Post on 14-Dec-2015
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Sailing WeatherPenny Tranter 1 February 2015
© Crown copyright 2009
Weather and Climate
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)
Warm air (holds more water)
Water condenses
out as cloud/rain
Up and down
Low pressure High Pressure
Climate zones
30°
60°
Low pressure
High pressure
Low pressure
Cold and dry
Changeable – often wet
Hot and dry
Hot, showers & thunderstorms
Climate - the average weather conditions of a particular part of the world
High pressure
Polar jet stream – cold air to north, warm air to southpolar cell
equatorial
cell
tropical cell
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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
Cold and moist – Arctic OceanWarm and moist – Tropical AtlanticWarm and Dry – North AfricaSiberia? Dry, Warm in summer but Cold in winter
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PolarMaritime
ReturningPolar Maritime
TropicalMaritime
TropicalContinental
PolarContinental
Arctic Maritime
British Isles Airmasses
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Weather and wind direction
SOUTH-WESTwarm and
cloudy
NORTH-WESTcloudy and
showery NORTH-EASTcold and showery
SOUTH-EASTwarm and dry
NORTHcold and showery
SOUTHwarm and dry
EASTcold and dry (winter)
Warm and dry (summer)
WESTcloudy
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
ONLY direction of movement determines the type of front
Which way is the low pressure and its fronts moving?
Warm tropical air
Cold polar air
Warm air moving north/east
Equator N. Pole
heig
ht
CirrusCirrostratus
Nimbostratus
Altostratus/Altocumulus
1200-1500 km750-900 miles
Cumulus
Stratus
Tropical air Polar air
“Ring around the moon … rain soon”
“Mackerel skies and mares’ tails make tall ships carry low sails”
A warm front passes
Warm air
Cool air
Cold air moving south/east
N. Pole Equator
heig
ht
300-500 km200-350 miles
NimbostratusCumulonimbus
Cirrus
Cumulus
Cumulonimbus
Stratus
Tropical airPolar air
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Cold Cool
Warm
One last thing…
Occlusion hidden
Occluded fronts
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Development of an occluding depression
Life cycle of a weather system
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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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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
Beaufort Scale and its meaning
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Fall or rise8mb in 3 hours almost certainly a Force 8 will follow 5mb in 3 hours 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 lullswith dark thunderclouds
Changes in barometric pressure
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Weather bomb!
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
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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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
• cols
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Winds and weather
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
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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
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)
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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 - closer the isobars the stronger the wind
Wind speed
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Wind - ChannellingGaps 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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Sea
Sea breezes
Land
Land heats up quicker than the sea – air expands and rises
Circulation develops
A simple view
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Sea breeze effects
Coasts are usually sunnier than inland!
The Sea Breeze
Exeter Torquay
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Sea Breeze Front
• Convergence zone/sea breeze front is generated when warm air from land meets cool air from sea
• If Humid and unstable – can trigger thunderstorms – often seen near the English south coast – but not right on the beach!
• 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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Any questions?