Get clear on mountain weather
Nowhere are we more vulnerable to weather than exposed on a mountaintop. Dr Mike Reading, Senior Operational Meteorologist at the Met Office, heads into the Cairngorms with Trail to share his insights into mountain weather.
Trail heads to the cairngorms with Met Office meteorologist Mike for a lesson in mountain weather.
It’s hard to imagine a nation more obsessed with weather than us Brits. But those of us who are also mountain lovers take our fixation with weather reports to another level altogether. And it’s no coincidence – notoriously unsettled, it’s in the mountains that the UK climate really unleashes its true potential...
Weather extremes
The lowest UK temperature of a bitter -27.2°C was recorded twice in Braemar in the Cairngorms (February 1895 and January 1982).
The highest monthly rainfall occurred on Crib Goch (Snowdon) in the December of 2015.
Seathwaite in the Lake District holds two records for the most rain in three and four consecutive days, when it lashed it down during the winter of 2009.
And the highest wind speed officially recorded was 150 knots (173mph) at Cairngorm Summit (1245m) on 20 March 1986.
No wonder, then, that us hillwalkers need to be prepared for every weather eventuality. So for some expert tips on this typically British pastime, it seemed only fitting to head to the Queen’s Balmoral Estate in the southern Cairngorms to meet the Met Office’s mountain meteorologist equivalent to Michael Fish, Mike Reading.
Britain’s coldest peaks
Our weather journey aptly began in the lovely mountain village of Braemar, although the temperature was fortunately a whole 45°C warmer than in the record-breaking low of January 1982! From there, a short drive took us past Balmoral Castle and to the Spittal of Glenmuick to meet our mountain weatherman before setting out to climb the mighty Lochnagar.
Contrary to the unconscious stereotype I had imagined, Mike wasn’t wearing large spectacles, and he wasn’t greyed and wrinkled by decades of scientific study. Mike was in fact young, fresh-faced, enormously passionate about the weather and a true mountainhead.
Mike had climbed Lochnagar many times before, but mostly in winter for some seriously hardcore winter mountaineering routes. It turned out his experience of the mountains was vast both in this country and all over the world, and entwined with his passion was always the weather – intrinsic to everything.
Used to putting his neck on the line with notoriously difficult to predict mountain weather forecasts, Mike handed us a printed copy of the Met Office Mountain Forecast for the day, which he’d prepared at work the night before. The Met Office now provides forecasts for temperature and wind at 300m intervals from valley to summit altitude. Mike predicted sunshine and blustery showers, with temperatures of between 8°C at the top and 18°C in the glens. The main story of the day, though, was the wind, due to increase and reach gale force on the summits in the afternoon.
Setting out across the glen, it was overcast but mild. Mike was armed with a device that measured wind speed, temperature and wind chill, to put his forecasting to the test. We didn’t need our meteorologist to tell us the dark menacing cloud spilling into the valley was one of the many isolated showers we would see that morning.
The altitude effect
There are three types of rain, Mike told us, and all start life as snowflakes. Warm air can hold more water vapour than cool air, so as warm air rises it cools and the moisture condenses into water droplets, forming cloud. “It’s very hard to get water to freeze unless it has a surface to freeze on,” says Mike. “Water in liquid form below 0°C is called supercooled water, and under laboratory conditions has been brought as low as -40°C without freezing. In the real world, supercooled water droplets often freeze around particles such as dust, pollen or sea salt in the atmosphere.
“Most clouds in the UK are made up of a combination of ice particles and supercooled water droplets. These grow until they form snowflakes which then fall once they become too heavy, melting to sleet then rain if the freezing level is high enough.
The exception to this is drizzle, which falls from low stratus clouds generally above 0°C.” All three of these can happen together over the mountains, which is why we see increased rainfall in the hills.
Out on Lochnagar, Mike showed us which types of cloud to look out for, as we dodged yet another heavy shower that looked sure to be heading our way. “You can see the thick heavy cloud bubbling upwards,” Mike pointed out. “Darkening, growing clouds are the main indications of impending rain from shower clouds. If they are growing rapidly in height, showers are more likely. If the cloud tops look ragged, the likelihood of rain could be reduced as this indicates strong high-level winds which can break up the vertical growth of the cloud.
