Change in the Weather

This is an extract from the above named book by Phillip Eden the meteorologist, first published in 2005. It's something that added more precise data to my knowledge & hopefully will add to whatever level you have, which is why I think it's worth posting.

Walking in the hills

Scarcely a single spring passes without newspaper reports of people walking in Britain's hill-country being caught out by a sudden change in the weather. All too often someone loses his or her life. Even when there are no fatalities mountain rescue teams and airborne search-and-rescue units are called out at great expense, and often at considerable risk to their own lives.

It is a sad fact that the majority of these dramatic and some-times disastrous events could be avoided if only the people involved listened to the experts before heading out into the hills. Even better, a little self-education on the meteorology of our upland regions would give first-time hill-walkers some knowledge of what to expect. It is no coincidence that the majority of rescues are of 'townies' who venture into the hills on a whim.

It is essential to take advice from those who know the district. First of all, get hold of an appropriate weather forecast. I don't mean listen to the girl on the telly, or look at the maps in the newspaper; these days there are specially the maps in the newspaper; these days there are specially written detailed forecasts available on premium-rate telephone and fax lines, and also on the Internet and mobile phone networks, designed for those heading for the hills. Some districts also have a local forecasting office where it is still possible to speak to a human meteorologist. Do not take the forecast as absolute gospel; rather, treat it as a reasonably accurate guide. Similarly, check for any avalanche warnings which may be in force, or predicted, for the district. Apart from seeking weather information, experienced walkers will also tell you to ensure you are suitably clothed and shod, have appropriate food and water, and carry a compass and mobile phone. It is also essential to leave an itinerary or map of your projected walk at the local mountain rescue centre or, if there is none, with a responsible friend or relative. All this may sound like common sense, but it is extraordinary how many folk are rescued from the deep snow and severe wind-chill of a mountainside clad in just a Marks and Sparks anorak and the latest fashion trainers.

And what of the weather? Spring is probably the most dangerous season of all for the casual walker in, for example, the Scottish highlands, the Lake District, the Pennines or Snowdonia. The first warm day of the season will draw forth thousands of city dwellers, blinking in the spring sunshine rather like moles waking from their winter hibernation. They head straight for moor and mountain expecting the sun to be shining just as warmly; instead they find winter still firmly in charge.

Meteorologically speaking, altitude changes everything. Climb a mountain and this is what happens: down go air pressure, temperature, sunshine hours, visibility; up go wind speed, cloudiness, rainfall and snowfall, and humidity. And, of course, the combination of lower temperature and a stronger wind means that the wind-chill factor increases sharply. Changes in the weather are also more sudden and more dramatic than they are at lower levels. With all this happening it is easy to see how an inexperienced hill-walker can get caught out.

On most days, the temperature falls systematically with height above sea level. On average the rate of decline in the British Isles is 1 deg C every 175 metres (1 deg F every 300 feet), but it can vary quite widely either side of this average. For example, in an unstable polar airflow, brought to us on a blustery north-westerly wind with a mix of sunshine and showers, the rate of decline is more like 1 deg C per 105 metres (1 deg F per 180 feet).

Imagine, for a moment, waking up on such a morning in Aberdeen, round about Easter; the sky is blue, the atmosphere is clean and pure, the air is mild although there is a fresh breeze, and the temperature stands at a near-average 10°C (50°F) — the perfect day for some healthy exercise. You jump in the car and head inland, aiming perhaps for Braemar or Aviemore. At Aviemore, some 230 metres (750 feet) above sea level, the sky is part-cloudy and the temperature is 8°C (46°F), but the snow-clad slopes of the Cairngorms gleam brightly in the fitful sunshine. Turning left onto the B970 you cross the River Spey, decide against the delights of the Whisky Centre at Inverdruie, and head through Glenmore Forest, eventually negotiating the hairpins as you climb towards the Cairngorm car park. There is a distinct nip in the air, you notice, as you lock the car, adjust your anorak, and re-tie the laces of those expensive trainers. You are now at 600 metres (2,000 feet) above sea level, the temperature is 4°C (39°F) and the wind is keen. No matter, a brisk walk will warm you up.

The main footpath paralleling the chair-lifts on the ski slopes is easiest, and in an hour or so you are crunching through snow as the Ptarmigan restaurant comes into sight. Already you have endured one or two brief but very sharp showers of hail and sleet, myriad tiny frozen missiles driven horizontally by the near-gale that is now blowing. At the restaurant, sadly closed today, you are just over 1,000 metres (roughly 3,500 feet) above sea level, the temperature is -1°C(30°F). The wind cuts through your woefully inadequate clothing like a knife, but your map shows you that the summit of Cairn Gorm is less than a kilometre (half a mile) away, before that map is snatched from your hands by the gale and disappears in the general direction of Arbroath. Standing on the top of Cairn Gorm always was one of your ambitions, and it is, after all, only four o'clock ...

You have a choice. Go back now and you will probably make it to the car park suffering no more than mild hypothermia and exhaustion. Carry on and you will likely end up just another statistic.

There is an electronic weather station on the mountain summit which is 1,245 metres (4,084 feet) above the sea. At four o'clock that afternoon it registered a temperature of -4°C(25°F), and a sustained wind speed of 70 knots (80 mph) gusting to 90 knots (103 mph).

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