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Chapter 6 Atmospheric Forces
We constantly attempt to control the motion of objects around us. We push
chairs, open doors, lift books, and throw balls. These actions require the application of a
force. Applying a force to an object often moves the object. Wind is air in motion that
arises from a combination of forces. Violent, destructive winds result from a complex
interplay of different forces, just as gentle summer breezes.
Weather involves the wind, or the lack of wind, and thus forces.
Complex
interactions link winds, temperature, pressure, moisture, clouds, and precipitation to one
another. These connections result in forces and provide the energy to drive various
atmospheric circulations. The purpose of this chapter is to explain the forces of nature
responsible for the winds and to illustrate how to use weather maps to interpret wind
direction and speed. This chapter is an introduction to atmospheric dynamics.
In introducing atmospheric dynamics we will make use of simple diagrams, or
models. While simple, and sometimes unrealistic, these models provide a conceptual
view of how the atmosphere works. We will use these models to explain observations of
the ‘real world.’ We will also begin to explore the coupling between circulations of the
upper troposphere and the surface wind. The different spatial scales of weather systems
are also defined in this chapter. Detailed discussions and examples of these scales of
motion are the topics of following Chapters.
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Magnitude and Direction of Forces
Weather results from the interaction of many forces. When discussing these
interactions we are concerned with the direction and magnitude, or strength, of the force
exerted on an object. These two factors, represented in diagrams by arrows, determine
what effect the force has on the object. The arrow points in the same direction as the
force. The length of the shaft represents the magnitude.
Two or more forces can act to pull or push at the same point on a body. A single
force, the net force or resultant, which reproduces the same effect on the body as the
separate forces, can replace these forces. Suppose we apply two forces to a body that will
only act to move the object. If the two forces act in the opposite direction and with
different magnitudes, the object will move in the direction of the stronger force (as in a
tug-of-war). A single force can represent these two forces whose direction is the same as
the stronger force and whose magnitude is the difference between the two forces (Figure
6.1).
Forces often act at an angle to each other. In this situation we need an approach to
determine the magnitude and direction of the net force.
One way is to graphically construct a parallelogram
(Figure 6.2) using two forces to represent two of the
Force exerted on an object equals the
body’s mass times the acceleration
that is produced. Force has a direction
and a magnitude.
sides. The diagonal of the parallelogram represents the net force of the two given forces.
The length of the diagonal represents the magnitude of the combined forces.
The
direction of the net force is along the diagonal and away from where the two forces are
applied.
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Laws of Motion
Applying a force often results in movement. When an object is changing its
position with reference to another body, we say it is in motion. As with force, motion has
a magnitude and a direction. The speed of the object, the distance traveled in a given
amount of time, is the magnitude of the motion.
Wind is air in motion. Weather reports include
wind speed and direction. Wind speed is reported on
Wind Gust is an abrupt and
momentary increase in the wind
speed.
weather maps in knots--one nautical mile per hour (equivalent to 1.1508 statute miles per
hour or 0.5144 meters per second). If the wind speed is strong (greater than 15 knots) and
highly variable, the weather report will include wind gusts, the maximum observed wind
speed.
Wind direction is reported as the direction from which the wind is blowing. It is
reported with respect to compass directions or the number of degrees east of north. A
north wind blows from the north to the south (Figure 6.3). Windward refers to the
direction the wind is coming from, while leeward denotes the direction the wind is
blowing towards. The prevailing wind direction of a region is the most frequently
observed wind direction during a given period of time.
Wind that changes speed or direction has undergone acceleration. If a body
increases its speed at a constant rate, it undergoes a uniform acceleration. A free falling
body is a good example of an object that undergoes
Acceleration is the time rate of
uniform acceleration. If we neglect air resistance, a change of velocity. An object that is
accelerated under goes a change in
falling body undergoes an acceleration of 9.8 meters speed and/or a change in direction of
motion.
per second for each second it falls. The object’s velocity increases 9.8 meters per second
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after each passing second.
This downward acceleration results from the earth’s
gravitational pull and is called the acceleration of gravity. Galileo discovered that the
acceleration of two falling objects is the same, even if their weights are very different.
Astronaut David Scott demonstrated this on August 2, 1971 by dropping a hammer and a
feather at the same moment while on the moon. The moon has no atmosphere so there
was no significant friction from air that would affect the fall of the feather more than the
hammer in our atmosphere. The objects hit the ground at the same time, though the
acceleration was less than 9.8 meters per second per second.
In the seventeenth century Sir Isaac Newton established the fundamental laws
which describe the motion of bodies: the law of inertia, the law of momentum, and the
law of reaction.
