Mountain Weather - The Basics

Misjudge the weather when you're popping down the shops for the papers and the worst scenario is a load of soggy newsprint and some soaked jeans. Get it wrong in the hills though and you're looking at possible hypothermia, death and at best, a pretty uncomfortable day.

As anyone who's spent time in the mountains will know, it can be warm and pleasant in the valleys yet blowing a hoolie up top. Temperatures drop around 1 degree C for every 100-150 metres of altitude gained and just to make things worse, wind massively reduces the effective temperature, a serious factor since wind speeds also increase - think about twice as fast on a typical English summit compared to ground level.

Simple really, mountains and hills stick up, so weather has to either go through or over them, so wind speeds rise and temperatures drop. Plus as clouds rise, they unleash all those water particles - hence the pleasantly damp demeanour of the western Pennines.

Fortunately there are several good mountain weather forecasts available on the web together with a detailed Scottish avalanche forecast - we link to them every Friday with our weekend weather outlook page - but that's just the start.

What really matters is what the weather forecast means to you on the mountains and being able to interpret it and alter your choices accordingly. So rather than get caught up in a complex whirl of meteorolgical theory, we've tried to keep things practical and comprehensible.

Air temperatures fall by 1 degree C every 100 to 200 metres of ascent. The variation depends on how moist or dry the air is. Dry air cools faster, damp, cloudy air less rapidly. So when it's cold and dry, the variation in temperature between top and bottom will likely be greater than in humid, more overcast conditions.

There are no definite rules for wind speeds, though as wind funnels through or rises over mountains it tends to increase. Local features make a huge difference though, in some areas, valleys funnel wind causing very strong winds at valley heads. As important for walkers is wind direction - walking into a headwind consumes huge amounts of energy.

So, if you're walking a circular route, it may be better to start off by heading into the wind and finishing with a tail wind behind you when you're most tired rather than the other way round. If you're considering a ridge route, beware of cross winds which will be accelerated as they rise over the ridge line and can make it almost impossible to keep your balance.

You might be better off tackling a sheltered buttress or gully line instead. We won't dwell on wind chill, but winds will strip away the layer of warm air heated by your body and effectively reduce the temperature significantly. It doesn't take much, so even in summer, you should carry a lightweight windshell.

Yay, good news, high ground statistically is wetter than anywhere else in the UK. Not only does it rain more frequently on hilly ground, but more heavily too. In very simple terms, as warm moist air rises, it cools, the moisture starts to condense and then often falls as rain.

For walker and climbers, that means poor visibility and less effective insulation, both because wet clothing is a less effective heat barrier and because the air itself is damper, so even if your clothing is dry, the air trapped in it transmits body heat more quickly.

Lightning is generated by electrical storms and is a bad thing. It strokes mainly at high points and protrusions - no, not those protrusions - ironically these actually also provide a dead zone around them as long as they're around 7 metres high,where you can shelter in relative safety. Don't sit against the summit lump or you're likely to cop earth currents draining through you instead.

Your natural instinct is to look for shelter, but in fact it's safer to sit in an open space on a dry rope or pack and minimise your contact with the ground. On rock faces, sit on a lege and stay out of chimnies.

The worst case scenario is to be caught on a sharp peak or ridge, in which case you should try and get off it quickly or risk becoming a natural earthing point for thousands of volts of electricity.

Decent mountain forecasts will include an projected visibility forecast with cloud levels. Poor visibility in itself - usually walking through clouds - shouldn't be a major problem, but you may want to adapt your route so it follows obvious major features like ridge lines or edges rather than meandering across featureless plateaux...

Okay, warm air rises, it gets cold, some of the water in it condenses out and forms ice crystals, bingo, rain... Public enemy number one are cumulonimbus clouds, the massive, towering ones that look like serious rain. They hold huge amounts of water and when they let go, things get nasty as in thunderstorms.

If you want to know more about clouds, see this BBC page.

Is that modern clothing and equipment has made it easier for us to cope with bad weather conditions. Our shell and insulation clothing is more effective and better designed than ever before.

Don't let that make you complacent though. High winds can make it impossible to walk in open areas - it's one of the reasons that the Cairngorm plateau is potentially lethal - and a combination of wind and rain can be deadly.

Forecasts are better and more accessible than ever before though, particularly through the web, and a combination of that knowlege and outdoors common sense will go a long way towards keeping you safe in the hills.