Download Earth Science 19 Review: Pressure Systems

Earth Science 19 Review: Pressure Systems
Review Ch 19
Pressure
Systems and
the Weather
Earth Science 19 Review: Pressure Systems
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Of the various elements of weather
and climate, changes in air pressure
are the least noticeable.
Although you might not perceive dayto-day or hour-to-hour changes in air
pressure, these changes are very
important in producing changes in our
weather.
Air pressure is simply the pressure
exerted by the weight of the air
above.
Average air pressure at sea level is
about 1 kilogram per square
centimeter. This pressure is roughly
the same pressure that is produced
by a column of water 10 meters in
height.
Earth Science 19 Review: Pressure Systems
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Air pressure is exerted in all
directions; down, up and sideways.
The air pressure pushing down on an
object balances against the air
pressure pushing up on the same
object.
When meteorologists measure
atmospheric pressure, they use a unit
of measure called millibars.
Standard air pressure is 1013.2
millibars.
A barometer is a device used for
measuring air pressure.
Earth Science 19 Review: Pressure Systems
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With some modern improvements, the
mercury barometer is the standard
instrument used today for measuring
air pressure.
The need for smaller and more
portable instruments for measuring
air temperature led to the invention
of the aneroid barometer.
The aneroid barometer uses a metal
chamber with some air removed.
This partially emptied chamber is
very sensitive to variations in air
pressure. This chamber changes
shape and compresses as the air
pressure is increases, and it expands
as the air pressure decreases.
ES 19 Review: Wind & Pressure Systems
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As important as vertical motion is,
far more air moves horizontally, a
phenomena called wind. What causes
wind however?
Wind is the result of horizontal
differences in air pressure. Air flows
from areas of high pressure to areas
of lower pressure.
The unequal heating of Earth’s
surface generates pressure
differences. Solar energy is
therefore the ultimate source of
energy for the creation of wind.
ES 19 Review: Wind & Pressure Systems
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If Earth did not rotate and their
were no friction what-so-ever
between moving air and earth’s
surface, air would flow in a straight
line from areas of high pressure to
areas of low pressure.
But both factors, Earth’s rotation
and friction, do exist and the flow of
air is therefore not so
straightforward.
Three factors combine to control
wind:
 pressure differences
 Coriolis effect
 and friction.
ES 19 Review: Wind & Pressure Systems
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Wind is created by differences in
pressure: the greater these
differences are, the greater the
wind speed is.
Over Earth’s surface, variations
in air pressure are determined
from barometric readings taken
at hundreds of weather stations.
These pressure data readings are
shown on a weather map using
isobars.
Isobars are lines on a map that
connect places of equal air
pressure.
ES 19 Review: Wind & Pressure Systems
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The spacing of isobars indicates
the amount of pressure change
over a given distance.
These pressure changes are
expressed as the pressure
gradient.
A steep pressure gradient, like a
steep hill, causes great
acceleration (stronger winds) of a
parcel of air.
A less steep gradient causes a
slower acceleration (milder
winds).
ES 19 Review: Wind & Pressure Systems
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The pressure gradient is the
driving force of wind.
The pressure gradient has both
magnitude (wind strength) and
direction.
It’s magnitude is reflected in the
spacing of the isobars. The closer
the spacing, the stronger the
winds.
The direction of wind force is
always going from
areas of high pressure
○ to areas of low pressure
○ and at right angles to the
isobars.
○
ES 19 Review: Wind & Pressure Systems
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The weather map at right shows
typical air movements associated
with high and low pressure
systems.
Air moves out of regions of
higher pressure and into the
regions of lower pressure.
However, wind does not cross the
isobars at right angles as one
would expect.
This change in rotation results
from the rotation of the Earth
and is called the Coriolis effect.
ES 19 Review: Wind & Pressure Systems
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The Coriolis effect describes
how Earth’s rotation affects
moving objects.
All free-moving objects or
fluids, including the wind, are
deflected to the right of
their path of motion in the
Northern Hemisphere.
In the Southern hemisphere,
they are deflected to the
left.
ES 19 Review: Wind & Pressure Systems
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Imagine the path of a rocket
launched from the North Pole
toward a target located at the
equator.
The true path of the rocket is
straight, however,
in the time it would take
for the rocket to fly from
the North pole to the
equator,
 the Earth would have
rotated underneath the
rocket by 15 degrees.
 The rocket arrives at a
spot to the left of where it
was intended because the
Earth moved under it while
it flew.
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The counterclockwise rotation
of the Northern hemisphere
causes this path to deflect.
