Beat the fog!

KNOW YOUR WEATHER No.1

As autumn closes in, fog can ruin a perfect day’s flying. Nick Heard reminds us how it forms - and what’s the best escape route.

OF ALL THE factors affecting flying in the UK, the weather is perhaps the most frustrating. You can have everything else sorted – aircraft booked, NOTAMs checked, passengers ready, perhaps even (apparently) a good weather forecast for your trip the next day. Then, on the day – bang! – some unexpected meteorological phenomenon hits. Your own airfield is in fog, summer thunderstorms pop up, or slow cloud rolls in off the sea at your destination airfield. Disappointed friends start to wonder what we see in such an activity; perhaps some of us pilots start to think the same way.

And that`s why we are going to have regular column on the weather. It`s not going to be a run through of the PPL Met syllabus, although a few bits of theory will pop up. We will take a look at all those weather systems that can cause us trouble. It would be interesting to find out about airfields with peculiar local effects. And we can widen the scope of this column, perhaps involving wider environmental aspects of GA flying. For example, seasonal sea temperature variations and survival in both land and sea forced landing situations.

Finally, each month it might be useful to think about weather conditions that we might expect for that month. So with October in mind, what might we expect?

As I stare out of the window this morning, I am reminded that autumn is the ‘season of mists and mellow fruitfulness’. The visibility is about 200 metres, thanks to classic radiation fog, something we will see more of as the winter approaches. So where does radiation fog come from? How long will it last? How do I avoid it as a pilot? And, perhaps most of all, if I am caught out with radiation for while flying, how can I sort myself out?

Radiation fog forms when reasonably moist and more-or-less stationary air is cooled below its dewpoint by contact with cold ground. So for radiation fog to form we need a slack weather pressure system to be in place, typically an anticyclone or a ridge of high pressure, which will give very light winds at the surface and clear skies overnight. As the sun goes down and daylight ends, the earth will cool. With the light winds encouraging very little mixing with warmer air above, the air in immediate contact with the cooling ground is cooled, eventually to its dewpoint. Water droplets will then form as mist or fog.

If the air in surface contact is completely stationary, the water will tend to materialise as a heavy dew; a small amount of wind (typically 3-5 knots) is the optimum amount to get fog forming. Much more wind and the surface air will tend to mix with warmer air above, precluding fog formation.

Radiation fog will occur more readily in winter than in summer. In winter the air is likely to be colder in the first place, and cooling of the earth will happen earlier in the day as the sun goes down earlier. The night is obviously longer, so there is all the more reason for fog to form. So in winter you can expect conditions suitable for radiation fog to form even from late afternoon, whereas summer fog is only likely to form later in the night.

There are also some geographic considerations. Fog will tend to form on lower ground first. The Vale of York is particularly susceptible to radiation fog, as well as much of Lincolnshire and East Anglia, and it can stick around! Higher areas, though, will often remain above the fog, giving us options for escape.

Radiation fog will clear either by the heating of the sun during the day or by a wind developing to blow it away. Clearly the sun’s heating is diminished during winter, so if radiation fog does form, it will be slower to clear. And if a bank of cloud moves in above the fog, this may well preclude fog dispersal.

So how are we going to avoid radiation fog. As we said earlier, a slack pressure gradient is fundamental, so watch for that first. Wintertime forecasts which have statements like BECMG 1518 2000 MIFG (‘from 1500 hrs to 1800 hrs becoming 2000 metres visibility in mist and fog’) are obvious indicators that the Met Man has spotted the conditions. A series of actual met reports will also show a sequence of decreasing visibilities, coupled with a dropping of the temperature towards the dewpoint. When the temperature and dewpoint are equal, expect fog to form any time.

So it’s easily forecast, but what if we are caught out? A low-time PPL is unlikely to get caught out in the summer as fog will probably form overnight, before most PPLs fly, although fog can form after dawn (remember, the coldest time of day is around two hours after sunrise).

A plausible scenario, though, is the PPL flying later on a winter’s afternoon, perhaps getting caught out with fog forming earlier than forecast. Let’s say that we are heading back to our home airfield after a day away, and we see fog start to form. Assuming we are at 2000 ft or so, the fog will form below us. What are the options?

We are technically a bit illegal now (not in sight of the surface), but let’s leave that for a while and keep ourselves safely in clear air above the fog (we definitely do not descend to get below the fog, since it’s probably on the surface!). Although we might have lots of fuel, time becomes important as the fog is only going get worse, and it might be foggy behind us as well! Consideration of alternate airfields (assuming we cannot continue to home base) should now move towards airfields with high elevation (Luton and Leeds spring to mind of the larger airports) or airfields closer to the coast.

But the best thing to do now is to call Distress and Diversion on 121.5, and declare a PAN. They will have all the information you need regarding weather at airfields, and they will sort you out – trust me! They are there for precisely this purpose and they will understand your experience level, so use them!

Your best alternate airfield might be some way away, and it will still be getting dark, of course, so you may be stuck with either some inadvertent night flying or an approach at a misty airfield while there is still some light. There is no right answer here, but I believe most PPLs would cope with night flying in clear skies better than trying an approach in mist at dusk, which is horribly disorientating.

So that’s it for the first edition. Watch out for the fog (which cleared very quickly after I saw it this morning, of course…).