As this bizarre winter turns reluctantly into spring, we still have occasional reminders of what winter should have brought us. Snow has just disrupted flying operations, most notably at Leeds Bradford, although other airports have been affected too at various times in the last few months. Why is it that the airports (and the country) grind to a halt the minute a snowflake falls?
The first thing to say is that we really, really do have the wrong kind of snow! We live on an island in the North Atlantic, and our weather is largely governed by this ocean – a ‘maritime climate’. Westerly winds bring moisture-laden air across an ocean which though cold is well above freezing point; the air mass reaches us at a similar, relatively mild temperature as a result. Our western shores are brushed by the Gulfstream, a warm sea current which originated in the Gulf of Mexico and flowed past Florida and up the eastern seaboard of the USA, and as a result Cornwall and the Scilly Isles are warm enough for palms to grow reasonably well.
When snow does fall here then, it is only just below freezing point – indeed it may have left the clouds in a frozen state, but is passing through air which is around freezing point so it is already partly-thawed ‘wet snow’, with its characteristically large flakes. Even if it reaches the ground in a frozen state, the ground itself will normally be only just at freezing point, and may well be above zero. If the air temperature rises even slightly, the snow will not settle for long and will be gone in days or even hours – which is normally what happens.
Looking at the weather report for Leeds at 0920 this Friday morning, snow is falling and reducing visibility, but the temperature and dew point are both zero: this is not really snow within the meaning of the Trades Description Act – it’s practically slush. The Leeds runway has been closed for snow clearing operations; it’s now open again and flying has probably resumed. The runway has been cleared of snow, and a layer of slush remains.
If you have been a skier unlucky enough to have your hard-earned annual holiday ruined by temperatures around or above zero, you will know all about wet snow and slush. It is infuriating to ski in; it is sticky, and readily turns to slush or water. So it is for aeroplanes (and cars). We can operate in wet snow and slush, but there is a noticeable performance penalty. On take-off the aircraft wheels have to push aside this sticky layer of snow or slush, which reduces our acceleration. When slowing down after landing it reduces the grip of the tyres and thus the effectiveness of the brakes. All this is strictly taken into account in the performance calculations that govern every take-off and landing, incorporating a large safety margin – but the inevitable result is a reduction in performance margin, and our ability to operate in crosswinds is also reduced. Clearing this wet snow off the runway is difficult and time-consuming – it sticks to itself and the equipment and soon starts making slush, which is even harder to remove. Taxiways are often not treated to the same extent as runways, so we have to taxi very slowly and cautiously. The result of all this is delay – and every time snow falls again, operations may well have to stop so that the runway can be swept and treated, meaning more delay.
In freezing conditions we will probably also need to remove snow and ice from our aircraft with de-icing fluid. This is sprayed on once doors are closed, and looks like a soapy detergent, often blue or straw-coloured. It removes the snow, and keeps the wing clear for a defined time. Although we close all external air intakes during de-icing, a small amount can nevertheless find its way in to the air-conditioning, resulting in a brief chemical smell – usually after we re-open the air intakes, or else just after take-off. It is completely harmless, and nothing to worry about – the air in the cabin is anyway continually changed and filtered, so the smell will quickly be removed.
I called this ‘the wrong kind of snow’ – so how does the rest of the world cope? Monarch has operated to many cold destinations, particularly in our charter past. A few years ago we made a series of flights to Helsinki, Finland, in December, in temperatures of typically -15C. Finland has a continental climate, their winter weather mainly influenced by freezing land temperatures in Russia. At these very low temperatures the snow remains as snow, falling in smaller, ‘dry’ flakes – it does not stick so readily to other snowflakes (or wings). As the Finns know there will be snow for most of the winter months they have the procedures and the equipment to deal with it. With snow falling, a long line of monstrous snow-clearing equipment would race down the runway once every hour or so, closing the runway only for ten minutes, and flying could be resumed. The ‘dry’ snow flakes would hardly settle at all.
North America has a continental climate – hot in the summer, very cold in the winter. Monarch used to operate to Calgary, Canada, where the winter temperature remains well below freezing. It is also 3600ft above sea level – and air temperature drops roughly two degrees per thousand feet above sea level, so the temperature is a further seven degrees cooler as a result. Although the land was deep frozen and snow-covered, any further falls of snow remained deep frozen and ‘dry’. The snow really behaved more like sand, and was relatively easy to sweep away provided the falls were not too intense.
Another feature of very cold air is that it holds hardly any moisture (- hence the reason why there are very few clouds at high altitude, in temperatures of -57C or lower). Little moisture means less snowfall and ice. A few years ago I took a Monarch aircraft to Kansas City, USA, in winter temperatures of around -15C. The Great Lakes were largely frozen as we crossed them, the land snow-covered for hundreds of miles, and we landed in brilliant sunshine. The aircraft was parked overnight in clear skies in even lower temperatures – but when we returned to the aircraft the next day for departure there was no ice or frost on the aircraft at all, and we did not need to de-ice it: very definitely the RIGHT kind of snow!