Download Coriolis Force

Chapter 4
Atmospheric Motion and Wind
© TAFE MECAT 2008
Introduction

We are all familiar with the notion, and motion, of
wind, but questions arise;
 What
exactly is it?
 What
causes it?
 How
does wind differ when we change the scale of our
surroundings?

These are the types of questions that will be answered
in this chapter.
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Introduction

It was mentioned in Chapter 1 that the weather plays
an enormous part in our lives for many reasons

So, along with temperature and precipitation, wind is a
major factor in determining whether we are enjoying
ourselves, or whether our lives are being blown apart
by a cyclone.
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Introduction

Once again we find that scale is very important when
examining meteorological phenomena

We find that there are different types of winds, along
with different causes, for each of the global, synoptic,
regional and micro scales.
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Horizontal motion

High and low pressure systems
 The
concept of how pressure changes in a vertical column
through the atmosphere was mentioned in Chapter 3,
 We
now need to apply that concept to the air around us on
the horizontal surface of the Earth.
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High pressure systems

A high pressure system results from the subsidence
(falling) of air.

For air to fall, it must lack buoyancy, and therefore be
denser (i.e. cooler) than the air around it.

When the air parcel subsides to the ground, it cannot
go further, and therefore it diverges along the surface
of the Earth in much the same way that water would if
it was poured onto the ground
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High pressure systems

A high pressure system generally exhibits dry, very
stable conditions with slight breezes.

These pressure systems create the perfect sunny
days that we crave in both winter and summer
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High pressure systems
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Low pressure systems

A low pressure system results from rising air.

For air to rise, it must exhibit buoyancy, and be less
dense (i.e. hotter) than the surrounding air.

Such circumstances are easily obtained when the
sunlight heats the air.

Figure 4.2 below
shows how this happens.
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Low pressure system
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Low pressure systems

Low pressure systems usually result in;

Wet conditions

Cool temperatures

Unstable weather

and are renowned for ruining a good weekend
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Pressure systems

The air molecules above the ground create a pressure
on the Earth’s surface (called Mean Sea Level
Pressure),
 On
average it is equal to 1013.25 hectapascals.
 The
problem is that this pressure measurement is not the
same all over the surface of the Earth, the pressure is
different at different locations relative the average value of
1013.25 hPa!
 The
important question therefore becomes “Why does the
air pressure differ over the Earth’s surface?” Fortunately, the
answer is quite simple!
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Pressure systems

The Sun’s light does not shine on the whole surface of
the Earth ‘evenly’.

As a result, the Equator is delivered more of the Sun’s
energy per unit of area than the poles

The result of this uneven distribution of the Sun’s
energy means that there is a temperature gradient
stretching from the north and south poles (relatively
cold) to the equator (relatively warm).
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Pressure systems
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Global Temperature Gradient
Example of a temperature profile for ambient air temperature
40
30
Temperature (degrees Celcius)
20
10
0
-10
-20
-30
-40
-50
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
Degrees Latitude
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30
40
50
60
70
80
90
100
Pressure Systems

Such a temperature gradient obviously causes the air
at the equator to heat up quicker, and to higher
temperatures,

This means that the equatorial belt is in a relatively
constant state of low pressure. But what happens to
the rising air?
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Longitudinal atmospheric motion

Warm, moist air that rises in the equator in what is
called the Inter-tropical Convergence Zone, or ITCZ

reaches troposphere heights where it hits a virtual
‘ceiling’ called the tropopause, slowly cooling as it
does so.

This blocks the upward movement and the air, which
is forced to move either north or south (it cannot fall on
top of itself as the air is constantly rising underneath
it).
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Longitudinal atmospheric motion

The air continues on its path towards the poles until it
starts to fall again at approximately 25-30° latitude.

This falling air causes a belt of high pressure systems
(called the mid-latitude highs) when it diverges on the
surface where
 the
air and is either ‘sucked’ back into the equator becoming
part of the trade winds to replace the air that is rising,
 or
diverges toward the poles as a westerly wind.
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Longitudinal atmospheric motion

This cycle of rising, moving and falling air is called the
Hadley cycle

When the air rises, it cools down and any moisture in
the air mass condenses out and becomes rain.

What this does is dry out the air that moves north and
south, creating phenomena known as mid-latitude
desertification.

The whole process is shown in the Figure 4.3 below.
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Longitudinal atmospheric motion
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Longitudinal atmospheric motion

The Hadley cell is one of three major latitudinal cells
that control the distribution of high and low pressure
systems on Earth.
 The
second cell is called the Ferrel Cell and its winds move
in the opposite direction to the Hadley (in the same way that
two meshed gears operate).
 Finally,
there is a Polar Cell, whose direction is opposite the
Ferrel Cell (and therefore exhibits the same north/south flow
as the Hadley Cell).
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Longitudinal atmospheric motion
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Longitudinal atmospheric motion

These three ‘super-cells’ go a long way in explaining
the presence of high and low pressure systems due to
the subsidence of air at the boundaries of each cell.

