NAVIGATION TRAINING

1
NAVIGATION TRAINING Section 11 Weather
2
Table of Contents

• Section 1 Types of Navigation
• Section 2 Terrestial Coordinates
• Section 3 Charts
• Section 4 Compass
• Section 5 Nautical Publications
• Section 6 Navigational Aids

3
Table of Contents

• Section 7 Buoyage
• Section 8 Position Lines and Fixes
• Section 9 Tides
• Section 10 Currents
• Section 11 Weather

4
Weather
5
Atmospheric Pressure
6
Atmospheric Pressure

• The standard atmosphere (symbol atm) is a unit
  of pressure and is defined as being precisely
  equal to 101.325 kilopascals, 1013.25 millibars,
  or 29.92 inches of mercury.
• The pressure gradient between a high pressure
  area and a low pressure area governs the strength
  of the wind, the wind blowing from high pressure
  to low pressure.
• The greater the gradient the stronger the wind.

7
Atmospheric Pressure

• An extreme example is the centre of a hurricane
  which can go as low as 94.8 kilopascals. The
  pressure gradient is huge, causing the winds to
  blow at 100 to 150 knots (nautical miles per
  hour).

8
Mean Sea Level Pressure

• Where air masses meet, there are well-marked
  boundary zones called fronts. This is where most
  cloud and precipitation occurs.
• In the northern hemisphere the circulation is
  anticlockwise around low pressure and clockwise
  around high pressure. The air flows almost
  parallel to the isobars but actually 10-15
  degrees inwards towards the low pressure.

9
Mean Sea Level Pressure
15 year average Mean Sea Level Pressure for June
July August 15 year average Mean Sea Level
Pressure for December January February
10
Global Circulation

• The Earth rotates at a constant rate, and the
  winds blow, the transfer of momentum between
  Earth/atmosphere /Earth must be in balance and
  the angular velocity of the system maintained.
• The atmosphere is rotating in the same direction
  as the Earth but westerly winds move faster and
  easterly winds move slower than the Earth's
  surface.

11
Global Circulation

• Remember winds are identified by the direction
  they are coming from, not heading to!

12
Weather Fronts

• Where air masses meet, there are well-marked
  boundary zones called fronts. This is where most
  cloud and precipitation occurs.
• In the northern hemisphere the circulation is
  anticlockwise around low pressure and clockwise
  around high pressure. The air flows almost
  parallel to the isobars but actually 10-15
  degrees inwards towards the low pressure.

13
Weather Fronts

• There are three types of front
• 1. Warm front
• 2. Cold front
• 3. Occlusions and Occluded Fronts

14
Warm Fronts

• When a warm moist air mass rises above a cold air
  mass, a warm front forms. The gradient of the
  front is very shallow. Warm fronts occur at the
  forward edge of a depression (a low-pressure
  system).

15
Warm Fronts
16
Warm Fronts
17
Cold Fronts
18
Cold Fronts
19
Cold Fronts
A cold front marks the advance of colder air
undercutting warm air. The gradient of the cold
front is steeper than that of a warm front, and
the rainfall is usually heavier. Thunderstorms
sometimes form along a cold front.
20
Occluded Fronts

• Depressions and other frontal systems have a
  three-dimensional structure.
• Most depressions weaken when the cold front
  catches up with the warm front and cuts it off
  from the ground.
• If the cold front rises over the warm front, this
  is a warm occlusion.
• If the cold front undercuts the warm front this
  is a cold occlusion.

21
Occluded Fronts

• Weather systems grow mature and decay and as
  they do, new ones are created. This creates
  families of weather systems.

22
Wind
23
Wind
Wind is primarily the result of uneven heating of
the earths surface, which causes large hotter
areas and large cooler areas.
24
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 0 0-1
0-1 Calm Sea like a mirror
25
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 1 1-3
1-3 Light air Ripples with the
appearance of
scales are formed, but without
foam crests.
26
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 2 4-7
4-6 Light Breeze Small wavelets, still
short,
but more pronounced. Crests
have a glassy appearance
and do
not break.
27
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 3 8-12
7-10 Gentle Breeze Large wavelets. Crests
begin
to break. Foam of glassy
appearance. Perhaps scattered
white
horses.
28
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 4 13-18
11-16 Moderate Breeze Small waves, becoming
larger
fairly frequent white horses.
29
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 5 19-24
17-21 Fresh Breeze Moderate waves, taking
a more
pronounced long form many
white horses are formed.
Chance of
some spray.
30
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 6 25-31
22-27 Strong Breeze Large waves begin to
form the
white foam crests are more
extensive everywhere.
Probably
some spray.
31
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 7 32-38
28-33 Near Gale Sea heaps up and white
foam
from breaking waves begins to
be blown in streaks along
the
direction of the wind.
32
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 8 39-46
34-40 Gale Moderately high waves
of greater
length edges of crests begin to
break into
spindrift. The foam is
blown in well-marked streaks
along the
direction of the wind.
33
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 9 47-54
41-47 Severe Gale High waves. Dense
streaks of
foam along the direction of the
wind. Crests of
waves begin to
topple, tumble and roll over.
Spray may affect
visibility.
34
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 10 55-63
48-55 Storm Very high waves with
long over-
hanging crests. The resulting
foam, in great
patches, is blown
in dense white streaks along the
direction of
the wind. On the
whole the surface of the sea
takes on a white
appearance.
The 'tumbling' of the sea becomes
heavy and
shock-like. Visibility
affected.
35
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 11 64-72
56-63 Violent Storm Exceptionally high
waves (small
and medium-size ships might be for
a time lost to
view behind the
waves). The sea is completely
covered with
long white patches
of foam lying along the direction
of the
wind. Everywhere the edges
of the wave crests are blown
into
froth. Visibility affected.
36
Wind Force
FORCE EQUIVALENT SPEED DESCRIPTION
SPECIFICATIONS FOR USE AT SEA 10 m above
ground miles/hour knots 12 73-83
64-71 Hurricane The air is filled with
foam and
spray. Sea completely white with
driving spray
visibility very
seriously affected.
37
Sea Breeze

