Transcript Sailing Weather Slide Show

Sailing Weather
Penny Tranter
© Crown copyright 2009
1 February 2015
Weather and Climate
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The difference between ‘weather’
and ‘climate’?
• Weather is the state of the atmosphere at a particular
place and time
• Climate is the average weather condition of a particular
part of the world (often over many decades)
Up and down
Cool air (holds less water)
Warm air (holds more water)
Up and down
Cool air (holds less water)
Water
condenses
out as
cloud/rain
Warm air (holds more water)
Up and down
Low pressure
High Pressure
Climate zones
Climate - the average weather conditions of a particular
part of the world
Polar jet stream
– cold
air to north, warm air to south
polar cell
Cold and dry
tropical cell
High pressure
Low pressure
Changeable – often wet
60°
equatorial
cell
High pressure
Low pressure
Hot and dry
30°
Hot, showers &
thunderstorms
Airmasses
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Airmasses need to sit quietly for
a long time over a large area to
develop…under high pressure
• Airmasses are characterised by temperature and
moisture content
• Cold areas lead to cold airmasses
• Warm areas lead to warm airmasses
• Development over oceans leads to moist airmasses
• Development over land lead to dry airmasses
• What about our area (mid-latitudes)?
• Not suitable, too much movement ie low pressure
areas
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Four types of source region
• Warm and moist - Tropical oceans
• Known as Tropical maritime
• Warm and dry - Desert regions
• Known as Tropical Continental
• Cold and moist – Arctic/Atlantic
ocean
• Known as Polar Maritime
• Cold and dry - Canada and Siberia
• Known as Polar Continental
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Source regions
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Siberia?
Warm
Cold
Warm
Dry, and
Warm
and
moist
Dry
in summer
– Arctic
Tropical
North but
Africa
Ocean
Atlantic
Cold in winter
British Isles Airmasses
Polar
Maritime
Returning
Polar
Maritime
Tropical
Maritime
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Arctic
Maritime
Polar
Continental
Tropical
Continental
Weather and wind direction
NORTH-WEST
cloudy and
showery
NORTH
cold and showery
NORTH-EAST
cold and showery
EAST
cold and dry (winter)
Warm and dry (summer)
WEST
cloudy
SOUTH-WEST
warm and
cloudy
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SOUTH-EAST
warm and dry
SOUTH
warm and dry
Clouds
CLOUDS
• What are they made of and what can they tell us about
the weather?
• 10 basic types – split into 3 categories
• Categories are high, medium and low
LOW CLOUDS Cumulonimbus
• Very high and large heaped cloud – water at bottom and
ice at top
• Characteristic anvil shape to the top
• Most dangerous cloud for anyone who works or is active
outdoors
• Source of heavy showers, thunderstorms, tornadoes and
hail
Clouds and low pressure systems
Climate zones
• Air descending down through the atmosphere usually
results in dry, settled conditions over the Earth’s surface
• Air rising upwards through the atmosphere leads to
disturbed weather, bringing rain
Which way is the low pressure
and its fronts moving?
