BROXBOURNE SCHOOL YEAR 12 EXPEDITION TRAINING

The weather controls an expedition more than anything else, it is the one thing that we cannot change. It is vital that we know what to expect in wild country.

Mountain weather conditions can change dramatically from one hour to the next. A morning that dawns clear and sunny can turn into an afternoon plagued by thunderstorms and gale force winds. In fact, gale force winds can be blasting the top of a mountain while the valleys below are calm. As a result, anyone planning to venture into a mountainous region, whether hill walking in the Peak District, or mountain climbing in the Andes or Himalayas, needs to know what type of weather to expect. It is vital that they check the forecast before they leave and, while in the mountains, that they pay constant attention to any changes in the weather.

Basic forecasts can be obtained from the TV, radio, and newspapers., but these are very general for mountain areas. There are websites that you can visit for a more detailed Forecast for a mountain area. Or you can phone; Met Office Snowdonia

Wind is simply air in motion, moving from high pressure to low pressure. Winds are described by the direction they blow from – an easterly wind lows from the east, a westerly from the west.

Beaufort number Wind speed DescriptionLand conditions kt km /h mp h m/s CalmCalm. Smoke rises vertically Light airWind motion visible in smoke Light breezebreezeWind felt on exposed skin. Leaves rustle Gentle breeze Leaves and smaller twigs in constant motion Moderate breeze Dust and loose paper raised. Small branches begin to move Fresh breezeSmaller trees sway Strong breeze Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult Near galegaleWhole trees in motion. Effort needed to walk against the wind GaleTwigs broken from trees. Cars veer on road Strong galeLight structure damage StormTrees uprooted. Considerable structural damage Violent storm Widespread structural damage. 12>63 >11 7 >7 2 >32.7Hurricane

Clouds Clouds form when rising air cools to a point where it can no longer hold its water vapour. The vapour condenses in the form of tiny droplets. Air is lifted because of: Widespread ascent i.e. up a warm/cold front Encountering barriers - mountains and hills Convection - when air is heated at low levels by contact with sun warmed ground – creating rising bubbles of air i.e. thermals..

How do we Describe Clouds? Clouds can be cirrus (wispy), cumulus (heaped) and stratus (in layers). They are classified into 10 main cloud types according to height and shape. Not all clouds bring rain, some are signs of fine weather. The word ‘nimbus' added to the beginning or end of a cloud type, e.g. cumulonimbus or nimbostratus, means that the cloud is a rain cloud and will usually appear to be dark grey. Generally cumulonimbus are the most dangerous clouds associated with hail, lightning, tornadoes, downdraughts, downbursts and flashflooding.

The first kind of cloud is a cirrus cloud. Cirrus clouds are thin and wispy, and are found very high in the sky. Cirrus clouds point in the direction the wind is blowing. No precipitation falls from cirrus clouds. They are a sign of fair weather for now and the very near future. Next are the altocumulus clouds. These clouds are like fuzzy bubbles in long rows or strings. If you see altocumulus clouds, it usually means that a cold front is coming. If it's summertime and warm and humid, altocumulus clouds may turn into thunderstorms when the cold front approaches. After that are the nimbostratus clouds. These clouds are dark and low-hanging clouds that cover the whole sky. They bring with them light to moderate precipitation. Cumulonimbus clouds are thunderstorm clouds. They are tall and wide clouds that have heavy rain, thunder and lightning. If you see these clouds on the horizon, it is likely it will rain very soon. Then there are fair weather cumulus clouds. These clouds look like big cotton balls in the sky. They have very large spaces of clear sky in between them. Normally they don't carry any rain, but during the spring and summer they can change into cumulonimbus clouds