Newton's First Law: Law of Inertia
Newton’s first law of motion states that a body at rest tends to stay at rest while a
body in motion tends to stay in motion traveling at a constant speed and in a straight line
until acted upon by an outside force. If a bus suddenly starts the passengers lurch
backward. The passengers are initially at rest and tend to remain at rest when the bus first
starts. The moving bus is exerting a force that eventually gets the passengers moving at
the same speed. If the bus suddenly stops, the passengers surge forward. They tend to
remain in motion. If the bus makes a quick, sudden right turn the passengers, who want
to continue traveling in a straight line, are crammed towards the left side of the bus. The
resistance of an object to changing its velocity is called inertia.
Air at rest will tend to stay at rest until a force puts it in motion. We will discuss
the forces that generate wind in the next section.
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Newton's Second Law: Law of Momentum
The momentum of an object is its mass multiplied by its velocity. Two objects
can have the same speed, but the one with more mass has greater momentum. Newton’s
second law states: When a force acts on a body, the body’s momentum is changed by an
amount that is proportional to the applied force and the amount of time the force acts on
the body. Hitting a baseball with a bat is good example of this law. If you want to hit the
ball far, you have to swing the bat hard to increase the force applied to the ball on contact.
You also want to follow through with your swing to keep the bat in contact with the ball
as long as possible.
Applying a force to an object changes its momentum by changing the speed at
which it travels. A light breeze will not move a sailboat as fast as a strong steady wind.
Back to our bus for a final example, as the bus accelerates, its momentum, and the
momentum of the passengers inside, increases.
Law of Reaction
Firing a gun moves a bullet forward, but there is an equal force in the backward
direction referred to as the ‘kick.’ Newton’s third law states that for every action (force)
there is an equal and opposite reaction (force). When a cup of coffee is placed on a
table, a downward force is exerted on the table because of gravity. The table exerts an
equal and opposite force in order to support the cup.
Forces that Move the Air
There are different forces that act to move air (Figure 6.4). The weight of air is a
force that always acts downward, towards the center of the earth. Other forces are the
pressure gradient force, Coriolis force, friction, and centripetal force. We must examine
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these four forces to determine which direction the wind will blow and which direction
storms will move.
Pressure Gradient Force
Spray paint cans exhibit warning labels that the contents are under pressure.
When the nozzle is squeezed, or the sides punctured, the large pressure difference
between the air and the inside of the can force the contents out of the can. Generating
pressure differences is also important in flying and sailing (Box 7.1) Pressure differences
exert a force and when not balanced by other forces cause movement. The force that
results from pressure differences in a fluid such as our atmosphere is called the pressure
force or the pressure gradient force. Air moves because of a pressure gradient force.
The existence of a pressure gradient force is essential for sustaining winds.
The pressure gradient force (PG) always acts from high pressure towards low
pressure. Its magnitude is equal to the pressure gradient, or the rate of change in pressure
with distance at a specific time divided by the air density.
PG 
Chang in Pressure
1
air density
Distance
When pressure changes rapidly over a small distance, the pressure gradient force is large.
Strong winds result from large pressure gradients.
On the surface weather map, measured atmospheric pressure at the surface is
converted to sea level pressure and analyzed. As noted in Chapter 1, when comparing the
pressure of two cities it is important to reference the pressure measurements to the same
altitude. If the pressures are not referenced to the same altitude, when analyzing
differences in pressure between the two cities, we will just see differences in the altitude
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of the cities, since atmospheric pressure always decreases with altitude. Measured surface
pressure is adjusted to what the pressure would be if the city were at sea level. To
accomplish this we need to determine the pressure that would be exerted by a column of
air that extends from the location of the weather station too sea level, and add this
pressure to the observed pressure (Figure 6.5). The web page discusses how these
corrections are made. On average, this correction is approximately 1 mb for each location
10 m of altitude
Isobars of constant sea level pressure are drawn at 4 mb intervals starting at 1000
mb. Figure 6.6 is an example of a surface weather map. The winds near Chicago are
greater than those winds near Oklahoma City. The isobars are spaced closer together over
the Great Lakes than over Oklahoma. Since the pressure difference between any two
isobars is fixed, the closer the isobars the larger the pressure gradient, the stronger the
pressure gradient force, and the greater the wind speed.
The surface map plots atmospheric pressure adjusted to sea level. Another type of
weather map plots the altitude of a given pressure
surface. This map is commonly used when analyzing
the weather above the surface and is called a constantpressure chart or isobaric chart.
Constant-pressure
Isobaric map is commonly used to
study the weather above the surface.