• A sea-breeze (or onshore breeze) is a wind from
  the sea that develops over land near coasts.
• It is formed by increasing temperature
  differences between the land (which heats up
  faster) and water (which warms slower) which
  create a pressure minimum over the land due to
  its relative warmth and forces higher pressure,
  cooler air from the sea to move inland.

38
Sea Breeze
It generally occurs in the afternoon.
39
Land Breeze

• A land-breeze (or offshore breeze) is a wind to
  the sea that develops over land near coasts.
• It is formed by increasing temperature
  differences between the land (which cools faster)
  and water (which cools slower) which create a
  pressure minimum over the sea due to its relative
  warmth and forces higher pressure, cooler air
  from the land to move offshore.

40
Land Breeze
It generally occurs in the very early morning.
41
Katabatic Winds

• A katabatic wind, from the Greek word katabatikos
  meaning "going downhill", is a wind that blows
  down a topographic incline such as a hill,
  mountain, or glacier.
• The cold form of katabatic wind originates in a
  cooling, either radiatively or through vertical
  motion, of air at the top of the mountain,
  glacier, or hill.

42
Katabatic Winds

• Since the density of air increases with lower
  temperature, the air will flow downwards, warming
  adiabatically as it descends, but still remaining
  relatively cold.

43
Wind Force Sea State

• The visible effects of the wind on the sea will
  be modified by the relative directions of wind
  and tide.
• If the wind and tide are in opposite directions,
  then a larger chop will be created, giving the
  impression of the wind being stronger.
• If wind and tide are in the same direction, the
  amount of sea will be reduced, giving the
  impression of the wind being less.

44
Sea and Swell

• Sea is the effect of wind passing over the water
  locally.
• Swell is only found in the open ocean and will be
  effects of weather systems, hundreds of miles
  away.

45
Fog
46
Fog
47
Fog

• Fog is a cloud in contact with the ground.
• Fog differs from other clouds only in that fog
  touches the surface of the Earth.
• The same cloud that is not fog on lower ground
  may be fog where it contacts higher ground such
  as hilltops or mountain ridges.
• Fog is distinct from mist only in its density.

48
Fog

• Fog is defined as cloud which reduces visibility
  to less than 1 nautical mile, where as mist is
  that which reduces visibility to more than 1
  nautical mile.

49
Fog

• Fog forms when water vapor in the air at the
  surface begins to condense into liquid water.
• Fog normally occurs at a relative humidity of
  100. This can be achieved by either adding
  moisture to the air or dropping the ambient air
  temperature.
• Fog can form at lower humidities, and fog can
  sometimes not form with relative humidity at 100.

50
Fog

• Advection fog occurs when moist air passes over a
  cool surface by advection (wind) and is cooled.
  It is common as a warm front passes over an area
  significantly cooler. It's most common at sea
  when tropical air encounters cooler waters, or in
  areas of upwelling.

51
Upslope Fog
52
Other Types of Fog
53
Fog Slight Sea, Low Swell, Cloudy, Fine and
Clear
54
Precipitation
55
Orographic Rain

• Orographic rain (or relief rain) is caused when
  the warm moisture-laden wind blowing in to the
  land from the sea encounters a natural barrier
  such as mountains. This forces the wind to rise.
• With gain in altitude, the air expands
  dynamically due to a decrease in air pressure.
• Due to this the wind experiences a decrease in
  temperature, which results in the increase of the
  relative humidity.

56
Orographic Rain

• This causes condensation of the water vapour into
  water droplets to form clouds.
• The relative humidity continues to increase until
  the dewpoint reaches the level of condensation,
  causing air to be saturated.
• This height where the condensation occurs is
  called the level of condensation.
• When the cloud droplets become too heavy to be
  suspended, rain falls.

57
Orographic Rain in Howe Sound