© TAFE MECAT 2008 Chapter 4 Atmospheric Motion and Wind.

Chapter 4

Atmospheric Motion and Wind

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.

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.

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.

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.

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

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


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.

Low pressure system

Low pressure systems usually result in;

Wet conditions

Cool temperatures

Unstable weather

and are renowned for ruining a good weekend

Low pressure systems

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!

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).

Pressure systems

Pressure systems

Global Temperature Gradient

Example of a temperature profile for ambient air temperature

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-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Degrees Latitude

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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?

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).

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.


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.


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).


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.


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.


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!)

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.

Latitudinal atmospheric circulation

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.

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.

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

Pressure gradient

Direction

The pressure gradient points from centres of lower pressure toward centres of higher pressure, as shown in Figure 4.1 below.

Pressure gradient

Pressure gradient

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.

Pressure Gradient Force (PGF)

PGF

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.


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.


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.

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.

Coriolis Force

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.

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'.

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’.

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.

Coriolis Force

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.

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.

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).

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!

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.

Friction

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

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.

Pressure gradient and the PGFVertical Pressure gradient force Vertical pressure gradient


This pressure gradient is about four times larger in magnitude than the horizontal PGF.

However, it is also balanced by gravity.

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.

Hydrostatic EquilibriumVertical Pressure

gradient forceGravitational 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?

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.

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.

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.

Other Factors

Terrain

The features of the land can also force air to move vertically.

This is called orographic lifting

© TAFE MECAT 2008 Chapter 4 Atmospheric Motion and Wind.

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Transcript of © TAFE MECAT 2008 Chapter 4 Atmospheric Motion and Wind.