This constant pressure chart includes
information on the temperature, wind
speed and direction, humidity, and the
altitude at a given pressure.
charts are commonly drawn for 850, 700, 500, 300, 250, and 100 millibars. Figure 6.7 is
an example of a 500 mb map corresponding to November 10, 1976. The units of altitude
are called geopotential meters and are nearly equivalent to geometric meters measured
with a ruler. Isobaric maps are useful for portraying horizontal pressure gradients. The
spacing between the lines of constant height (isoheight) portray the pressure gradient
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force. To see this consider Figure 6.8. The colored surface is a constant pressure surface
of 850 mb. Anywhere on this surface the pressure is 850 mb. The altitude of this 850 mb
surface varies, the altitude is higher at City A than at City B. Above this figure is an
analysis of the pressure gradient force along this 850 mb surface. Atmospheric pressure
always decreases with increasing altitude, so the steeper the pressure surfaces the greater
the pressure gradient force. The magnitude of the pressure gradient force is weak near
City B and stronger near City A. Also shown in Figure 6.8 are constant-height lines of
the 850 mb pressure surface. The magnitude of the pressure gradient force is proportional
to the spacing of the contour lines. The closer the constant-height lines the greater the
pressure gradient force. The direction of the pressure gradient force is perpendicular to
the lines of constant-height, and point towards lower heights.
To summarize, on an isobaric map the lines of constant-height of the pressure
surface indicate the direction of the pressure gradient force and the magnitude. Referring
back to Figure 6.7, the stronger winds are located in regions where the spacing of the
constant-height lines is a minimum, as expected because this is where the pressure
gradient force is the largest. However, the winds are not in the direction of the pressure
gradient force! The winds, in general, are blowing to the right of the pressure gradient
force, and are nearly parallel to the isoheight lines! This suggests that there must be more
than one force acting on the winds.
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Coriolis force
Newton’s laws of motion are defined with respect to a fixed reference frame.
However, Earth is moving and so we must include an
apparent forces that accounts for our moving reference
frame. This apparent force is called the Coriolis force.
The magnitude of the Coriolis force is proportional to
the speed of the wind. If the wind speed is zero, there
Coriolis force is an apparent force
used to describe the observed
deflection of moving objects caused
by the observer’s moving frame of
reference. The magnitude is
proportional to the wind speed and the
latitude. It always acts to the right of
the wind.
is no relative motion and the Coriolis force is zero. An object’s inertia increases with
speed, so a larger force is required to change its direction of travel. The Coriolis force
increases with increasing wind speed.
The Coriolis force acts perpendicular to the
direction of motion (to the right of the wind in the Northern Hemisphere) and therefore
cannot change the wind speed. The Coriolis force cannot generate a wind, it can only
change its direction.
The magnitude of the Coriolis force depends also on of latitude. At the equator the
Coriolis force is zero and it increases towards the poles. To conceptualize this consider
launching a rocket from city X to city Y (Figure 6.9). As the rocket heads northward
towards Y the earth is moving beneath it. Yet, the rocket will land to the east of Y! Both
cities make one complete revolution each 24 hours. Each city is moving eastward at an
angular velocity of 15 per hour (360 in 24 hours). X is closer to the equator where the
latitude belt is largest. Thus, while the two cities have the same angular velocity, X is
moving eastward faster, because it has more distance to travel in the same amount of time
(Figure 6.10). The launched rocket carries an eastward velocity component as it streaks
towards Y, a city that is moving eastward at a slower rate than the rocket. Because of this
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difference in latitude, the launched rocket lands off course, to the east of Y and appears to
have been deflected to the right of its motion. If the rocket were launched from Y to X, it
would still appear to veer to the right of its path.
We have to consider the Coriolis effect only when dealing with winds that travel
over long distances. It takes only a few seconds for a thrown ball to reach its destination.
During this time the earth has hardly moved and the deflection of the ball’s path is
insignificant. Since we do not have to adjust for the Coriolis force when playing football,
soccer, tennis, or Ping-Pong, there is no reason to assume that the Coriolis force has
anything to do with the spinning motion of a flushed toilet. However, play catch on a
rapidly revolving merry-go-round, with one person in the center and the other on the outer
edge. When thrown, the ball will appear to curve away from its intended target.
Centripetal force
Air often travels in a curved path and is therefore, according to Newton's First
Law, being accelerated. An acceleration involves a change in speed, a change in direction,
or a change in speed and direction. According to Newton’s first law a net force must be at
work causing the air parcels to turn. This net force is called a centripetal force. When
the centripetal force is removed from a body, or reduced in magnitude, the object ceases
traveling along a curved path and moves in a straight line because of inertia.
