Transcript 07a.6 Weather - Canadian Coast Guard Auxiliary (Pacific)

NAVIGATION TRAINING
Section 11
Weather
CANADIAN COAST GUARD AUXILIARY - PACIFIC
Table of Contents
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Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Types of Navigation
Terrestial Coordinates
Charts
Compass
Nautical Publications
Navigational Aids
CANADIAN COAST GUARD AUXILIARY - PACIFIC
Table of Contents
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Section 7
Section 8
Section 9
Section 10
Section 11
Buoyage
Position Lines and Fixes
Tides
Currents
Weather
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Weather
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Atmospheric Pressure
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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.
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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).
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Mean Sea Level Pressure
•Where air masses meet, there are wellmarked 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.
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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
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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.
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Global Circulation
Remember winds are identified by the direction
they are coming from, not heading to!
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Weather Fronts
•Where air masses meet, there are wellmarked 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.
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Weather Fronts
• There are three types of front:
1. Warm front
2. Cold front
3. Occlusions and Occluded Fronts
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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 lowpressure system).
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Warm Fronts
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Warm Fronts
Weather
Phenomenon
Prior to the
Passing of the
Front
Contact with
the Front
After the
Passing of the
Front
Temperature
Cool
Warming suddenly
Atmospheric
Pressure
Decreasing steadily
Levelling off
Warmer then
leveling off
Slight rise followed
by a decrease
Winds
S to SE
Variable
S to SW
Precipitation
Showers, snow, sleet or
drizzle
Cirrus, cirrostratus,
altostratus, nimbostratus,
and then stratus
Light Drizzle
None
Stratus, sometimes
cumulonimbus
Clearing with
scattered stratus,
sometimes
scattered
cumulonimbus
Clouds
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Cold Fronts
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Cold Fronts
Weather
Phenomenon
Prior to the
Passing of the
Front
Contact with
the Front
After the
Passing of the
Front
Temperature
Warm
Cooling suddenly
Atmospheric
Pressure
Decreasing Steadily
Levelling off then
increasing
Cold and getting
colder
Increasing steadily
Winds
S to SE
Variable and Gusty
W to NW
Precipitation
Showers
Clouds
Cirrus and cirrostratus,
changing later to cumulus
and cumulonimbus
Heavy rain or snow,
sometimes hail
Cumulus and
cumulonimbus
Showers then
clearing
Cumulus
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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.
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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.
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Occluded Fronts
• Weather systems grow mature and decay and
as they do, new ones are created. This creates
families of weather systems.
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Wind
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Wind
Wind is primarily the result of uneven heating of
the earth’s surface, which causes large hotter
areas and large cooler areas.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
0
0-1
0-1
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Calm
Sea like a mirror
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
1
1-3
1-3
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Light air
Ripples with the appearance of
scales are formed, but without
foam crests.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
2
4-7
4-6
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Light Breeze
Small wavelets, still short,
but more pronounced. Crests
have a glassy appearance and
do not break.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
3
8-12
7-10
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Gentle Breeze
Large wavelets. Crests begin
to break. Foam of glassy
appearance. Perhaps scattered
white horses.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
4
13-18
11-16
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Moderate Breeze
Small waves, becoming larger;
fairly frequent white horses.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
5
19-24
17-21
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Fresh Breeze
Moderate waves, taking a more
pronounced long form; many
white horses are formed.
Chance of some spray.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
6
25-31
22-27
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Strong Breeze
Large waves begin to form; the
white foam crests are more
extensive everywhere.
Probably some spray.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
7
32-38
28-33
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Near Gale
Sea heaps up and white foam
from breaking waves begins to
be blown in streaks along the
direction of the wind.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
8
39-46
34-40
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
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.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
9
47-54
41-47
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
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.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
10
55-63
48-55
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Storm
Very high waves with long overhanging 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.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
11
64-72
56-63
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
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.
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Wind Force
FORCE EQUIVALENT SPEED
10 m above ground
miles/hour knots
12
73-83
64-71
DESCRIPTION
SPECIFICATIONS FOR USE AT SEA
Hurricane
The air is filled with foam and
spray. Sea completely white with
driving spray; visibility very
seriously affected.
CANADIAN COAST GUARD AUXILIARY - PACIFIC
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.
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Sea Breeze
It generally occurs in the afternoon.
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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.
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Land Breeze
It generally occurs in the very early morning.
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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.
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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.
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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.
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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.
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Fog
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Fog
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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.
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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.
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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%.
CANADIAN COAST GUARD AUXILIARY - PACIFIC
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.
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Upslope Fog
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Other Types of Fog
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Fog
“Slight Sea, Low Swell, Cloudy, Fine and Clear”
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Precipitation
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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.
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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.
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Orographic Rain in Howe Sound
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