This results in a global semi-permanent pattern of high
and low pressure areas, one of which has already
been mentioned, the mid-latitude highs.
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Longitudinal atmospheric motion

Overall, we observe low pressure systems at the
equator (0° latitude), and in the sub-polar low pressure
belts located at approximately 60° north and south.

We observe high pressure systems at the poles (90°)
and in the mid-latitudes at 25-30 ° latitude north and
south.
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Latitudinal atmospheric circulation

It is not only the longitudes that see dramatic
circulations of air masses.

A very similar phenomenon occurs latitudinally as well.

The most significant of these is;
 the
Walker Circulation and
 the
Jet Streams (not forgetting the winds at the surface, but
these come in detail later!)
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Latitudinal atmospheric circulation

The Walker Circulation is described as being thermally
driven, as opposed to the Hadley-Ferrel-Polar cells
which are temperature gradient driven.

This circulation consists of three super-cells that cover
the entire equator, with each cell stemming from one
of three major lands; the Central Americas, Africa
(Middle East) and Australasia

Generally speaking, to make something move, we
need a phenomenon called force, and for simplicities
sake, we shall view force as a simple kind of energy.
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Horizontal Forces

The primary forces which affect horizontal motion are;
 Pressure
gradient force (PGF) which is a consequence of
a pressure gradient
 Coriolis
force which is a consequence of the rotation of the
Earth
 Friction
force which is a consequence of the roughness of
the Earth surface and the difference between solid Earth and
the gaseous atmosphere.
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Pressure gradient

The distribution of pressure in the atmosphere is not
the same all over on the surface of the Earth, nor is it
the same through a vertical slice of the atmosphere
 The
increase in pressure over a distance is called a
pressure gradient.
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Pressure gradient


A pressure gradient has two significant attributes;

Magnitude

The magnitude is based on the distance between the high and low
pressure centers, and how strong the high and low pressure centers are.
This results in;

an increase in the magnitude of the pressure gradient if the distance
between high and low pressure systems is close, and the pressure
difference between high and low pressure systems is large,

and a decrease in the pressure gradient when the opposite applies
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Pressure gradient

Direction
 The
pressure gradient points from centres of lower pressure
toward centres of higher pressure, as shown in Figure 4.1
below.
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Pressure gradient
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Pressure Gradient Force

The pressure gradient is associated with the Pressure
Gradient Force, or PGF.
 Its
magnitude is directly proportional to the magnitude of the
pressure gradient, but its direction is the opposite.
 We
know heat moves from an area of higher temperature to
the area of lower temperature so does pressure behaves
similarly.
 Pressure
is directed from areas of higher pressure toward
areas of lower pressure.
 The
pressure here is a measure of air pressure, so the
pressure gradient force drives air outward from high
pressure regions to low pressure regions.
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Pressure Gradient Force (PGF)
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Pressure Gradient Force

You should notice that the pressure gradient force is
perpendicular to the isobars (the black lines). If the
isobars (lines of equal pressure) were closer together,
then the PGF would be greater, and the wind would
blow harder.
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Pressure Gradient Force

The figure above implies that the PGF forces the air to
‘fall’ in a straight line from the high to the low pressure
system, and if the earth did not rotate, that is exactly
what would happen.

Horizontal air motion in the Southern Hemisphere
sees the air spiral out of high pressure areas in an anti
clockwise direction and spirals into low pressure areas
in a clockwise direction.

This spiralling is due to the Coriolis force.
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Coriolis Force

Any one point on the Earth’s surface completes one
full rotation in a west-to-east direction in one day.

This rotation creates the Coriolis Force. It is strange
because it is based only on latitude, not any
meteorological phenomena.
 The

Coriolis force increases as latitude increases.
At the equator, it has no effect on horizontal flow.
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Coriolis Force

The reason for this is simple, if you roll a ball across a
disk, that ball will travel in a straight line.

But if you spin the disk clockwise (like the earth, if you
were looking down on the South Pole) then the ball
will appear to curve.

We say ‘appear’, because the ball actually goes
straight, but you continue to move.
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Coriolis Force
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Coriolis Force
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Coriolis Force

The atmosphere spins with the earth as though it were
a solid body.

If it didn't there would be extremely strong winds,
particularly at the equator, where a point on the earth
is moving at 1670 km/hr because of the earth's
rotation.

The deflections that are imperceptible for objects like
footballs traveling short distances, but they are
important over long distances.
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Coriolis Force

The Coriolis force can therefore balance the pressure
force so that, in the southern hemisphere, the air will
flow clockwise around a centre of low pressure and
anticlockwise around a centre of high pressure.

When the Coriolis force and the pressure force are in
balance, the wind blows along the isobars and not
across them.

