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CGS Ground School
Meteorology
Wind
© Crown copyright 2012.
No part of this presentation may be reproduced without the permission of the issuing authority.
The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.
Fluid pressure
Air is a fluid and like all fluids will flow from high
to low pressure.
A good example would be a canal lock:
Gates closed.
A has more water and
therefore more pressure
than B.
Gates open.
Water flows from A to B
(high to low pressure) until
the water levels (pressures)
are equal.
A
B
Fluid pressure
Air is a fluid and like all fluids will flow from high
to low pressure.
A good example would be a canal lock:
The greater the pressure
difference, the faster the
flow from high to low.
Large pressure difference
A
B
Fluid pressure
Air is a fluid and like all fluids will flow from high
to low pressure.
A good example would be a canal lock:
The greater the pressure
difference, the faster the
flow from high to low.
Small pressure difference
A
B
Air pressure
In the same way air will flow from high pressure
to low pressure.
Again, the greater the pressure difference the
greater the flow.
The wind speed can
be calculated from a
synoptic chart by
measuring the
pressure gradient
1024
1028
(change of pressure
per unit distance).
The tighter the isobars,
the stronger the wind.
High
1000
1004
Low
Geostrophic force
As the air begins to move it is acted on by
another force, the geostrophic force.
The geostrophic force is due to the difference in
velocity of points near the equator compared with
points near the poles.
This is caused by the
Coriolis effect.
1024
High
1004
Low
Coriolis effect
Coriolis effect produces a force that prevents the
wind flow directly from high to low pressure.
Consider two identical objects placed on the
surface of the Earth; one at the North
Pole, the other at the equator.
Both objects (ball
bearings in this case),
are stationary relative
to the Earth beneath
them, but when we
consider how far each
one travels in a day,
we would see that they
were travelling at very
different speeds.
0 mph
0 mph
Coriolis effect
In 24 hours the object at the North Pole will rotate
through 360° but will not have any forward speed.
0 mph
360° per 24 hrs
The object at the equator will
0 mph
also rotate 360° but will travel
approximately 24,000 miles in
the same 24 hour period,
therefore it has a forward speed
of 1,000 mph.
Any object (including air) will
initially carry the momentum from
the point on the Earth from which
it originated.
0 mph
Therefore, if we now push our
360° per 24 hrs
objects towards each other, there
1000 mph
is a strange effect.
Coriolis effect
Both objects are travelling at their original
speeds over parts of the Earth’s surface that
are travelling at different speeds.
The ball bearing heading towards
the equator is going slower than
the Earth it travels over, and is
deflected to the right.
The ball bearing heading towards
the pole is travelling faster than
the Earth it travels over and is
also deflected to the right.
This is known as the Coriolis
effect. In the Northern
hemisphere, objects are deflected
to the right, in the Southern
hemisphere, they’re deflected to
the left.
0 mph
360° per 24 hrs
0 mph
0 mph
360° per 24 hrs
1000 mph
Coriolis effect
If we replace the ball
bearings with parcels
of air, the same effect
can be seen.
The overall effect in
the northern
hemisphere is that air
flows clockwise
around an
anticyclone,
and anti-clockwise
around a depression.
1024
High
1004
Low
Coriolis effect
Buys Ballot's Law is a
practical application
of the previous
information.
It states –
“In the Northern
hemisphere, with your
back to the wind, the
low pressure system
is on your left”
1024
High
1004
Low
Coriolis effect
Another application of
this information
comes during cross
country flying.
If the aircraft drifts
right it is flying from
high to low pressure.
If it drifts left it is flying
from low to high
pressure.
1024
High
1004
Low
Coriolis effect
We have seen that the Coriolis effect deflects the wind
to the right.
Since winds near the ground are slowed by friction,
obstructions etc., they travel more slowly and are less
affected by the geostrophic force. They are therefore
deflected less, travel more directly from high pressure to
low pressure and are said to have "backed".
It is for these reasons that
the wind appears to
increase and veer with
increases in height.
L
H
Sea breeze
A
the
name
occurs
to theto
Assea
At
altitude
thebreeze,
air rises
the as
airflow
the
pressure
is fromimplies,
near
the land
the
surface,
(high close
pressure)
over
coast.
It(low
is usually
light (10
kts) and
extends
land,
the
sea
reduces.
pressure).
However
therarely
airflow
is spread
Air starts
to20nm
flow
in
from
the band
sea toand
equalise
theis
more
inland.
over
athan
much
greater
height
its effect
pressure
difference.
Thisaislight
the off-shore
sea breeze.
The
day often
starts with
breeze.
negligible.
As
the
sunaair
rises
thethe
land
to heat
up fasteratthan
In
the
UK
sea
breeze
isbegins
The
rising
over
land
causes
the pressure
altitude
the
most
likely
to occur
in the
oversea.
the
land
to increase.
H
summer,
the
weather
The air flowing
inwhen
contact
in from
with
the
is
settled
with
land
the
sea
begins
causes
to warm
warm
the days
and
and
cloudless
nights.
therefore
airmass
over
rises.
the
sea to
subside and the
pressure at altitude to
reduce.
L
L
Sea breeze weather
The onset of a sea breeze is often sudden and may be
marked by a squall accompanying the change in wind
direction.
If an area of sea fog or low cloud exists just off shore then
the sea breeze will carry this inland, causing a rapid
deterioration in cloud base and/or visibility.
Coastal airfields
Consider the effect of a sea breeze at a coastal airfield.
Initially the sea breeze blows perpendicularly to the coast.
But towards the end of the afternoon the Coriolis effect
causes the wind to veer.
As night falls the land cools quicker than the sea.
This often leads to the process being reversed and a light
off-shore breeze developing during the night.
This whole cycle is then repeated the following day.
Winds and terrain
Valley winds
When a wind meets a range of mountains its progress is
impeded much like a river being blocked by large boulders.
Where a valley breaks this barrier the wind will flow at a much
increased velocity.
The Mistral is a typical example, which flows along the Rhone
valley at speeds of up to 75 kts.
Winds and terrain
During a sunny day the side of a hill facing the
sun will warm up and conduct this heat to the air
above it.
The warm air then rises
and flows up the hill.
The resulting wind is
termed an Anabatic wind.
Winds and terrain
At night the side of a hill will cool down and
conduct this heat loss to the air above it.
The cool air then
descends and flows
down the hill.
The resulting wind is
termed a Katabatic wind.
Katabatic winds are
usually more significant
than anabatic winds.
They are particularly
significant in fjords and
steep valleys.
Describing winds
The wind is usually described by giving its
direction of origin and its speed.
In aviation it is normally given in degrees and
knots, separated by a "/" and with the units
omitted.
200/20
350/10
Describing winds
When the wind direction changes clockwise it is
said to have veered.
Veered
270/20
200/20
Describing winds
When it changes direction anti-clockwise it is
said to have backed.
Backed
270/20
200/20
Describing winds
When the wind direction changes clockwise it
is said to have veered.
When it changes direction anti-clockwise it is
said to have backed.
Sudden, brief increases in wind velocity are
called gusts. They are extremely important to
pilots of light aircraft.
Temporary increases in wind velocity which
are of a longer duration are called squalls.
They are usually associated with rapidly
moving cold fronts, troughs or thunderstorms.
THE END
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