Transcript 14 03 06 Understanding the Weather

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?
Artic Maritime
Polar maritime
Polar
Continental
Tropical
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
Air pressure determines wind and weather patterns.
High Pressure
Warm in summer
Low Pressure
Colder
Cold in winter
Clear skies
Cloudy
Calm
Windy; unsettled
Sunny
Rain or snow
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!
•
Wind chill based on an ambient temp of 5 degrees C.
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
Do not underestimate wind chill
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
700m
1°
-1°
600m
500m
0°
3°
1°
400m
300m
2°
4°
3°
200m
100m
4°
5°
Cloudy and Wet
5°
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, curlyshaped 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