If there is one word to sum up the British weather it would be 'changeable' - one minute you can be basking in sunshine in a t-shirt and next you're looking for a pully and a raincoat as the sky clouds over and the rain starts to fall. Because the weather is so changeable it presents specific problems to walkers. Here's just a few pointers as to weather types and the issues they pose:
HOT
When Experienced: Late spring, summer, early autumn.
What the Sky Looks Like: Clear azure skies, possible high wispy clouds or sparse lower fluffy white clouds.
Other Features: Little or no wind, air temperature above 25C/70F.
Main Risks to Walkers*: Sunburn, dehydration, heat exhaustion, sunstroke.
Advice: Protect exposed skin with a high factor sunscreen - do not forget thighs and calves if wearing shorts, tops of feet if wearing walking sandals on easy or moderate walks. Protect top of head with a hat. Carry plenty of fluids (plain water is best, though consider isotonic fluids on longer walks). Drink regularly.
CLOUDY
When Experienced: All year round.
What the Sky Looks Like: Lots of fluffy white clouds with clear azure skies behind. Sun pops out from time to time.
Other Features: Little or no wind, air temperature typically above 0C/32F in winter and below 25C/70F in summer.
Main Risks to Walkers*: Little risk in winter as long as wind speed is low, though can be some dehydration and heat exhaustion due to 'over-layering' on intermediate, harder and severe walks. In summer risks of sunburn, dehydration, heat exhaustion and sunstroke are greatly reduced but still present.
Advice: In winter use layering and take off/add on layers as necessary. In summer protect yourself as you would if it were hot.
WINDY
When Experienced: Late autumn, winter, early spring.
What the Sky Looks Like: Most likely accompanied by storm conditions so expect sky to be dark and covered in low, black cloud.
Other Features: Actual air temperature can be significantly above 0C/32F but perceived temperature can be well below zero.
Main Risks to Walkers*: Two risk factors to consider; the risk of wind affecting your movement and the risk of wind reducing your body temperature. If the wind is strong enough to physically move you this is a severe risk at altitude where, depending on wind direction, it can impede your progress or blow you off your feet. If you are on a rock face, an edge or an arette a high-speed gust can have the force to scoop you up and throw you to your death. Due to the chilling effect of the wind, if the three factors of low temperature, wetness of clothing and high wind come together it can reduce your body temperature very quickly freezing exposed skin and/or leading to exposure.
Advice: Use windproof and waterproof technical clothing, especially if walking at height. Protect hands and face. Avoid elevated routes where loss of balance or being slightly off course could lead to injury or death. Avoid walking where constant wind speed will be >40mph or where gusts of >60mph are indicated.
RAINY
When Experienced: Late spring, summer, early autumn.
What the Sky Looks Like: Dark sky and low, black cloud. Sun might pop out from time to time.
Other Features: Duration and type of rain can be very variable. Heavy rainfall usually has a duration of under an hour whereas fine drizzle can persist for several hours. Can turn to hail in colder months.
Main Risks to Walkers*: Flash flooding is a risk where there is a sudden heavy downpour; streams can quickly become uncrossable torrents and low lying areas can quickly become waterlogged and/or boggy. In mountainous areas, rock can become slippy. Drizzle has the potential to be more penetrating than normal rain, especially when blown by a light breeze or wind.
Advice: Use waterproof clothing. Protect contents of rucksack with drysack and/or poly bags. Avoid routes where a sudden increase in water volume could be an issue, such as fording points and stream/river gullies.
STORMY
When Experienced: All year round.
What the Sky Looks Like: Dark sky and low, black cloud.
Other Features: A storm has an accompanying wind with much greater strength than a breeze. Thunder and lightning can also be present (see below). In late autumn, winter and early spring accompanying rain can turn to driving sleet, especially at altitude.
Main Risks to Walkers*: Stormy weather is a combination of rain with wind, hence the risks that apply to both those types of weather apply to stormy weather. Risks pertaining to thunderstorms can also apply if present.
Advice: As per rainy and windy weather types. In event of thunderstorm, advice pertaining to that weather type also applies.
THUNDERSTORM
When Experienced: Late spring, summer, early autumn.
What the Sky Looks Like: Very dark sky and big, black, 'broiling' clouds, often tall with a typical 'anvil' shape.
Other Features: More often than not accompanied by rain, the approach of a thunderstorm can usually be seen and heard for several minutes before it actually arrives overhead.