ES 19 Review: Wind & Pressure Systems
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This apparent shift in
direction is attributed to the
Coriolis effect.
This deflection
 Is always directed at right
angles to the direction of
airflow
 Affects only wind direction
and not wind speed
 Is affected by wind speed;
the greater the wind speed
the greater the amount of
deflection
 Is strongest at the poles and
weakens toward the equator,
becoming nonexistant at the
equator
ES 19 Review: Wind & Pressure Systems
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Now, consider that the amount
of deflection in a new direction is
affected by wind speed (the
greater the wind speed the
greater the amount of
deflection) ; anything that slows
the speed of the wind also
affects it’s direction.
This is where the difference in
the altitude of the wind currents
play a role. Currents that are
high in the atmosphere, such as
the jet streams, do not
encounter resistance.
Wind currents that are close to
the ground however may be
slowed down by friction from
objects such as mountains and
hills.
ES 19 Review: Wind & Pressure Systems
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The pressure gradient (PGF) and
Coriolis effect (CF) balance in
high-altitude air, and wind
generally flows parallel to isobars.
For air close to Earth’s surface ,
the roughness of the terrain
determines the angle of airflow
across the isobars.
 Over the smooth ocean surface,
friction is low, and the angle of
airflow change is small.
 Over rugged mountain terrain
however, where friction is higher,
winds move more slowly and cross
the isobars at a change in angle.
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Friction causes winds to flow
across the isobars at angles as
great as 45 degrees.
Slower wind speeds caused by this
friction decreases the Coriolis
effect and thereby affects the
wind direction.
ES 19 Review: Wind & Pressure Systems
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Pressure centers are among the most
common features on any weather
map. By knowing just a few basic
facts about centers of high and low
pressure, you can increase your
understanding of present and
upcoming weather.
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Lows, or cyclones, are centers of low
pressure.
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Highs , or anticyclones, are centers
of high pressure.
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In cyclones, lows, the pressure
decreases from the outer isobars
toward the center.
In anticyclones, highs, just the
opposite is the case: the values of
the isobars increase from the outside
toward the center.
ES 19 Review: Wind & Pressure Systems
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Winds move from higher
pressure to lower pressure
areas and are deflected to
the right or left by the
Earth’s rotation.
When the pressure gradient
and the Coriolis effect are
applied to pressure centers
in the Northern
Hemisphere;
 winds blow
counterclockwise around
the low.
 Around a high, the winds
blow clockwise.
ES 19 Review: Wind & Pressure Systems
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In the Southern
Hemisphere, the Coriolis
effect deflects the winds to
the left.
Therefore, the winds around
a low, south of the equator,
move clockwise while the
winds around a high blow
counterclockwise.
In either hemisphere,
friction causes a net flow of
air inward around a cyclone
and a net flow of air
outward around an
anticyclone.
ES 19 Review: Wind & Pressure Systems
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Rising air is associated with cloud
formation and precipitation,
whereas sinking air produces
clear skies.
Imagine a low pressure surface
system where the air is spiraling
inward.
Here the net inward movement of
air causes the area occupied by
the air to shrink; a process called
horizontal convergence.
When air converges (comes
together) horizontally, it must
increase in height to allow for the
decreased area it now occupies.
This increase in height produces
a heavier and taller column of air.
ES 19 Review: Wind & Pressure Systems
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In order for a surface low to
exist for very long, converging
air at the surface must be
balanced by outflows aloft.
For example, surface
convergence could be
maintained if divergence, or
spreading out of air aloft,
occurs at an equal rate. (as in
the cyclone at right)
The figure at right shows the
relationship between surface
convergence (inflow), uplift of
the air, and divergence aloft in
the cyclonic flow of low
pressure system.
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ES 19 Review: Wind & Pressure Systems
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Lows move in roughly west to
east direction across the
United States, and require a
few days (often up to a week0
for the journey across the
country.
The paths of lows can be
unpredictable and making
accurate calculations as to
their movements can be a
chore.
Before surface conditions can
be linked to the conditions of
air higher up, it is important for
us to understand the circulation
patterns of the total
atmosphere.
ES 19 Review: Wind & Pressure Systems
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The underlying cause of the
wind is the unequal heating of
Earth’s surface.
In tropical regions, more solar
radiation is received than is
radiated back into space. In
regions near the poles the
opposite is true; less solar
energy is received than is lost.
The atmosphere balances these
differences on a global scale by
acting as a giant heat-transfer
system.
This system moves warm air
toward high latitudes and cool
air toward the equator.
ES 19 Review: Wind & Pressure Systems
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The figure at right shows three sets
of cells that carry out this function;
the Polar cell, Ferrel cell, and Hadley
cell.
