Fog and Visibility

For the non instrument rated pilot,one of the most important things will be visibility and cloud base. Without having a clear understanding of this, the NON IR pilot will place himself and others in grave danger. We have discussed how clouds form in other parts of the course. I will now discuss fog and other hazards that make visibility difficult. Hopefully after this a pilot can make a good balanced decision whether to fly or divert to avoid danger.

As we know fog or cloud forms depending on how much humidity is in the air. 75% of the world has water over the surface. Fog or cloud forms when the air is saturated at the dew point temperature. Pilots will check carefully weather forecasts for their route and destinations. If the temperature drops to the dew point temperature and both are the same then fog could form. If the temperature and dew point are very close the pilot will need to monitor this especially after mid afternoon.

Things that cause fog could be a parcel of warm air moving over the sea. The lower part of the air cools and the dew point is reached. Coastal Fog then forms. This is sometimes known as advection fog. It is a bit like you breathing onto a cold mirror it steams up. Fog cannot form if the wind is strong but is ideal if it is one or two knots as it will swirl the air. 

In the evening, during the night and early morning on a clear night heat radiates into space the air on the surface cools rapidly to dew point temperatures. Fog and cloud can therefore exists during these times at a greater risk than say the middle of the day. Pilots should be aware of this when planning arrival times. This is called Radiation Fog.

Mountains and Hills can cause air to rise forming clouds above them. The katabatic wind can force cold air into valleys this air if it has crossed snow and ice can cause valley fog.

Fog is defined as anywhere where the visibility is less than 1000 metres.

In aviation forecast the following terms are used :-

At airfields control towers can have fixed reference points at different distances to measure visibility. At major airfields they have equipment that looks a little like the shape of the msn messenger symbol. They are normally painted yellow. They are put at the end of the runway. These can measure the exact distance. Sometimes different runways have different Runway Visual range distance at the same time.

Pilots should be warned that sometimes you can fly over an airfield and see it directly from above vertically below you. However when you position onto finals and look diagonally at the runway the visibility may well be below the approach capabilities of the airport.  Bombay in India is a prime example of that. Pilots circle above for a short time in the morning and as the ground heats the visibility improves and a whole stream of aircraft land one behind the other when the safe approach minimums are reached.

All instrument charts show the approved level for landing in terms of visibility and cloud base for each airport. The pilot should never descend below decision height. If the runway cannot be seen then the pilot must carry out a missed approach and possibly divert. You will hear mentioned CAT1 Cat 2 Cat 3. A lecture on this is elsewhere which explains the cloud base and visibility restrictions. Some major airports have CAT 3 conditions. Equipment on the aircraft allows the aircraft to land automatically. The pilots not having to do anything other than monitor. This is very eerie using this. The problem is after landing finding the gate to park on or the taxi way. Some pilots have got lost on the airfield and there have been a few bad accidents where aircraft have strayed onto a runway without permission. follow me trucks go out and help aircraft find their way to the gate.

In dense fog departing aircraft are normally grounded. This is in case an aircraft has an engine failure on take off. The pilot will need good visibility to correct the steering as an aircraft will swing towards the side of the failed engine. 

As the ground heats or the wind increases fog may lift and form low Stratus clouds and then eventually disperse into a nice warm day,with ideal weather for non instrument rated pilots to fly. 

Dust & Haze and the Sun

Dust and smoke particles can be blown into the air to any height the sun can reflect off of these or illuminate them. The result is haze forms and can affect the pilots visibility. For example one afternoon my friend a keen photographer wanted to take some photos of his house and London from the air. It was a lovely clear day at Biggin Hill when I set off towards Tower Bridge in London 20 miles to the North. As we got to a 1000 ft in the climb it started to get hazy. I levelled at 2000 ft I could only just make out the ground in front of me. The trip had to be abandoned after one pass I therefore took him to Brands Hatch motor racing circuit and the coast. By then we had started hitting the anabatic winds and bumping and it put him off flying in light aircraft forever. I dared not mention the word air sick bag. So please do not assume that because it looks clear on the ground it will be in the air. This flight was done mid afternoon on a hot day with what appeared to be clear skies from the ground.. Sea salt blown into the air can also cause poor visibility. 

Massive continents such as America can also have adverse weather effects. Sometimes the sea is warmer than the land and fog can move in rapidly closing all the Eastern Coast Airfields. Pilots flying to these airfields monitor the airfields whilst crossing the Atlantic. Sometimes New York Aircraft may have to divert to Montreal. Extra fuel should be carried for holding and diversion when these conditions are expected.  The problem is other aircraft have the same idea. Traffic congestion can be massive. Allow for this when planning the flight.

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