Main Risks to Walkers*: A thunderstorm may have elements of rainy, windy and stormy weather associated with it, hence the risks that apply to those types of weather apply to thunderstorms. Additional risks pertaining to lightning will also apply.
Advice: As per rainy, windy and stormy weather types. Specific to lightning, if you are near to a building or structure then take shelter inside it. If are near to a vehicle get in it. If you are on, or within 20ft (7m) of ascent of a rock prominence (e.g. a summit) get off it. If you are near to the top of a peak, fell, or hill descend at least 100ft (30m) quickly, then sit the storm out somewhere safe, which often is out in the open. Avoid sitting under trees, large boulders or other natural projections from the general surroundings. Lower down a dry stone wall on a slope or in a depression in the ground can be good.
SNOWY
When Experienced: Late autumn, winter, early spring.
What the Sky Looks Like: Dark, greying sky. Cloud shape and colour can vary but will be low lying
Other Features: Can be a progression from rain to sleet to snow as you climb higher up an ascent.
Main Risks to Walkers*: On easy and moderate level walks snow presents no greater risk other than the possibility of slipping where snow has been compacted then become wet. On intermediate, hard and severe walks snowfall can distort vision and distance perception, make traversing surfaces more hazardous, produce dangerous features such as cornices, and present potential for avalanche.
Advice**: Use waterproof clothing together with layering. If walking in heavy snowfall at altitude, especially if significant wind is present, consider use of mountain glasses or goggles. If walking across old snow or ascending snow at altitude consider using walking crampons. Avoid steep traverses (contouring) and never walk over a cornice - you could kill yourself or, worse still, kill someone else further down the mountain.
FROSTY
When Experienced: Late autumn, winter, early spring.
What the Sky Looks Like: Most likely clear blue sky, but can also be frosty when overcast if temperature is low enough.
Other Features: Can be a progression from soft, damp ground to hard, frozen ground as you climb higher up an ascent.
Main Risks to Walkers*: On easy and moderate level walks frost presents no greater risk other than the possibility of slipping in areas where surface water has frozen. On intermediate, hard and severe walks ice can make traversing surfaces more hazardous and can make activities such as scrambling very difficult. Traversing of exposed, elevated paths, arettes and edges becomes much more hazardous. Possibility of verglas on rock paths and climbs making movement very hazardous, especially if exposed to sunshine.
Advice**: Use layering with hat and gloves. If walking in mountainous areas avoid exposed paths, scrambles, arettes and edges. Consider carrying walking crampons, especially where there is the possibility of ice fields/verglas. If temperature is below -5C, especially if significant wind is present, avoid over-arduous activity, avoid long stops and consider use of mountain glasses or goggles. If walking across an ice field or verglas use walking crampons.
NOTES
* Risks to walkers due to weather are reasonably low on easy and moderate grade walks. The risks become more significant to those on intermediate grade walks as altitude and strenuousness increase and are very significant to those on hard and severe grade walks due to their altitude, difficulty and remoteness in mountainous areas.
** The safe use of full crampons (C1 to C3) and ice axes requires a degree of individual training. The leading of groups in conditions whereby use of such equipment might occur requires the leader to be a holder of the UK-MLTB Mountain Leader (Winter) Certificate. LYPWG will not, therefore, authorise walks where the use of full crampons and ice axes might be required.
The BBC Weather Department
The BBC has an excellent online series on understanding the weather as well as its own interpretation of the Met Offices weather data
Understanding the weather: http://www.bbc.co.uk/weather/features/understanding/
BBC Weather Main Page: http://www.bbc.co.uk/weather/
The UK Meteorological Office
The 'Met Office' site is a one stop shop for all your forecasting needs including localised forecasts and mountain weather
Main Page: http://www.metoffice.gov.uk/
UK Weather Map with Selectable Regions: http://www.metoffice.gov.uk/weather/uk/uk_forecast_weather.html
UK Mountain Areas Forecast: http://www.metoffice.gov.uk/loutdoor/mountainsafety/index.html
The Mountain Weather Information Service
MWIS is an excellent site with detailed three-day weather forecast reports, most updated daily, for each of Britain's mountain areas
Main Page: http://www.mwis.org.uk/
MetBrief
Are you a weather afficionado? Know your Lows from your Highs; know the difference between an isobar and an occluded front? Get the computer modelled North Atlantic Area synoptic charts for the next five days from this site
Synoptic Charts T to T+120: http://www.metbrief.com/EGRR.html