The direction of the centripetal force is always towards the center of curvature
(centripetal means seeking a center) perpendicular to the direction of motion. A
centripetal force simply states that a net force points radially inward along a curved path.
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Friction
Friction opposes, or decelerates, the wind. Friction is largest in the atmospheric
boundary layer, the air layer nearest the ground.
Friction always slows down the wind, and therefore
Boundary layer is the atmospheric
layer that is within 1 to 4 kilometers of
the surface.
also reduces the Coriolis force. The roughness of the
surface and the speed of the wind determine the magnitude of frictional force. The force
of friction over still water or an ice pond is small. Trees in a forest strongly oppose the
wind, so the frictional force of forests is large. While the wind may be blusterous over an
ice pond, it will be calmer in the woods.
We shall consider the effects of friction when dealing with the air within 20 to
200 meters of the surface. This layer is called the surface layer; it is here that frictional
drag is important in determining wind direction and speed. In the surface layer friction
acts to slow the wind, and thus always acts opposite to the wind direction. Its magnitude
increases with increasing wind speed.
Friction near the surface not only slows the wind, it mixes the air. This mixing
produces turbulence. Air motions that are turbulent are highly erratic, and therefore
difficult to predict.
We shall now make observations of winds in the atmosphere. We will then
explain these observations by applying what we have learned about forces. Remember
that only a pressure gradient force can generate a wind. Friction can slow the wind speed.
The Coriolis forceacts to change the direction of the wind, but has no effect on the wind
speed.
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Hydrostatic Balance
As we've seen, horizontal differences in pressure generate a horizontal pressure
gradient force. Powerful hurricane force winds are generated with pressure changes of
only 50 mb over a distance of a few hundred miles. Atmospheric pressure decreases with
altitude generating a vertical pressure gradient. Near the earth's surface the pressure
decreases by about 10 mb for every 100 meter increase in elevation, a much stronger
pressure gradient than in a hurricane! Why don’t we have strong vertical winds as a
result of this strong vertical pressure gradient? Because motion results from the
imbalance of forces. The weight of air is a force that always acts downwards, towards the
center of the earth, and usually balances the upward pressure gradient force. This is
referred to as hydrostatic balance (Figure 6.11). The atmosphere is in approximate
hydrostatic balance, explaining why vertical motions are typically small.
Air motion and Energy balance
An energy imbalance can lead to a force that generates a wind. Energy imbalances
are the impetus of atmosphere and ocean circulations.
Consider the air between two pressure surfaces, say 850
mb and 300 mb (Figure 6.12). Initially the isobars are
The thickness of a layer between two
fixed pressure levels increases as the
air is warmed.
parallel, and the vertical separation between the surface is 700 meters. Now we heat the
column of air. Since we are concerned with the air between the 850 and 300 mb layers,
the vertical pressure difference has not changed; it is still 550 mb. Heating the air lowers
the density and therefore, to maintain equality, the vertical height difference between the
two pressure surfaces must increase.
Because atmospheric pressure decreases with
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altitude, an outward pressure gradient force is generated near 300 mb and an inward
pressure gradient force near 850 mb.
Observations of Upper Air Winds
Let’s return to Figure 6.7 and make three observations.
1. The winds are fastest where the spacing of the isolines is smallest. This is expected,
as it is in these regions where the pressure gradient force is largest. But the wind is
not blowing in the direction the pressure gradient force is acting. Another force must
be acting to change the wind direction.
2. The winds blow parallel to the isolines of constant height. Imagine putting your back
to the wind, low heights, and thus low pressures, are always to your left (since we are
in the Northern Hemisphere). This relationship between wind and pressure is called
Buys-Ballot’s Law.
3. The winds, while meandering, are blowing counterclockwise around the North Pole.
To explain the first two observations consider a simplified situation demonstrated
in Figure 6.13. The isolines of constant height are parallel to one another with lower
heights at the top of the page. By our observations, the wind is blowing parallel to these
lines as drawn. The wind is moving in a straight line parallel to these isolines. Also
shown in the figure are the forces acting on a parcel of moving air. There are only two
forces acting: the pressure gradient force and the Coriolis force. The pressure gradient
force acts towards low pressure perpendicular to the lines of constant height while the
Coriolis force acts to the right and perpendicular to the wind direction. The two forces
are acting in opposite directions and are of equal magnitude. A wind that results from a
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balance between the pressure gradient and Coriolis forces is called a geostrophic wind.
When forces are in balance, there is no net force. How does the air move if the two
forces are balanced?
To answer this question imagine an air parcel released from rest as in Figure 6.14.
