Summer has now well and truly been and gone, and with the nights drawing in and the clocks just gone back, pilots’ thoughts turn to the subtle changes in operations we can expect in coming months.
More take-offs and landings will be during the hours of darkness. This presents absolutely no problems – it just emphasises how the year is rushing by. The first autumnal change we will feel is the increase in low pressure systems – lows – that affect the UK, with the attendant rain and stronger winds. (Memo to self: remember to start bringing my coat to work.)
Our autumnal weather actually starts in the spring, off the coast of W. Africa. Fluffy, cotton wool-like cumulus clouds form over the tropical Atlantic coastal waters, and start to drift lazily westwards on the prevailing winds. Warm equatorial sea water fuels them, and by the time they have reached the S. American coast some of them will have grown into large, cumulonimbus thunder clouds that bring the summer rainy season to the Amazon. Others drift clockwise around the Caribbean, growing in size all the time, before the prevailing winds take them northeast towards Florida and the eastern seaboard of the USA. By this stage some of them may even be fully-fledged hurricanes. They continue northeast, battering the east coast of the USA, but as the ocean temperature below them drops they lose their power and degenerate. By the time they reach our side of the Atlantic again they are vigorous lows, passing usually (but not always) to the north of Scotland. They bring a day or so of strong winds and rain, usually southwesterly veering to northwesterly, stripping the dead leaves from the trees (and depositing most of them in my garden).
We do not need to take off or land exactly into wind – our Airbus aircraft are certified to accept a considerable wind from the side, or ‘cross wind component’. There is a maximum component we can accept however, and very occasionally in very strong winds we might need to divert to somewhere more into wind. (Thinking back over my almost 29 years with Monarch I have only needed to divert in the UK once or twice due to cross-winds.)
Most major UK runways are westerly in direction (Heathrow, Gatwick, Luton) or southwesterly (Stansted, Manchester, Newcastle, Glasgow, Edinburgh). A few are northwesterly (Birmingham, Leeds, Prestwick, Cardiff). Only Aberdeen is (almost) northerly – otherwise there’s Amsterdam. The runway direction reflects the wind direction predominantly experienced at these airports as a result of the local geography – and some airports will be affected more by strong crosswinds than others. Because these lows often pass quickly the wind direction usually veers (and reduces) in a matter of hours, before returning to the usual westerly or southwesterly direction.
As the low approaches it drags in warm, damp southwesterly air. At the boundary where this meets the colder air it produces extensive sheets of (nimbostratus) cloud – a ‘warm front’ – which results in prolonged and extensive rain. If the air below is cold enough this will fall as snow, but as the base of the front passes overhead this will usually turn back to rain; shortly afterwards the wind will generally abate and veer more westerly, and the rain (or snow) will peter out. In other words, if you don’t like the weather brought by winter lows – just wait a few hours and it will all be different!
The exact opposite of these lows is high pressure, or ‘anticyclones’. In summer these bring us calm, settled weather, often with clear, cloudless skies and hot days; sometimes there will be a little haze around dawn. In autumn this haze becomes fog, and as the air mass is colder and the sun weaker, the fog can be slow to clear. Statistically the worst month for fog in the UK is November, but it can occur throughout any of the cold winter months.
Fog is no longer the problem it was when I started flying. All our Airbus aircraft are equipped to operate in visibility as low as 75 metres at suitably-equipped airports. (Imagine driving your car along a road at 150mph, in fog, with just 75 metres visibility, and you will understand how much better things are now.) Because the autopilot is controlling the aircraft right down to touchdown, the transmissions it receives must be very pure, and uncontaminated by nearby aircraft (or vehicles on the ground): consequently the rate of take-offs and landings in fog can be much reduced, resulting in holding and delays.
With cold, still air comes ice and frost. This needs to be cleared from the aircraft wings before flight, and is done with a chemical spray. Modern de-icing fluids are very effective, but the procedure takes up more time and can cause delays, particularly if ice was not forecast. Winter skiing destinations mean we may also need to consider de-icing when landing down route. I hope not to need to de-ice for a few more months yet though, so I’ll save talking more about it until next time.
I hope you’ve enjoyed your summer flights with us then, and I hope too we’ll see you on board over the winter.
Meanwhile – where did I put my uniform coat …?
Thanks to: Brett Westcott for the main image
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