Fog is a challenge generally in aviation because it has a profound effect on safety – pilots and air traffic controllers still often rely on seeing each other, especially when landing and taxiing on the ground.
Heathrow Airport, October 2014
Fog – some clarity on how it forms
Fog is a suspension in the air of very small water droplets which reduce visibility to less than 1000 metres. There are several types of fog but the principle two which affect Heathrow are radiation fog and advection fog. The former occurs when, under clear skies and light winds, the rapid cooling of land overnight reduces the overlying air’s ability to hold moisture, so it condenses to produce fog. The latter refers to the movement of either a warm, moist air-mass over a cold surface (warm advection) or a cold, dry air-mass over a warm, wet surface (cold advection) such as a sea, lake, river or any body of water. Both warm and cold advection similarly allow for the deposition of water droplets, and therefore fog formation.
How does fog impact the airport and what challenges does it bring?
Most planes and airports are equipped with high tech radar and Instrument Landing Systems (ILS), and major airports such as Heathrow have the highest category of these – Category III C – which allows for aircraft movements even when the visibility is below 50 metres. However, just like when driving a car in fog, the critical aspect of maintaining safe operations is to increase the spacing of aircraft. The spacing for aircraft coming in to land during foggy conditions currently doubles from 3 miles to 6 miles but with the implementation of a new Enhanced Instrument Landing System (EILS) this should hopefully decrease to 5 miles. Nonetheless, the extra spacing significantly reduces the ability to run a full schedule and so can lead to cancellations, especially if the fog is persistent.
What are we doing to improve the accuracy of forecasting fog at Heathrow?
At Heathrow there is a dedicated team of embedded Met Office forecasters who have built up local knowledge of fog formation in the surrounding area and advise on the likelihood and duration of fog affecting the airport itself. The results shown below, of a recent study into LVP (Low Visibility Procedure) events at Heathrow over the course of 5 years (2005-2009), illustrate that by far the greatest occurrence of fog occurred under near calm (light and variable wind) and clear conditions – suggesting radiation type fog.
However a trend for a light WNW to NE flow to also be present suggests that the Chiltern Hills to the NW may have an influence. Cold air ‘pooling’ in valleys such as the River Thames to the WNW & Colne Valley to the NNW as it drains off the Chilterns at night-time allows fog to form more readily here. This can then drift into Heathrow, especially just after dawn when initial heating creates a small scale surface low, drawing in the fog enriched air. Also evident is the secondary peak in LVP conditions with a SSW flow which could locally relate to the occurrence of cold advection fog from a body of water, especially in Autumn and early Winter when the temperature difference between the water and overlying air is greatest.
To further study these subtle movements of air around Heathrow a network of Wind Profilers that measure the ‘Doppler shift’ is being introduced which, aided by local knowledge, will help the forecaster identify in good time such foggy situations and more accurately assess the risk for Heathrow itself.
Ross Mitchell, Senior Operational Meteorologist, Heathrow Airport