Initially, since the parcel is at rest, the Coriolis force is zero. The pressure gradient
results in a force that accelerates the parcel of air towards lower pressures. Initially the
parcel moves in the direction of the pressure gradient forceperpendicular to the
isoheights. Once the parcel begins to move the Coriolis force acts to deflect the parcel’s
path to the right, assuming we are in the Northern Hemisphere. As the wind speed
increases because of the pressure gradient force, the Coriolis force strengthens and moves
the parcel causing the parcel to travel in a curved path. Eventually the Coriolis force and
the pressure gradient force are equal and acting in opposite directions. Although the net
force acting on the parcel of air is zero, the parcel continues to move because of Newton’s
First Law of Motion. The parcel is in motion and tends to remain in motion. Lower
heights are to the left when you face downwind.
As observed in Figure 6.7 the winds blow parallel to the isoheights. The winds
change direction to remain parallel to the constant-height lines. According to Newton’s
first law of motion, forces are acting to change the direction of the wind. To explain this
consider a simplified situation represented by Figure 6.15. In this case the lines of
constant-height are curved and the pressure gradient and Coriolis force are not in balance.
The net force is the centripetal force, which results in changing the wind direction. The
wind is now three forces involved, the pressure gradient, Coriolis and centripetal forces.
This type of balance results in a gradient wind.
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When the Coriolis force is greater than the pressure gradient force, the wind
curves clockwise (in the Northern Hemisphere). The centripetal force points towards
regions of higher pressure. When the pressure gradient force exceeds the Coriolis force,
the air curves counterclockwise in the Northern Hemisphere (clockwise in the Southern
Hemisphere). In this case, the centripetal force points towards regions of lower pressure.
Thus, in the Northern Hemisphere, winds flow clockwise around regions of high pressure
and counterclockwise in regions of low pressure, in agreement with our observations.
Why, in general, do the global winds in the midlatitudes blow from west to east?
We can explain this third observation by noting that the polar regions are colder than the
tropics and conceptualize this using a simple model (Figure 6.16). In this model we
examine a vertical slice of the atmosphere between the North Pole and 30N latitude.
The polar region is colder and so the thickness between the 500 mb and 850 mb surface
must be greater in the warmer sub-tropical regions. As a result, the 500 mb pressure
surface is at a higher altitude at 30N than near the pole, and a pressure gradient force acts
from the right side of the conceptual diagram to the left. To determine the wind direction
along the 500 mb pressure surface we apply the Buys-Ballot’s Law. To do this, imagine
floating in a balloon on the 500 mb pressure surface. Extend your arms up from your
sides and point to lower pressure with your left hand. The wind will be at your back, or
in this example into the pagethe wind blows counterclockwise around the pole.
The geostrophc wind is proportional to the tilt of the pressure surfaces. The
greater the tilt the stronger the geostrophic wind. Figure 6.16 shows that the thickness
between pressure surfaces in warm air are greater than in cold air. The pressure surfaces
tilt with increasing altitude. So, the geostrophic wind increases with altitude when the
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temperature changes horizontally. The relationship between the vertical changes in the
geostrophic wind and the horizontal temperature gradient is called the thermal wind
relationship.
The thermal wind of a layer represents the difference between the
geostrophic winds at the top and bottom of a layer. The geostrophic wind increases with
altitude when a strong horizontal temperature gradient exists. In the vicinity of fronts,
where the temperature gradients are strong, we will observe strong winds aloft.
Observations of Wind near the Surface
Consider the surface map shown in Figure 6.6. The winds tend to blow counterclockwise around regions of low pressure and clockwise around the high. The fastest
wind speeds are located in regions where the spacing of the isobars is a minimum, which
are around low pressure systems. We also observe that the winds blow across the isobars.
Friction causes the winds at the surface to cross the isobars and flow inward
towards a low pressure region and outward from a high pressure region. To explain this
consider a simplified situation depicted in Figure 6.17. In the absence of friction we have
a geostrophic wind. The pressure gradient and Coriolis forces are in balance and the wind
flows parallel to the isobars. Friction acts to reduce the wind speed. Depending on the
surface, friction slows the wind by 25% to 75% of what the speed would be in the
absence of friction. Slowing the wind has no effect on the pressure gradient force;
however, by slowing the wind the Coriolis force is reduced, since the magnitude of the
Coriolis force is dependent on the wind speed. With a weaker Coriolis force the pressure
gradient force pulls the wind toward the low pressure region.
