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

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Presentation on theme: "© Crown copyright 2009 Sailing Weather Penny Tranter 1 February 2015."— Presentation transcript:

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

2 © Crown copyright 2009 Weather and Climate

3 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)

4 Up and down Cool air (holds less water) Cool air (holds less water) Warm air (holds more water) Warm air (holds more water)

5 Up and down Cool air (holds less water) Cool air (holds less water) Warm air (holds more water) Warm air (holds more water) Water condenses out as cloud/rain

6 Up and down Low pressure High Pressure Low pressure High Pressure

7 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 south polar cell equatorial cell cell tropical cell

8 © Crown copyright 2009 Airmasses

9 © Crown copyright 2009 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

10 © Crown copyright 2009 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

11 © Crown copyright 2009 Source regions Cold and moist – Arctic OceanWarm and moist – Tropical AtlanticWarm and Dry – North AfricaSiberia? Dry, Warm in summer but Cold in winter

12 © Crown copyright 2009 Polar Maritime Returning Polar Maritime Tropical Maritime Tropical Continental Polar Continental Arctic Maritime British Isles Airmasses

13 © Crown copyright 2009 Weather and wind direction SOUTH-WEST warm and cloudy NORTH-WEST cloudy and showery NORTH-EAST cold and showery SOUTH-EAST warm and dry NORTH cold and showery SOUTH warm and dry EAST cold and dry (winter) Warm and dry (summer) WEST cloudy

14 Clouds

15 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

16 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

17 Clouds and low pressure systems

18 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

19 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

20 Warm air moving north/east Equator N. Pole height Cirrus Cirrostratus Nimbostratus Altostratus/Altocumulus 1200-1500 km 750-900 miles Cumulus Stratus Tropical air Polar air

21 “Ring around the moon … rain soon” “Mackerel skies and mares’ tails make tall ships carry low sails”

22 A warm front passes Warm air Cool air

23 Cold air moving south/east N. Pole Equator height 300-500 km 200-350 miles Nimbostratus Cumulonimbus Cirrus Cumulus Cumulonimbus Stratus Tropical air Polar air

24 © Crown copyright 2009 Cold Cool Warm One last thing… Occlusion  hidden Occluded fronts

25 © Crown copyright 2009 Development of an occluding depression Life cycle of a weather system

26 © Crown copyright 2009 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

27 © Crown copyright 2009 Barometric pressure and winds

28 © Crown copyright 2009 Force 4seen as limit of safety for many sailing boats and motor boats Force 6known as the ‘yachtsman’s gale’ Force 8usually when the wind starts to become a hazard for commercial shipping Beaufort Scale and its meaning

29 © Crown copyright 2009 Fall or rise 8mb in 3 hoursalmost certainly a Force 8 will follow 5mb in 3 hoursalmost 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 Changes in barometric pressure

30 © Crown copyright 2009 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

31 © Crown copyright 2009 Analysis and forecast chart interpretation



34 © Crown copyright 2009 Analysis and forecast charts Analysis What actually happened The analysis chart is produced for 00Z, 06Z, 12Z and 18Z A combination of actual data, NWP (numerical weather prediction) data and human analysis

35 © Crown copyright 2009 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

36 © Crown copyright 2009


38 Winds and weather

39 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

40 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

41 © Crown copyright 2009 Which way do the winds blow?

42 © Crown copyright 2009 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

43 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)

44 © Crown copyright 2009 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

45 © Crown copyright 2009 Wind speed - closer the isobars the stronger the wind Wind speed

46 © Crown copyright 2009 Wind - Channelling Gaps in barrier strengthen wind flow e.g. Strait of Dover, Central Scotland

47 © Crown copyright 2009 Forth Road Bridge - Channelling example W’ly Winds strengthen

48 © Crown copyright 2009© Crown copyright Met Office Coasts

49 © Crown copyright 2009 Sea Sea breezes Land Land heats up quicker than the sea – air expands and rises Circulation develops A simple view

50 © Crown copyright 2009© Crown copyright Met Office Sea breeze effects Coasts are usually sunnier than inland! The Sea Breeze Exeter Torquay

51 © Crown copyright 2009 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

52 © Crown copyright 2009 Any questions?

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