Download Unit 6

What is wind velocity, and how is it
measured and reported?
■ What causes the wind to blow?
■ What is a pressure gradient, and how is it
related to the wind speed?
■ How do thermal wind systems (such as the
Hadley cells, sea breezes, land breezes,
etc.) develop?
■
Unit 6
Wind - Causes
Wind velocity is a vector quantity
Adding vectors
A vector has magnitude and direction
■ Wind is always reported as a speed
AND a direction
■ 230 at 35 kt means the wind is coming
FROM 230 degrees true (southwest)
with a speed of 35 kt
■ Wind from the west is a “westerly” wind
■ Wind is mostly horizontal, with a small
vertical component
■
–
Aircraft true
heading &
airspeed
Resultant true
course &
ground speed
Measured with an anemometer and
directional wind vane as a two-minute
average (sustained wind) at 10 m (30 ft)
■ Met winds always reported in deg.
■ TRUE whereas ATC gives wind as deg.
MAGNETIC unless stated otherwise
Wind representation on
surface and upper air charts
■
■
Wind
Vertical velocity is very important, however
Surface wind measurements
–
=
+
Given in METAR as 36020G27KT means
wind is from 360 TRUE at 20 kt gusting to
27 kt
See 4-4 and 4-5, and Gleim
Flag=50 kt
Barb=10 kt
■ Half-barb=5 kt
■ Circle=Calm
■ Shaft points in
direction from
which wind is
coming
■
290 at 35 kt
■
What is this
wind velocity?
What about this
one?
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Pressure gradients
Causes of wind
■
Wind caused when forces act on air parcels
(theoretical volumes of air)
–
Forces are vectors, and according to Newton:
1. Unless a force acts on a object (or parcel)…
–
–
if at rest, it remains at rest
if in motion, it continues in motion at same velocity
2. The sum of the forces on the parcel equal its mass
times its acceleration: Σ F= ma
■
The three forces that can act on a parcel:
–
–
–
Pressure gradient force
Coriolis force (discussed in next unit)
Friction force (discussed in next unit)
Difference in pressure between two points
divided by the distance between the points
(see Fig. 4-5 on p. 4-7)
■ Produces force from HIGH TO LOW pressure
■ Only horizontal press. gradients cause wind
■ Vertical pressure differences are normal and
occur because of gravity
■
–
–
–
Pressure (increases/decreases) with height
Upward pressure gradient balanced by gravity.
What is this balance called?_______________
In this case, would a parcel accelerate up or
down?
Causes of horizontal pressure differences
Example of pressure gradient
PG points toward
low pressure
■ PG perpendicular
to isobars
■ Largest where
isobars are closest
together
Low pressure
aloft
High pressure
aloft
Low pressure
aloft
■
L
980
990
100
0
How does the wind circulate around Earth?
L
H
Cold
Warm
1000 mb
1000 mb
Cold
L
Air
H
Warm
Cold
H
Rising P
1010 mb
L
Falling P
990 mb
Horizontal differences in warming or
cooling will produce pressure changes
which can lead to thermal circulations
Thermal circulation (local)
Air
L
Deeper
column
because of
expansion
High pressure
aloft
Air
H
Warm
L
Cold
L
H
L
Cold
Warm
Cold
H
L
H
H
L
H
RIsing P
1010 mb
Falling P
990 mb
Rising P
1010 mb
Rising P
Falling P
Rising P
Cold sea
Warm land
Cold sea
North Pole
Tropics
South Pole
A: Heating differences (in the above example,
the circulation cells are called Hadley cells)
•Called a sea breeze. How would a land breeze
develop (i.e., when land is colder than the sea)?
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