A new balance is
established between the pressure gradient, Coriolis and friction forces. The new balance
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has cross-isobaric flow. The angle at which the winds cross the isobars depends on the
type of surface and the latitude. Over open water, where friction is low, the winds
typically cross the isobars at an angle between 15 and 30. Over land the angle is
usually between 25 and 50.
Friction is important in slowing and turning the wind. Friction is also important
in shaping many of the earth’s geological features. Dunes of deserts and ripples in sand
are examples of how the wind and friction combine to sculpt the land. Wind speed and
duration are also important in generating waves (Box 6.2).
Convergence and Divergence
At the surface the wind in the Northern Hemisphere flows counterclockwise
around a low pressure region and clockwise around a high. Unlike the upper air, the wind
crosses the isobars and flows inward towards lower pressures. Because of friction, air in
the surface layer cannot spiral around low pressure centers, but moves inward to fill them.
In both hemispheres air flows inward, or converges towards a low pressure region.
Surface winds flow outward or diverge from a high pressure region.
Cyclonic
circulation refers to convergent air flow around a region of low pressure. Cyclonic flow
is counterclockwise in the Northern Hemisphere and clockwise in the Southern.
Anticyclonic circulation refers to diverging air flow around a high pressure system.
Coupling of upper and lower air
The importance of divergence and convergence of air cannot be over stressed.
Developing winter storms are marked on maps by a low pressure at the surface. Air is
moving in to 'fill-in the low pressure', this is convergence. By converging air molecules
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over a given region, we are adding molecules to our column and so the pressure should
increase. So how does the low pressure continue to decrease, 'develop', 'spin-up', or
'deepen'? Because divergence is occurring above the surface low. Divergence above
removes air molecules from our column of air, dropping the pressure at the surface. If the
upper level divergence is removing molecules faster than the low level convergence, the
pressure at the surface will decrease even further. The pressure gradient force will
increase, which increases the wind speed at the surface and causes the air to rush in faster
towards the low pressure center. How can air in the upper tropopause diverge if there is
no friction? One way is to change the speed of the wind. If the wind near the tropopause
where to slow down, air molecules would diverge more slowly, causing the pressure at
the surface to rise. Conversely, if the wind speed where to increase, divergence would be
more rapid and the atmospheric pressure at the surface would drop.
Chapter 7 will discuss in detail the relationships between air flow near the surface
and winds higher in the troposphere. For now it is useful to consider one simple diagram.
Figure 6.18 presents a top and a side view of surface high and low pressure regions in the
Northern Hemisphere. Surface air that diverges from a high pressure region is replaced
by air above that sinks.
formation.
Sinking motion, or subsidence, is unfavorable for cloud
High pressure systems are often cloud free. Daily surface weather
observations show this (Figure 6.6). Air converging in towards the low pressure rises.
This is favorable for cloud formation and explains why approaching low pressure weather
systems are often accompanied by clouds and precipitation. How do regions of high and
low pressures form? A good example is the sea breeze.
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Putting it all together - the Sea Breeze
If you have spent much time at the beach during the summer you've probably
noticed that at around 3:00 p.m. there often is a strong steady breeze blowing in from the
water. This steady wind, the sea breeze, is a result of the uneven heating during the
daytime between the land and the adjacent water. At night the wind often reverses
direction and blows from the land to the water (a land breeze). Land and sea breezes are
referred to as direct thermal circulations. Let's examine the sea breeze.
During the day the land, which has a low specific heat and is a poor conductor
(Chapther 2), heats much more quickly than water. As the land warms up, the air next to
it heats by conduction and rises, warming the air above the land by convection (Chapter
2). As the air rises it generates a pressure gradient, and thus a pressure gradient force,
generating the thermal circulation. To understand this, let's first imagine that the land,
sea and the air above them are at the same temperature and that the isobars are parallel.
As pressure is defined as the weight of the air molecules above us (Chapter 1), the isobars
must decrease in magnitude with increasing altitude (Figure 6.19). In this simplified
model, the temperature of the ocean and the air above the ocean are not changing
(Chapter 3) and thus initially no air molecules are moving. As air over the land warms
(due to absorption of solar energy, and conduction to a thin air layer above), the air rises
and there is a vertical displacement of air molecules to a higher altitude. These rising
thermals of air change the initial isobars.
