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CPL Meteorology
Wind
Prepare to be blown away!
Wind
The term wind refers to the flow of air across
the earth’s surface.
Measuring Wind
An anemometer and wind vane are used to
measure the direction and speed of the wind.
Measurement of the wind is complicated by surface
features such as terrain, buildings and trees.
Wind Reporting
Wind direction and strength are significant and are reported
together as wind velocity.
Example
25005KT or 250/05
In forecasts, meteorologists relate wind direction to true
north due to the large geographic areas involved, often
encompassing areas of different magnetic variation.
Wind reported by ATC or ATIS relates to a local area and so is
given in magnetic direction.
Changes in Wind Direction
• A clockwise change in wind direction is called
a veering wind.
• An anticlockwise change in wind direction is
called a backing wind.
Changes in Wind Speed
• A gust is a momentary increase in wind speed above
the mean reported speed.
– Gusts greater than 10K above the mean wind speed are
included in forecasts:
35025G40KT
• A lull is a momentary decrease in wind speed below
the mean reported speed.
• A squall is a sudden increase in wind speed by at least
16 knots, the speed increasing to at least 22 knots and
lasting for not less than 1 minute.
Changes in Wind Velocity
• Any change in wind velocity (speed or direction) is the
result of an acceleration of the air mass. In order to
accelerate an air mass, a force must be applied to it.
• When all forces acting on an air mass are balanced
there is no acceleration and the air mass will continue
its current motion.
• When all forces acting on an air mass are balanced a
steady wind velocity results, which is known as
balanced flow.
Pressure Gradient Force
• The pressure gradient force acts to move air
from areas of high pressure to areas of low
pressure.
Pressure Gradient Force
• Isobars are lines used on meteorological charts to
join areas of equal pressure.
Pressure Gradient Force
• The greater the pressure difference over a given
distance, the stronger the pressure gradient force.
Pressure Gradient Force
• Observation of the true flow of the wind shows that it does
not simply flow from areas of high pressure to areas of low
pressure.
Coriolis Effect
• The Coriolis Effect is an apparent force that
acts on a moving parcel of air.
• It is the result of the
conservation of
momentum of a
parcel of air as it
moves relative to
the equator.
Coriolis Effect
• The circumference of the earth
at the equator is 40,075km.
• Any point on the equator
therefore has a rotational
velocity of about 1,700km/hr.
• The circumference of the earth
at 60 degrees latitude is
19,981km.
• Any point at 60 degrees of
latitude therefore has a
rotational velocity of about
833km/hr.
Coriolis Effect
Geostrophic Wind
• The PGF starts a mass of air moving.
• The Coriolis “force” causes the air to turn.
• When these two forces are in equilibrium the wind blows
parallel to the isobars in a balanced flow called the
geostrophic wind.
Buys Ballot’s Law
If you stand with your back to the wind, the
lower pressure is on your right in the southern
hemisphere.
H
L
Gradient Wind
Isobars are rarely straight, but usually follow a
curved path around high and low pressure
systems.
The balanced flow of air around curved isobars
is called the gradient wind.
Surface Wind
• Friction slows the wind down in the layer below 2000ft AGL.
• Slower moving wind reduces the Coriolis effect.
• Pressure gradient force is no longer balanced by the Coriolis effect
resulting in a tendency for the wind to diverge from higher pressure
systems and converge into lower pressure systems.
Surface Wind
Converging and diverging surface and upper winds in the
northern hemisphere.
Surface Wind
Gradient wind compared
to surface wind (in the
northern hemisphere).
In the southern
hemisphere, the surface
wind veers clockwise
when compared to the
gradient wind.
Surface Wind
• The nature of the surface determines how much friction is produced and
therefore how great the effect is.
• Land surfaces tend to cause the wind to slow to about half of the
strength of the gradient wind and veer about 30°.
• Ocean surfaces produce less friction and cause the wind to slow to about
two-thirds of the gradient wind strength and veer only about 10°.
Diurnal Variation in Surface Wind
• Changes in surface temperature over a 24 hour period cause
variations in surface wind.
• During the day convection (vertical mixing of the air)
promotes a greater mixing of the warmer surface air with the
higher levels – reducing the temperature gradient.
• This results in the surface winds resembling the gradient wind
more closely by day than when compared to the surface
winds by night.
• That is to say that the surface wind by day will be both
stronger and have backed when compared to the surface
wind by night.
Sea Breeze
• The sea breeze is a local, small scale wind.
• Advection caused by the different rates at which the
air is heated over different surfaces.
Sea Breeze
• Land surfaces have a lower specific heat than water
surfaces – it takes less energy to heat the land by 1°C
than it does to heat the water by 1°C.
• The land therefore heats up much faster than the
water during the day.
• This produces a temperature and pressure gradient
between the land and sea air masses.
• Sea breeze usually occurs between
noon and 1 hour after sunset.
Land Breeze
• Similar process to the sea breeze.
• Advection caused by the different rates at
which the air is cooled over different surfaces.
Katabatic Winds
• Caused by the cooling of the air at night over
sloped surfaces.
• Air near the top of the slope is cooled greater
than the air at the same level but further from
the surface.
Katabatic Winds
• Gravity “pulls” the cold air mass down the
slope resulting in winds that can exceed 30
knots.
• Strength of the Katabatic wind depends on:
– The length of the slope
– The smoothness of the slope
– The steepness of the slope
– The length of the night
– Season (summer/winter)
Anabatic Winds
• Heating of the mountain slopes by day causes
air in contact with the surface to become
warmer than air at the same level but further
form the surface.
Anabatic Winds
• The density of the air near the surface becomes
lower than the surrounding air and so it rises up
the slope.
• Weaker than the katabatic wind as it is opposed
by gravity rather than assisted by it.
• Observed more frequently on westward-facing
slopes during summer
afternoons.
Föhn Wind
• Unsaturated air is forced up a slope and cools to the point of
becoming saturated and produces cloud.
• As the Saturated air continues to be forced up the slope the cloud
precipitates on the windward side, losing its moisture.
• As the now unsaturated air descends on the lee side of the slope, it
warms adiabatically producing a warm dry wind.
• DALR = 3°C/1000ft
• SALR = 1.5°C/1000ft
Föhn Wind
Low-Level Jetstream
• When the leading edge of a high pressure
system or ridge of high pressure approaches a
mountain range it’s migration is obstructed
and a low-level jetstream is produced.
Low-Level Jetstream
• Requirements to produce low-level jetstream:
– Strong surface inversion
– Mountain range barrier north-south
– Ridge of high pressure approaching range from
the west
– Subsidence inversion to prevent the flow escaping
over the range
Low-Level Jetstream
• Characteristics of a low-level jetstream:
– Strongest in the early morning (0600-0900)
– Can reach up to 70kt
– Usually a southerly wind
– Western side of ranges
– Strong vertical and lateral windshears
– Normally confined to within 3000 ft of the surface
– Breaks up when surface inversion dissipates
DIY Wind…
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