Bronze Group

Understanding the Weather

Aims of Session

•Understand the effects that the weather has on you and your activities.

•Where to look for weather information

•How to interpret that information to make an informed decision about your activities

Why do we need to know about the weather?

• Weather has a profound effect on the walking environment.

• Knowing what the weather is going to do is an essential part of the expedition planning process.

• Final decisions on itinerary, route etc can ultimately depend on the weather.

How does Wind Direction Effect the Weather?

• The air can swirl towards Britain from different directions. As it comes towards us it picks up the properties of whatever it is crossing.

• If the air comes from over the sea (wet/rain).

• If the air comes over land (dryer).

• Most common air flow to the UK is polar maritime (cold and wet).

• Then Tropical maritime (warmer but still wet).

Where does our weather come from?

Polar maritime

Polar Continental

Tropical

Maritime

Artic Maritime

Tropical Continental

Air Pressure

• The air flows seen in the previous slide are generated by pressure.

• Low Pressure is turbulent and volatile, producing bad weather.

• High Pressure is slow and stable, generally producing good weather.

Note: high pressure can also produce bad weather ie. Electrical storms etc.

• The wind runs roughly along the isobars – the closer the isobars, the stronger the wind, similar to contour lines.

• Lows circulate anti-clockwise

• Highs circulate clockwise

(see high and low pressure areas on following slide)

Air Pressure

Effects of Air Pressure

High Pressure Low Pressure

Warm in summer Colder

Cold in winter  

Clear skies Cloudy

Calm Windy; unsettled

Sunny Rain or snow

Air pressure determines wind and weather patterns.

The Effects of Wind

• Wind can have a tremendous effect on morale and safety.

• Strong winds can make conditions feel considerable colder.

• Wind behaves differently in hills & mountains; influenced by the shape of the land.

• Wind accelerates uphill due to it being squeezed between the earth and the troposphere.

• Its also accelerates through valleys etc for the same reason.

The Effects of Hills on the Wind

Wind and its effect on us!

• Do not underestimate wind chill

Beaufort Scale (Gale

Force)MPH Effect

How cold does it feel

1 - 3 2 - 20 Nil, good walking day +5

4 21 - 29 Wind chill cools you fast on the hill tops -3

5 30 - 39 Extra care needed on ridges -5

6 40 - 50 People lose balance and fall in gusts -8

7 51 - 61 Horizontal rain -12

8 62 - 74 Doubled over into wind -15

9 75 - 87 Streams blow back up hill -18

10 88 - 100 Effect trebled, you may be crawling -20

• Wind chill based on an ambient temp of 5 degrees C.

Rain & Hail

Rain or hail preceding or during a walk will have a profound effect on the party:

•Lowers morale

•Increases the risk of cold injury - especially if you do not have good quality waterproofs

It will also effect the going underfoot, making for:

•Slippery rocks

•Slower progress

Temperature

Knowing how cold or hot it going to be is important and will influence:

•Choice of clothing and spares

•Choice of route – particularly in hilly or mountainous terrain.

It is important to note that temperature changes with altitude especially in the wind.

Temperature with Altitude

600m 0°

500m 3° 1°

400m 2°

300m 4° 3°

200m 4°

100m 5° 5°

700m 1° -1°

Cloudy and Wet Clear and Dry

Sources of Information

• Internet - www.metoffice.gov.uk

• BBC weather app

• Tourist information

• Mountain rescue bases

• Newspapers

• Television & radio

• Look around!

• Check the forecast as close to the time of the expedition as possible to obtain the most accurate information.

Cloud Types

Cirrus - thin, wispy, curly-shaped clouds

Cumulo-nimbus- large, dense, towering clouds that cause rain and

thunderstorms

Cumulus - puffy clouds Stratus - Layered, horizontal clouds with a flat base

Clouds can indicate a change in the weather

Cloud formations are the main indicator of approaching bad weather.

• Cirrus clouds indicate fair weather in the immediate future – they can also be an indication of bad weather within the next 36 hours

• Stratus are low-lying solid clouds they bring drizzle or light snow.

• Cumulus clouds – if low clumps floating across the sky, there will be fair weather; vertical growth can indicate the start of a large storm.

• Cumulonimbus clouds are cumulus clouds that have grown vertically. These clouds bring stormy weather such as rain, lightning and hail.

Predicting Bad Weather

Predicting Bad Weather

• Weather systems bring a sequence of descending clouds as the bad weather approaches.

• Observation of devolving clouds can help you anticipate weather changes.

• A progression from high wispy or cirrus clouds through to lower layers or stratus clouds is a clear indication of an approaching warm front and subsequent unsettled weather.

• Cirrostratus clouds often form a halo around the sun or moon, so a halo around the moon is often a good early warning sign of rainfall arriving in the morning so don’t leave your stuff outside the tent!

Remember

• Hills are more likely to have their own micro-climate and localised wind.

• High pressure:

Summer – protect yourself against the sun

Winter – wrap up warm

• Low pressure:

Layers; wet weather gear

• Check the forecast before going into the hills

• Forecasts do not take account of wind chill

• Be prepared to change your plans in bad weather