee of high ground aided by weak mountain wave activity, and output from the Euro 4 model suggested that this would continue to break-up through the early evening. Trajectories taken at 2000 ft from the Euro 4 (see Figure 2) show how the StCu associated with a second occlusion was likely to move into the north of the area of interest overnight and likely preclude much nocturnal cooling (and therefore fog) in this region - and this was correctly identified by the Euro 4 (see attached output from post processed Euro 4 in Figure 3) which shows predicted visibility at 0600 Z on 27th October 2016, and limits the fog to the far south of England, mainly (but not exclusively) south of the M4 corridor.

Forecast for Wattisham:

The meteorologist at Wattisham is responsible for providing a forecast of conditions at the station and in a radial area that extends 40 km around the base, this is provided for Apache Attack Helicopter pilots who regularly train for night operations in and around the base. On this occasion the Euro 4 was not indicating any significant change to visibility (limits overnight are visibility < 3 km), with the UKV indicating a fall in visibility to 5 km after around 2200 Z. So far, so good for the night flying story - however as is often the case the meteorologist 'sanity checked' the NWP output with the adapted Middle Wallop technique to get an idea of how the visibility could fall with their temperature curve and whether there would be any likelihood of fog overnight.

Utilising back-trajectories from the Euro 4 (see Figure 4) the meteorologist was able to identify the source region for the air overnight, and that this was likely to be across the Thames corridor, close to the station of Benson. Using the data from stations in this area they were able to attain a representative airmass dew point (10.9C at Benson, 9.7C at Northolt) and maximum visibility (25 km at Benson, 20 km at Northolt). They could then use these to find a forecast of the fog point and how quickly the visibility was likely to fall overnight.

To attain the visibility at airmass dew point you need to forecast the mean windspeed overnight. This was 6-9 knots; and therefore required a halving of the maximum visibility diagnosed earlier, to therefore give around 10 km at the airmass dew point of 10C. You then simply halve the visibility for each subsequent degree fall in temperature (e.g. 5 km at 9C, 2.5 km at 8C and approximately 1 km at 7C), therefore giving a fog point of approximately 7C - note that this was almost 3 degrees below the Saunders' derived fog point from the Watnall and Herstmonceux 11Z ascents (see Figure 5a/5b), though these were in unrepresentatively moist locations.

The meteorologist then created a cooling curve (using Barthram and McKenzie) and used this to forecast the fall in visibility overnight - this gave a fall in temperature to approximately 9C by 21Z - this was then used for the TAF and cross-section forecast to show to the customer that the visibility would fall to around 5 km (still above limits, but close) by this time. This cooling curve also then predicted the temperature would fall to 8C by 24Z with visibility therefore down to 2.5 km, and to 7C by 03Z and therefore close to, or in, fog limits. Using this information they then issued a fog warning for the overnight period, from midnight, assuming the worst case scenario of the fog point being slightly higher at 8C.

Conclusions:

During the evening skies cleared as expected from the model output, though the temperature fell much faster than expected - this due to the fact that the topography surrounding Wattisham favours rapid cooling in a slack west to southwest flow. As the temperature fell away the visibility also decreased and matched well the fall in visibility predicted from the adapted Middle Wallop technique (see forecast and observations in Figure 6), with fog forming around 0040 Z at a temperature of just below 7C.

This event demonstrates how by using a mix of empirical techniques, model output and local forecasting skill a reasonable forecast can be drawn up providing good advice to the customer - in this event the customer was able to carry out night flying operations ahead of the poorer conditions that developed by 21 Z.