“Look out for really big dark clouds with an anvil on top [see images of cumulonimbus] as these can indicate there’s a risk of hail and lightning,” he warned.
Reaching the beginning of the final push to the summit, the view of the majestic corrie of
Lochnagar yawned and protected us from the wind. A menacing-looking tarn at the feet of Lochnagar’s cliffs lay perfectly still. But as we
scrambled over the short boulder field and onto the summit plateau we were hit with 30mph gusts of wind.
“Generally wind speed tends to be 2-3 times stronger on mountain tops, but dependent on atmospheric conditions, can be a lot more,” shouted Mike over its roar. “The strength of the wind today is being caused by low pressure north-west of Scotland and high pressure over France giving a steepening pressure gradient
– like a steep slope between a ridge and a valley bottom on an OS map – resulting in strengthening south-westerly winds.”
Lots of factors affect wind in the mountains, but the most common factor for UK upland is when a temperature inversion (temperature increasing with height) sits just above the summit. “Wind cannot easily pass through the temperature inversion, so it acts like a wind tunnel, squeezing the wind between the mountain top and the inversion,” explained Mike.
“The shape of the mountain has a big effect too, with the effect most pronounced across summit plateaus, like the Cairngorm plateau, or where the wind blows perpendicular to a ridge. As walkers, we experience this as a rapid increase in wind speed as you approach the summit or ridge crest.”
On the summit plateau of Lochnagar it was also noticeably much cooler. Mike explained that on average the atmosphere cools by 1°C for every 100m you climb. Combined with the chilling effect of wind, it meant that the temperature at the top of the mountain was not only half of that in the glen, but wind chill meant it actually felt three times colder.
Predicting conditions
Mike’s forecast was pretty spot on – we’d battled the wind, summiting before it was at its worst, and we’d been lucky enough to avoid the isolated showers. He was within a degree in most of his temperature predictions, and the wind increased to gale force later on that day as expected. But how exactly was such an accurate forecast produced?
The Met Office is the UK’s flagship forecasting organisation. Previously under the Ministry of Defence, the Met Office now operates as a commercial agency of the government under the Department for Business, Energy and Industrial Strategy, and uses some of the most advanced systems in the world, not to mention one of the
on average, the atmosphere cools by 1°c for every 100m you climb
most powerful computers ever produced.
As always, the ability to predict the weather helps us make decisions about what to wear (or where to avoid!) when we’re heading outdoors. But today, weather forecasting is also a serious commercial business and an indispensable function of global travel, energy supply, space travel, military defence, the economy, public health, disaster prevention, the study of climate change… the list of industries and organisations reliant on accurate forecasts is endless.
From when the Met Office first began in 1854 (when it supplied shipping forecasts) the observations and technologies that model and predict the weather have exploded.
Ten million weather observations a day now feed into the system, and meteorologists like Mike analyse the many data sets to produce the public forecasts we find on the Met Office’s website and app, as well as those it produces for airlines, governments and businesses. Human knowledge and experience applied to computer models is still an important part of the increasing accuracy of forecasts, particularly when it comes to forecasting on all the UK’s mountain summits.
Mountain weather: the future
With increasingly frequent weather anomalies, such as last winter’s ‘Beast from the East’– which resulted from a sudden warming of the stratosphere over the North Pole, followed by the freakishly hot summer we experienced – weather forecasting for mountains and everyday life becomes increasingly important.
Mountain Rescue statistics demonstrate that there are still plenty of people heading to the hills unprepared. The Met Office has been working hard to educate the public on the weather likely to be encountered in mountain environments. As well as specific reports for mountain ranges at various altitudes, they now have hundreds of web resources – from how to read synoptic weather charts and videos demonstrating conditions likely to be faced on the mountains for those less experienced.
It’s true we’ll never be able to predict the mountain weather with 100% accuracy, but with a decent weather forecast, the appropriate kit, mountain skills and a touch of common sense, we can all weather-proof our adventures.
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