Air Pressure and Winds II
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RECAP
Ideal gas law: how the pressure, the temperature and the
density of an ideal gas relay to each other.
P  T   C
• measurements
Pressure and pressure
•
♦ Constant height
charts: isobars
♦ Constant pressure
charts: contour lines
Note wind direction in NH
♦ Low P -> Cyclones
♦ High P -> Anticyclones
Isobaric chart
• Constant
pressure, P=500 mb,
then plot heights: contour
lines
• Pressures
on surface maps <->
Heights on isobaric charts
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•
♦ Low <-> Low
♦ High <-> High
Pressure <-> Temperature
♦ Low <-> Low
♦ High <-> High
Note wind direction here
♦ Different from surface
winds
♦ Along the contour lines,
no crossing
• will
Newton’s Laws of Motion
Newton’s First Law: an object
•
♦ stay at rest, or
♦ maintain its motion at a
constant velocity and in a
straight line
as long as
♦ no force is exerted on the
object, or
♦ all forces cancel each other
Says who? An observer at rest
F
G



Ftotal  Fdrag  G
•
Newton’s Second Law
The force (F) experienced by an object is equal to its
mass (m) times the resulting acceleration (a).


F  ma
• Acceleration: change in velocity (magnitude or direction)
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•
Examples of accelerated motion
♦ Speeding up
♦ Slowing down
♦ Making a turn
A body with a large mass is more difficult to accelerate
(or stop). Objects with large masses are more inertial.
Forces acting in the atmosphere.
• Gravity force.
♦ Vertical force in a downward direction
• Atmospheric drag force (friction).
•
•
•
G  mg
1
 C air A 2
2
♦ Acts against the motion
Fdrag
♦ Proportional to velocity squared
Pressure force.
♦ From high to low pressure regions FP   pressure gradient
♦ The bigger the pressure gradient the larger the pressure
force
Centripetal force
♦ Acts perpendicular to the wind velocity,
V2
F
towards the center of the curvature.
r
♦ It changes the wind direction not the magnitude.
♦ Larger velocity and larger curvature (smaller radius)
correspond to a bigger centripetal force.
Coriolis force: due to the Earth’s rotation
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•
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Pressure gradient
force
Example: two tanks
♦ Tank A: full
♦ Tank B: half-full
Pressure at the base of
tank A is higher than the
pressure at the base of
tank B
The pressure gradient
force causes the water
to flow from A to B
(from high pressure to
low pressure)
Analogous phenomenon in
the atmosphere
Pressure Gradient
•
• given direction.
Vector: It has a magnitude and direction.
Magnitude: how fast the pressure is changing in a
P
PG 
d
• Direction: the direction
•
of the fastest increase of
the pressure.
Two examples:
♦ Color gradient
♦ Mountain slope
What is the direction of the color
gradient?
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Topographic maps
Elevation contours, index
contours (bold), slope
angle.
Steep slope: dense
contours
Gentle slope: contours
are further apart
Elevation -> pressure;
slope -> gradient
In which direction is the pressure gradient?
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•
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Pressure Gradient Force
The pressure gradient force is analogous to the
gravity force on a mountain slope
♦ Its magnitude is proportional to the pressure
gradient.
♦ Its direction is opposite to the direction of the
pressure gradient. It is from HIGH to LOW
pressure.
♦ It is perpendicular to the lines of constant
pressure (isobars).
The closer to each other the isobars are the larger
the gradient is and the larger the pressure force is.
The pressure force along an isobar is ZERO!
In which direction is the
pressure gradient force?
Wind and Pressure Map
Coriolis Force (Effect)
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• Due to the rotation of the coordinate system (Earth);
•
It is an apparent force;
It makes a moving object deflect from a straight line
even in the absence of any forces acting on it.
The Magnitude of the Coriolis Force
• The rotation of the Earth
•
•
♦ The faster the planet
rotates the bigger the
force
The speed of the object
♦ Bigger V -> bigger
effect
The latitude:
♦ Min. at the equator
♦ Max. at the poles
Fco  2mV sin 
Coriolis force
as a function of:
• The speed of the object
• The latitude:
♦ Min. at the equator
♦ Max. at the poles
Fco  2mV sin 
The Direction of the Coriolis Force
• right of the direction of motion.
•
• deflected to the right and in the Southern
In the Northern hemisphere the deflection is to the
In the Southern hemisphere the deflection is to the
left of the direction of motion.
The winds in the Northern hemisphere will be
hemisphere they will be deflected to the left.
♦ Hurricanes spin differently in the Northern and
Southern hemisphere
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The Coriolis Force and the Earth
The Coriolis effect is important when moving over
LARGE distances (air plane travel), with large
velocities, away from the equator.