Weather at Sea

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Weather at Sea

• Clive Mason

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Why Do We Need Forecasts?

• Safety of passage
• Speed of passage
• Time available
• Wind direction and force
• Comfort of passage
• Sea state
• Heading

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Sources of Forecasts

• Observation
• Keeping a log and relating it to
• Forecast conditions
• Sail plan
• Boat speed
• Sailing directions
• Yachtsmens
• Admiralty
• Other

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FUNDAMENTAL CAUSE OF 'WEATHER'
Low pressure
High pressure
High pressure
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Differential Heating
Sun
The Suns energy heats the Earths surface
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Differential Heating

• Caused by different surface materials
• Land
• Water
• Snow
• Rock
• Vegetation

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WORLD WIDE EFFECTS
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CORIOLIS EFFECT
Deflected to the right
Equator
Deflected to the left
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CORIOLIS EFFECT
Northern Hemisphere
High Pressure
Winds blow clockwise and out of high pressure
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CORIOLIS EFFECT
Northern Hemisphere
Low Pressure
Winds blow anti-clockwise and in to low pressure
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Gas Laws 1
As air is heated, it expands
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Gas Laws 2
As a body of air expands, it cools
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Gas Laws 3
P X V N X T (N quantity)
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Gas Laws 4
A body of warm air tends to rise
Sinking air is compressed and warmed
Rising air expands and cools
A body of cool air tends to sink
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Water Vapour
A body of warm air tends to rise

• Water vapour is an invisible gas and comes from
  evaporation of
• sea
• lakes
• moist land surface
• snow
• ice

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Water Vapour
A body of warm air tends to rise
Cloud and fog are not water vapour They are
visible droplets of water suspended in the
atmosphere Water droplets often form on
particles of dust suspended in the
atmosphere 1,000,000 particles of water vapour
make a raindrop 2mm in diameter
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Dry and Saturated Air

• The amount of water vapour contained in a volume
  of air is governed by its temperature
• Warmer air holds more water vapour then cool air

• Dry air contains water vapour to some degree

• Saturated air contains the maximum amount of
  water for its temperature

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Dry and Saturated Air

• At 0C, 1m3 of air can contain 5g of water
• At 20C, 1m3 of air can contain 20g of water

• As air is warmed it can absorb more water
• As air is cooled it becomes saturated and the
  water condenses out as droplets

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Stability
On average the air temperature reduces
approximately 0.6ºC/100m of altitude
A body of warm air tends to rise
Sinking air is compressed and warmed
Pocket of air becomes stable and stops rising
5C
Rising air expands and cools
7C
A body of cool air tends to sink
10C
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Environmental lapse rate
On average the air temperature reduces
approximately 0.6ºC/100m increase of altitude
A body of warm air tends to rise
Sinking air is compressed and warmed
Lapse rate of dry air is 1.0ºC/100m Therefore
rising dry air cools faster than the environment
around it and soon becomes stable and stops
rising
Rising air expands and cools
A body of cool air tends to sink
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Stability
On average the air temperature reduces
approximately 0.6ºC/100m of altitude
A body of warm air tends to rise
Sinking air is compressed and warmed
Lapse rate of saturated air is about 0.5ºC/100m
at low altitudes The lapse rate of saturated
air is always less than dry air Therefore
saturated air cools slower than the surrounding
environment and is unstable and continues to rise
Rising air expands and cools
A body of cool air tends to sink
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Lapse Rates

• It is warmer at A or B ?
• On which side of the mountain is the dew point
  highest?

A
B
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Stratus Clouds
Cloud formation
Wind strength remains relatively constant
Large area of uplift
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Cumulus Clouds
Very tall cumulus clouds bring gusts, showers and
squalls
Rising air and moisture
Wind increases and decreases at the surface
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Cumulus Clouds
Fair weather cumulus Clouds are thinner than the
height of the base
Showers are possible Cloud height and thickness
are equal
Showers and gusts are probable Cloud thickness
greater than height of base
Localised uplift
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The Weather Atmosphere
Troposphere
Stratosphere
6 Miles
Tropopause
Weather happens here
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HIGH AND LOW PRESSURE
High
Low 50N
Low pressure forms under rising air
High 20N
Low
High 20S
Low 50S
High pressure forms under descending air
High
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HIGH AND LOW PRESSURE
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Pressure and Tides
Average UK air pressure is 1013mb
Sea level drops 1cm for each mb the air pressure
is above 1013mb
Sea level rises 1cm for each mb the air pressure
is below 1013mb
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PRESSURE BANDS
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PRESSURE ZONESNORTHERN SUMMER(June/Oct)
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PRESSURE ZONESNORTHERN WINTER(Dec/Apr)
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WIND PATTERNSNORTHERN SUMMER(Jul-Sept)
Trades
Trades
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WIND PATTERNSNORTHERN WINTER(Jan-Mar)
Trades
Trades
Trades
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Jan, Feb and March currents
1
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July, Aug and Sept currents