© Crown copyright 2009 Sailing Weather Penny Tranter 1 February 2015.

© Crown copyright 2009

Sailing WeatherPenny Tranter 1 February 2015


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


Airmasses


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


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


Source regions

Cold and moist – Arctic OceanWarm and moist – Tropical AtlanticWarm and Dry – North AfricaSiberia? Dry, Warm in summer but Cold in winter


PolarMaritime

ReturningPolar Maritime

TropicalMaritime

TropicalContinental

PolarContinental

Arctic Maritime

British Isles Airmasses


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


Cold Cool

Warm

One last thing…

Occlusion hidden

Occluded fronts


Development of an occluding depression

Life cycle of a weather system


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


Barometric pressure and winds


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


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


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


Analysis and forecast chart interpretation


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


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


Which way do the winds blow?


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)


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


Wind speed - closer the isobars the stronger the wind

Wind speed


Wind - ChannellingGaps in barrier strengthen wind flow

e.g. Strait of Dover, Central Scotland


Forth Road Bridge- Channelling example

W’ly Winds strengthen


Sea

Sea breezes

Land

Land heats up quicker than the sea – air expands and rises

Circulation develops

A simple view


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


Any questions?

© Crown copyright 2009 Sailing Weather Penny Tranter 1 February 2015.

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Transcript of © Crown copyright 2009 Sailing Weather Penny Tranter 1 February 2015.