For example, the height of the 980 mb isobar in Figure 6.19 must increase if we
are putting air molecules above this height. The result is that above the surface, the
isobars begin to slope upward. Now, at a given altitude (say 100 meters), the pressure is
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no longer the same over the ocean as it is over land, it is higher over land. Note, at this
time the surface pressures over the land and ocean are the same as we have not
transported molecules horizontally. But now we have a horizontal pressure gradient
above the surface and thus a pressure gradient force generating a wind from over the land
out towards the ocean. This wind removes air molecules from over the land lowering
pressure at the surface. The air that moves out over the ocean increases the surface
pressure over the water. At the surface over land low pressure is developing while over
the ocean high pressure is developing, generating a horizontal pressure gradient force at
the surface acting from over the ocean towards the land. Air moves toward the land
creating a sea breeze. To replace this surface air that is moving from over the water
towards land, air sinks from above, completing the circulation. This sinking air moves air
molecules towards the surface, causing the pressure above the surface to lower as the
molecules from above descend. Notice the slope of the pressure surfaces in Figure 6.19.
As the temperature difference between the land and water increases throughout the
afternoon (Chapter 3), the circulation increases in strength and winds pick-up reaching a
maximum in the middle to late afternoon. Over land the distance between two isobars
(i.e., 980 and 960 mb) is greater than over the ocean. This difference is what keeps the
circulation moving and is due to the air over land being warmer than the air over the
ocean.
If you are not at the beach to feel the sea breeze, you can monitor its existence and
intensity by analyzing satellite imagery (Figure 6.20). A rising parcel of air expands, and
cools and the relative humidity increasesconditions favorable for the formation of
clouds (Chapter 4). For this reason, the upward branch of the sea breeze is often visible
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from satellite pictures (Chapter 5) in the form of cumulus clouds. During the day, the
upward branch moves inland and is an indication of the strength of the sea breeze. If the
atmospheric conditions are favorable for the formation of thunderstorms, the sea breeze
may provide just enough lifting to cause thunderstorms to develop.
Whenever large land and water bodies are adjacent to one another, sea breezes
may develop and may cause thunderstorms. Florida's abundant summertime rainfall is a
result of sea breezes. One sea breeze front advances from the east and one from the Gulf
of Mexico side.
The important concept is that heating (or cooling) of a column of air leads to
horizontal differences in pressure, generating a pressure gradient force, which causes the
air to move and a circulation to develop. During the evening, the land cools faster than
the water and the process is reversed (Figure 6.21). The net result is a land breeze,
surface winds blow from the land out to sea.
Scales of Motion
Atmospheric motions span an enormous range of space and time. The size of an
atmospheric weather system is related to how long it exists. Small swirls of wind may
last for a few seconds while a hurricane may last several days. In general, as the size of
the phenomena increases so does its life span. The size and life spans of different
atmospheric phenomena are shown in Figure 6.22. Meteorologists refer to weather
systems with reference to three primary size categories:

Microscale is generally applied to circulations that are less than 1 km in size.
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
Mesoscale systems, such and thunderstorms, range in size between 1 km and
1,000 km.

Macroscale systems, like winter storms, are larger than 1,000 km.
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Summary
The wind is air in motion. Newton’s three laws of motion describe the physical
laws that govern the movement of objects, including air. Bodies at rest tend to stay at rest
until acted upon by a force. The important forces at work in the atmosphere are:
I. Pressure Gradient Forcedirected from higher pressure towards lower pressure at right
angles to the isobars. Contour lines on a constant pressure chart, or isobaric chart, tell us
the altitude above sea level at which one obtains a given pressure reading (i.e., 500 mb
isobaric map). Cold air aloft is normally associated with low heights as indicated by
hydrostatic balance. The pressure gradient force acts perpendicular to lines of constant
height and from high heights towards low heights. The greater the change in pressure
over a given distance, the stronger the pressure gradient force.
II. Coriolis Forcedescribes an apparent force that is due to the rotation of the earth. The
Coriolis force acts to the right in the Northern Hemisphere and to the left in the Southern
hemisphere. As a result of the Coriolis force, winds blow counter clockwise around a
low pressure system and clockwise around a high in the Northern Hemisphere. The
Coriolis force is zero at the equator and increases in magnitude as the poles are
approached. This force is zero if the velocity of the object is zero and increases as the
velocity increases.
III. Frictionalways acts in the direction opposite to movement. The magnitude of this
force depends on the type of surface. Friction is small over a frozen lake and strong in a
forest.
IV. Centripetal forcethe direction of the centripetal force is always towards the center
of the curvature and increases as the speed of the object increases.
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Winds arise as a result of a combination of these forces. Geostrophic winds arise
from the balance between pressure gradient and Coriolis forces. Geostrophic winds blow
parallel to the isoheights, and in the northern hemisphere lower pressure is to the left.
