Fog Blog


Fog Blog

Ever wondered why fog causes such chaos for the aviation industry? This blog reveals all!

Why is Southampton Airport affected by fog?

Southampton airport is unique in many ways. One of those ways is our location. South Downs to the East, Sea to the South and New Forest to the West – all features that make the South a great place to visit. This fantastic location does bring its challenges. Geographically Southampton Airport is in a bowl and thus has its own micro-climate which does bring some pretty challenging weather conditions ranging from terrain induced turbulence (that’s a whole other subject!) to fog.

Fog, I don’t see what the issue is?? Something we hear from our passengers on a regular basis! The problem is exactly that – you can’t see. I will explain more later…

What is fog?

So what is fog? There are many different types of fog and all form under very different conditions but generally speaking, we see radiation fog. Radiation fog is formed by the cooling of land after the sun has set. Heat rises through the ground and meets the cooling air of the evening thus creating a layer of condensation. Fog will mainly occur when the sky is clear and atmospheric conditions are calm but fog levels can be exacerbated by turbulent air. Fog at Southampton mainly occurs either late in the evening or early in the morning and usually dissipates as the sun rises and begins to heat the air and ground.

However, this is where the unique location of the airport comes into play – the bowl effect means that the rising sun takes longer to heat the ground around the airport so fog can linger late into the morning. When the sun is lower in the sky as it is in winter, this process can take longer leading to slower dissipation rates.

So what is the problem?

But aircraft can fly at night with no issues, so why does fog in daylight equal so much disruption? Each airport in the country has AGL or Aeronautical Ground Lighting (our industry loves acronyms!) which guides pilots into the airport. These lights include approach lights and runway and taxiway edge lights which demarcate the safe operational areas of the airport. You can see these lights for miles during darkness but the way light beams refract in foggy conditions means that the lights are not as effective. Lights in foggy conditions are only effective at low levels, hence why the fog lights on your car are so low. This doesn’t really help aircraft at height!

Why not use an Instrument Landing System (ILS)?

'Do you not use ILS?!' we hear you cry! Well, during periods of fog the airport uses Low Visibility Procedures (LVP). In these situations, pilots rely on their cockpit instruments to attempt a landing at an airport. They have no visual references outside of the cockpit and are required to be on a safe heading and height to avoid any obstacles in the vicinity of the airport. These type of approaches are called Precision Approaches. A Precision Approach is an approach using instruments and landing using precise horizontal and vertical guidance with minimum visibility determined by the category of operation.

In order for pilots to land at Southampton Airport, they have strict visibility criteria that they must take into account during periods of bad weather. For most of our arrivals they use a CAT 1 instrument ILS which is specific instrumentation that advises the pilot on the aircrafts horizontal and vertical positioning in the sky as it approaches to land. If the visibility is below the required criteria (200ft for Southampton Airport) then the aircraft will have to either hold in the sky until visibility improves so it can land or divert to an airport with better visibility.

Well then how come some flights can still take off?!

In order for aircraft to take off they need to have a certain amount of horizontal visibility. At Southampton this is 200m and is referred to as Runway Visual Range or RVR. It is usually very unlikely that the weather gets that bad, but when it does aircraft will have to wait until the visibility improves before departing. As landing is much more restrictive it is often the case that even though an aircraft can take off it cannot get in in the first place and this is what usually causes onward delays.

Anything else?

Another of the main disruption points caused by fog is the ground maneuvering to and from the runway. During fog, Air Traffic Control (ATC – acronym bingo anyone?!) place restrictions on ground and air movements to allow for an increased separation between take offs and landings. It is this extra separation that causes the delays, for safety reasons. This of course has a knock on effect across the whole route network, not just on local operations so even if Southampton airport is clear, fog at other airports can also have a knock on effect.

During WW2, the RAF employed the skils of a FIDO unit – Fog Investigation and Dispersal Operation (Bingo!) Their main dispersal method was the installation of enormous pipes containing aviation fuel which ran along the length of the runway. The pipes were pierced with holes from which a fine jet of fuel was emitted when the pumps were fired up. These jets were then lit by a lucky volunteer in a bid to burn off the fog to ensure the bombers could land safely. Anybody care to volunteer for that risk assessment…?!