Cold polar air
Warm
tropical air
ONLY direction of movement determines the type of front
Warm air moving north/east
Polar air
height
Tropical air
Cirrus
Cirrostratus
Altostratus/Altocumulus
Stratus
Equator
Cumulus
Nimbostratus
1200-1500 km
750-900 miles
N. Pole
“Ring around the
moon … rain soon”
“Mackerel skies and
mares’ tails make tall
ships carry low sails”
A warm front passes
Cool air
Warm air
Cold air moving south/east
height
Polar air
Cumulonimbus
Tropical air
Cirrus
Cumulus
Nimbostratus
Cumulonimbus
Stratus
N. Pole
300-500 km
200-350 miles
Equator
One last thing…
Occluded fronts
Warm
Cold
Cool
Occlusion  hidden
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Life cycle of a weather system
Development of an occluding depression
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Fronts are …
• usually associated with a band of thick cloud
and rain
• a change of air mass / weather conditions
• a warm front marks a change from cool, dry air to
warm, moist air
• a cold front marks a change from warm, moist air to
cold, dry air
• showers often happen after a cold front has
passed
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Barometric pressure and winds
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Beaufort Scale and its
meaning
Force 4
seen as limit of safety for many sailing boats
and motor boats
Force 6
known as the ‘yachtsman’s gale’
Force 8
usually when the wind starts to become a
hazard for commercial shipping
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Changes in
barometric
pressure
Fall or rise
8mb in 3 hours
5mb in 3 hours
almost certainly a Force 8 will follow
almost certainly a Force 6 will follow
if Force 3 or less when you see this –
you have about 4 to 8 hours notice
Not the time to be caught on a ‘lee’ shore – eg a southerly
on the south coast!
1 or few mb erratic indicative of squall lines, sudden
change
strong gusts or lulls
with dark
thunderclouds
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Weather bomb!
The scientific term for a weather
bomb is an ‘explosive deepening’
The phenomenon happens in a
rapidly deepening area of low
pressure and is characterised by a
decrease in atmospheric pressure
of at least 24 millibars in 24 hours
The lower the pressure, the
stronger the winds become. A
Scottish storm which had a drop of
44mb - gusts of 165mph were
recorded over the Highlands
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The North Atlantic is
particularly prone to
weather bombs thanks to
the Gulf Stream, which
pits a reliable source of
warm air against cold air
Analysis and forecast chart
interpretation
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Isobars
• Lines joining points of equal mean sea level
pressure
• Average pressure in UK ~ 1013 hPa
• Unusual to be above 1050 or below 950 hPa
• Used to identify
• wind speeds/directions
• anticyclones (highs)
• depressions (lows)
• troughs
• ridges
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• cols
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Winds and weather
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Wind direction
• The direction that the wind is blowing from eg a northerly
wind is coming from the north
• Most common direction in southern England is a south
westerly
Wind direction
• Backing/turning left – an anti-clockwise change in direction
eg from N to NW through NNW
• Veering/turning right – a clockwise change in direction eg
from N to NE through NNE
Which way do the winds blow?
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Depressions, lows, cyclones
• Winds blow anti-clockwise around a low (in the
northern hemisphere)
• Depressions are associated with unsettled
weather
• Air is generally rising
• Rising motion generates cloud and precipitation
• Often fronts are associated with low pressure
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Winds around low pressure
• Buys-Ballot law – when you are standing with your back to the wind the
area of low pressure is on your left (in the Northern Hemisphere)
• Around a low pressure area winds go in an anti-clockwise direction (in
the Northern Hemisphere)
Highs, anticyclones
• Winds blow clockwise around a high (in the northern
hemisphere)
• Anticyclones are associated with settled weather
• Air is generally sinking
• Sinking motion causes clouds to disperse
• In summer: fine weather
• In winter: fog/frost
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Wind speed
Wind speed - closer the isobars the stronger the wind
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Wind - Channelling
Gaps in barrier strengthen wind flow
e.g. Strait of Dover, Central Scotland
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Forth Road Bridge
- Channelling example
W’ly Winds strengthen
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Coasts
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Met Office
Sea breezes
A simple view
Land heats up
quicker than the
sea – air expands
and rises
Circulation
develops
Land
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Sea
Sea breeze effects
Coasts are usually sunnier than inland!
The Sea Breeze
Exeter
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Met Office
Torquay
Sea Breeze Front
• Convergence zone/sea
breeze front is generated
when warm air from land
meets cool air from sea
• Cornwall can have at least 2
sea-breezes meet and converge
on A30 – cloud and rain.
• Gliders use them very
effectively
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• If Humid and unstable –
can trigger thunderstorms –
often seen near the English
south coast – but not right
on the beach!
Any questions?
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