Near the surface, friction reduces the wind speed, which reduces the Coriolis force. The
weaker Coriolis force no longer balances the pressure gradient force and so the wind
blows across the isobars toward lower pressure. In the northern hemisphere, surface
winds blow counterclockwise and into a low, and flow out of a high in a clockwise
direction. A gradient wind results from a balance between the pressure gradient, Coriolis,
and centripetal forces. Like the geostrophic wind, the gradient wind flows parallel to
isolines of pressure altitude.
Convergence and divergence of air can occur anywhere in the atmosphere. The
development of a major storm requires convergence near the surface and divergence
above.
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Terminology
You should understand all of the following terms. Use the glossary and this Chapter to
improve your understanding of these terms.
Acceleration
Inertia
Anticyclonic flow
Isobaric map
Boundary layer
Leeward
Buys-Ballot’s law
Newton’s laws of motion
Convergence
Pressure gradient force
Coriolis force
Prevailing wind
Cyclonic flow
Sea breeze
Divergence
Thermal wind
Force
Wind gust
Friction
Windward
Geostrophic wind
Gradient wind
Hydrostatic balance
6-2
Review Question
1. Why might a sea breeze develop on one summer day and not another?
2. The sea breeze is one example of a circulation caused by differential heating between
two regions. Give examples of other circulations that are driven by differential
heating.
3. Describe the pressure gradient and Coriolis forces.
4. Why is friction important in weather systems?
5. Draw a picture of the forces acting on the winds about a low pressure region in the
Southern Hemisphere.
6. Draw the wind flow around an anticyclone in the Southern and Northern Hemisphere.
7. What would happen to converging air at the surface if convergence also occurred in
the upper troposphere?
8. Explain the relationship between the pressure gradient force and the slope of isobaric
surfaces.
9. Is the following statement true: For a fixed pressure gradient the geostrophic wind
will be stronger at 45N than at 55N. Justify your answer.
10. What is a wind gust? Why is gust important to report?
11. Explain why the Coriolis force does not influence how your bath tub drains.
12. Does Buys-Ballot’s law have to be modified to apply to winds in the Southern
Hemisphere?
Web Activities
Web activities related to subjects in the book are marked with subscript . Activities
include:
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Explorations of pressure gradient, Coriolis, and frictional forces on wind speed and
direction
Wind rose
Contouring upper air weather maps
Practice multiple choice exam
Practice true/false exam
6-4
Box 7.1 Planes and pressure differences
Airplane wings have a particular shape. They are designed this way so they can
lift the plane off the ground. The forces responsible for lifting the plane off the ground
are produced by the wind flow over the wing. You can demonstrate this by shaping a
piece of cardboard like a wing, securing it to a table and blowing air over it (See below).
The cardboard will lift off the table!
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Bernoulli's Principle (named after the Swiss scientist Daniel Bernoulli) states that
when a fluid in steady flow speeds up, the pressure lowers and where the fluid slows
down the pressure is high. As the air flows over the wing it speeds up as it travels over
the top of the wing, lowering the pressure causing a pressure difference between the
bottom and top of the wing, resulting in an upward force called the lift. There is also a
drag force acting to slow-down the moving wing. The net force, if the wing is designed
correctly, lifts the wing and the plane off the ground. The key to designing wings is
reducing the drag while increasing the lift.
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Box 7.2 Wind generated waves
The storm shown in Figures 6.6 and 6.7 represent a significant maritime event.
On November 10, 1975 the 729-foot Great Lakes freighter Edmund Fitzgerald sank in
Lake Superior. Can you determine the direction of the surface winds over Lake Superior
from Figure 6.6?
In this chapter we have emphasized how friction at the ground modifies the wind.
The wind also modifies the condition of the surface. A good example of this is wind
generated waves. Waves form as the wind’s energy is transferred into the water. The
size of a wind generated wave depends on:
1. The wind speed: The stronger the winds, the larger the force and thus the bigger the
wave. The wind must also be steady - a constant wind speed.
2. The duration of the winds: The longer the wind blows over the open water, the larger
the waves.
3. The fetch: This is the distance of open water over which the wind blows. The larger
the fetch the larger the waves.
Figure B6.1 below plots observations of wind speed and wave height observed on Lake
Superior on this fateful day. Also shown is the theoretical maximum wave height for
different wind speeds. In this particular storm not only where the wind speeds large, but
also the fetch was long for this lake. Figure B6.2 shows the path of the Edmund
Fitzgerald (thin black line) and the wind direction (heavy blue lines) at the time the great
ship sank. Chapter 10 describes the storm that generated these wind.
6-7
High waves are not only hazardous to shipping, but are also perilous to coasts.
Waves can cause flooding, crush buildings, and scour the soil from under structures. The
rapid rise in sea level associated with a storm is called the storm surge.
6-8