This is called the 'geostrophic wind'.
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Coriolis Force

The geostrophic wind is a theoretical wind and is
only an approximation to the actual wind

In reality friction, pressure differentiation and nearby
air masses with other physical and mechanical
attributes all contribute to throw the approximated
geostrophic wind ‘off course’.
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Coriolis Force

The PGF is the primary force pushing the air from high
to low pressure centres,

The Coriolis force that makes air spiral out of high
pressure systems and into low pressure systems.

The PGF and the Coriolis forces play significant roles
in atmospheric motion, but there is one more factor we
need to consider, and that is the rough surface of the
Earth.
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Friction Force

Friction is a force which impedes motion. Because our
atmosphere is not a vacuum

Objects in motion will eventually slow down because
of collision with other matter,

The friction forces point opposite the direction of
motion which slows an object down.
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
Friction is based on two factors:
 the
speed of the object in motion,
 and
the relative viscosity of the fluid through which the object
travels which means that friction is greater the denser the
fluid.

Friction is an important force on air near the surface
because of the rough terrain, but is insignificant above
about 1 km.
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
Therefore, the layer of air in which friction is important
is known as the friction layer.

The air above the friction layer is known as the free
atmosphere because there is no significant friction.

The height of the friction layer varies greatly, but is
usually between 1 and 2 km. The friction layer is also
known as the atmospheric boundary layer (ABL).
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
But why is friction important?
 Friction
‘stuffs up’ the whole balance of the other two forces
(PGF and Coriolis) by undermining the Coriolis force.
 It
does this because friction reduces the velocity (wind
speed), which in turn reduces the Coriolis force,
 Ultimately,
if there is a balance between all three forces, and
you reduce two of them, then the third one has to give out as
well!
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
Friction, Coriolis, and the PGF are summed together
to get the net force placed on a parcel of air.

If this net force is zero, there is said to be a balance
of forces.

If there is a net force greater than zero, then the parcel
of air is said to have an acceleration.
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Friction
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Vertical Motion

Air doesn’t only move horizontally across a surface, but can
also move vertically from the surface.

There are three major forces that influence vertical motion;


Vertical PGF

Gravity

Friction
There are also three other factors influencing vertical motion;

Convergence

Buoyancy

Terrain
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Vertical pressure gradient

Pressure changes dramatically with height, forming a
pressure gradient directed toward higher pressure
values which are of course near the Earth’s surface

If the pressure gradient follows the direction of the
increase in pressure, then there must be a very strong
vertical pressure gradient force (VPGF) directed
upward.

The figure below shows the actual pressure gradient,
as well as the directions of the gradient and the force.
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Pressure gradient and the PGF
Vertical Pressure gradient force
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Vertical pressure gradient
Pressure Gradient Force

This pressure gradient is about four times larger in
magnitude than the horizontal PGF.

However, it is also balanced by gravity.
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Gravity
 Gravity
is that familiar force that is produced by the earth (or
any mass) and pulls all objects towards its center.
 Gravity
causes an acceleration which is equal to the familiar
9.8 meters per second squared.
 This
force is approximately the same magnitude as the
vertical PGF, but points in the opposite direction and
therefore produces a net force near zero.
 When
gravity and the PGF perfectly balance each other, we
call the situation hydrostatic equilibrium.
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Hydrostatic Equilibrium
Vertical Pressure
gradient force
Gravitational force
When the net force between the balance of the VPGF and
the force of gravity = 0, then the atmosphere is said to be in
a state of hydrostatic equilibrium. But what happens when
the forces are not equal to 0?
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Friction

Friction influences vertical motion just as it does
horizontal motion.
 If
a parcel is moving up, friction is directed downward, and
vice versa.
 However,
remember that parcels can reach a level where
the frictional influence is approximately zero.
 This
occurs in the free atmosphere above the friction layer.
 In
the layer below the free atmosphere, friction plays a major
role in air motion.
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Other Factors

Convergence
 We
have mentioned that air spirals into low pressure centers
due to the earth's spin.
 But
where does the air go then? It cannot go into the ground,
and so must compress and move upwards rapidly. T
 The
zone where the air meets is known as a convergence
zone. Convergence can happen in the horizontal direction
as well.
 Air
converges at the surface in low pressure systems and
diverges aloft.
 The
opposite (divergence) takes place in high pressure
system. Convergence is one method of vertical air motion.
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Other Factors

Buoyancy
 As
air warms, it expands, becoming less dense.
 If
the air around it is sufficiently denser, then the air parcel
will become buoyant and rise, or "float" upward.
 Since
density changes as temperature changes, the
temperature profile of a layer is very important.
 If
the surface is warm, and it is very cold aloft, then warming
air parcels will be able to rise rapidly to great heights, since
the temperature of the surrounding air decreases more
rapidly than the temperature of the warm air parcel.
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Other Factors

Terrain
 The
features of the land can also force air to move vertically.
 This
is called orographic lifting
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