Near the equator, rising air produces
a pressure zone known as the
equatorial low; a region characterized
by abundant precipitation (heavy
rain).
As we see in the figure at right, the
upper level flow from the equatorial
low reaches about 20 to 30 degrees
latitude (about the Tropic of Cancer
or Capricorn) and than sinks back
toward the surface in a loop called
the Hadley cell.
ES 19 Review: Wind & Pressure Systems
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The sinking of air, and it’s
associated heating due to
compression, produce hot dry
conditions.
The center of this zone of sinking
dry air is the subtropical high
around 30 degrees north and south
latitude; areas known as the Tropic
of Cancer and Tropic of Capricorn.
The great deserts of Australia,
Arabia, and North Africa exist
because of this stable dry air
system associated with sub-tropical
highs.
ES 19 Review: Wind & Pressure Systems
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At the surface, airflow moves
outward from the center of the
subtropical high.
Some of the air travels toward
the equator and is deflected by
the Coriolis effect, producing the
trade winds.
Trade winds are two belts of
winds that blow almost constantly
from easterly directions.
The trade winds are located
between the subtropical highs
and the equator.
ES 19 Review: Wind & Pressure Systems
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The remainder of the air
travels toward the poles and is
deflected, generating the
prevailing westerlies of the
middle latitudes.
The westerlies make up the
dominant wet-to-east motion of
the atmosphere that
characterizes the regions on
the polarward side of the
subtropical highs.
As the westerlies move toward
the poles, they encounter the
cool polar easterlies in the
region of the Polar cells
(subpolar lows).
ES 19 Review: Wind & Pressure Systems
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Circulation in the middle latitudes is
complex.
Between 30 and 60 degrees latitude, the
general west to east flow, known as the
“westerlies”, is interrupted by migrating
cyclones (Lows) and anticyclones (Highs).
In the Northern hemisphere, these
pressure cells move from west to east
around the globe.
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On a regional level, small-scale winds
produced by locally generated pressure
gradient are known as local winds.
The local winds are caused by either
topographic effects (such as mountains) or
by variations to the surface composition
(land or water).
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4 types local wind patterns
ES 19 Review: Wind & Pressure Systems
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In coastal areas during the warm summer
months, the land surface is heated more
intensely during the daylight hours than an
adjacent body of water is heated.
As a result, the air above the land surface
heats more, expands and rises; creating an
area of lower pressure.
A sea breeze than develops because the
cooler air over the water at higher
pressure moves toward the land to fill in
the low pressure area.
This breeze starts developing shortly
before noon and generally reaches it’s
greatest intensity during the mid to late
afternoon.
These relatively cool winds can have a
moderating influence on afternoon
temperatures in coastal areas.
ES 19 Review: Wind & Pressure Systems
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At night, the reverse may take place.
The land cools more rapidly than the
sea, and a land breeze develops.
The cooler air, at high pressure over
the land, moves to the sea, where the
air is warmer and at a lower pressure.
Small scale breezes can also develop
along the shores of large lakes.
People who live in cities along the
Great Lakes, such as Chicago,
recognize the lake effects of winds.
This is why their will be cooler
temperatures along the shores of the
lakes in the summertime as breezes
off the water bring heat relief.
ES 19 Review: Wind & Pressure Systems
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A daily wind similar to land and ocean
breezes occurs in many mountainous
regions.
During daylight hours, the air along
the slopes of mountains is heated
more intensely than the air at the
same elevation over the valley floor.
Because this warmer air on the
mountain slope is less dense, it glides
up along the slope and generates a
valley breeze.
The occurrence of these daytime
upslope breezes can often be
identified by the cumulus clouds that
develop on adjacent mountain peaks.
ES 19 Review: Wind & Pressure Systems
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After sunset, the pattern may
reverse.
The rapid cooling of the air along the
mountain slopes produces a layer of
cooler air next to the ground.
Because cooler air is denser than
warmer air, it moves downslope into
the valley.
Such a movement of air is called a
mountain breeze.
In the Grand canyon at night, the
sound of cool air rushing down the
sides of the canyon at night can be
louder than the sound of the Colorado
River running.
ES 19 Review: Wind & Pressure Systems
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The same type of air drainage can be
found in places that have slopes less
steep.
The result is that the coldest pockets
of air are usually found in the lowest
spots.
Like many other winds, mountain and
valley breezes have seasonal
preferences; they happen more at
certain times of the year.
Although valley breezes are most
common in the warmer seasons when
solar heating is most intense,
mountain breezes tend to be more
dominant in the colder weather
seasons.