GPH113: Circulation LAB

Description

The lab sheet will be in the attachment.

Unformatted Attachment Preview

Name_______________________
Exercise 4 Introduction
GPH 113
INTRODUCTION TO PHYSICAL GEOGRAPHY LABORATORY
THE ATMOSPHERE: WIND AND CLIMATE
CONTROLS
•
•
•
•
•
•
TARGET SKILLS
Define air pressure and explain why it exists
Name the driving forces of wind and explain how they
influence wind speed and/or direction
Derive the major global wind belts
Name types of local winds and describe how they occur
Discuss the five basic climate controls
Explain why Phoenix, Arizona, has a desert climate
TOOLS YOU’LL BE USING:
PART 1:
Heat lamps
Aluminum foil
Tissue, boxes, misc.
Small fans
Turntables
Colored pencils and rulers
PART 2:
Heat Lamp
IRT / thermometer
KEY TERMS
PART 1:
Mass
Weight
Air pressure
Sea level air pressure
Millibar
Driving forces
Pressure gradient force
Convergence
Divergence
Wind
Friction
Coriolis effect
PART 2:
Weather
Climate
Climate control
Annual temperature range
Specific heat
Mechanically induced
Thermally induced
Intertropical Convergence Zone
Subtropical highs
Subpolar lows
Polar highs
Continentality
Shortwave radiation
Longwave radiation
Land-sea breeze
Onshore breeze
43
Offshore breeze
Mountain/valley breeze
Mountain breeze
Katabatic wind
Monsoonal wind
Exercise 4, Part 1
EXERCISE 4, PART 1:
AIR PRESSURE AND MOTION
AIR PRESSURE
Although we can’t see the air around us, it comprises an essential part of the Earth system: the
atmosphere. The atmosphere consists of billions of gas molecules, gathered together into a
nearly 500-km thick blanket around Earth. Without the atmosphere, life on Earth would not
exist. It shields us from dangerous cosmic and ultraviolet radiation, regulates the temperature at
the surface, and facilitates weather that distributes water around the planet.
All of the molecules in the atmosphere are held by the gravity exerted by Earth. This results in
several important features of our atmosphere. First, the majority of molecules are pulled down,
near the surface, so that density in our atmosphere decreases as height above the surface
increases. At the furthest outreaches of the atmosphere, gas molecules grow scarcer and scarcer
until the atmosphere eventually tapers off into the vacuum of outer space. Second, gravity acting
on the mass (the amount of physical matter) of the gas molecules causes them to have weight (an
expression of the strength of the gravitational force).
B
A
Figure 4.1: Dark gray represents the
densest concentration of molecules, and
white represents an absence of
molecules in the atmosphere. Point A
experiences the weight of all the
molecules in the atmosphere, while the
atmosphere above Point B is low in
density and exerts much less pressure.
The weight of the atmosphere exerts a tremendous
amount of pressure on the surface of the Earth. This
pressure is called air pressure, defined as force exerted
by the atmosphere per unit area of surface. In this course,
we will use the units of millibars to measure air pressure.
One millibar of pressure represents 100,000 Newtons of
force in one square meter, which is enough force to
accelerate 100,000 kg of mass by 1 m/s2. The average
sea level air pressure on Earth is 1013 mb. As elevation
above sea level increases, air pressure drops because the
density of the atmosphere overhead decreases (Figure
4.1). Even without a change in elevation, the spatial
variation in air pressure can be dramatic. For example,
very warm air rises away from the surface, lowering the
air pressure, while cold air sinks towards the surface and
raises air pressure. Differences in air temperature, along
with other factors like the activity of weather systems,
upper atmospheric wind patterns, and the presence of
land or water can also cause air pressure to vary.
There are several principle global belts of pressure. At
the equator, a concentration of heat energy from the sun’s direct rays causes warm air
temperatures and rising air, resulting in a low pressure belt at 0° latitude that is thermally
induced (caused by consistently warm or cold air temperatures). This low-pressure belt is known
as the Intertropical Convergence Zone, and it is almost continuously cloudy because of the
warm, moist air that is always rising here. Thermally induced pressure belts also exist at the
North and South Poles, where cold air sinks and generates the polar highs at 90°S and 90°N. As
air rises from the equator, it eventually reaches the top of the atmosphere, cools, and sinks back
down towards the surface at approximately 30°S and 30°N. These belts of high pressure, called
2
Exercise 4, Part 1
the subtropical highs, are said to be mechanically induced because they exist due to the
mechanics of the atmosphere, not because of consistently cold air temperatures. These large
regions of subsiding air tend to inhibit any other movement of the atmosphere below them, so
winds are usually very calm beneath the subtropical highs. In addition, sinking air suppresses
cloud development, so the subtropical high belts experience frequently clear skies. When the air
sinking in the subtropical highs reaches the surface, it spreads out. Some of it heads back
towards the equator, completing one circulation cell. The rest of it heads towards the poles,
where it collides with air that is spreading away from the poles. This converging air has no
where to go but up; thus, mechanically induced belts of low pressure called the subpolar lows
form at about 60°N and 60°S. Rising air in the subpolar low belt eventually reaches the top of
the atmosphere, spreads out, cools, and sinks back to earth either at the polar high or the
subtropical high, thus completing the second and third circulation cells of Earth’s atmosphere.
These belts of pressure will be important in determining the dominant surface wind patterns
across our planet.
DRIVING FORCES
There are several different forces at work in our atmosphere that combine to dictate the speed
and direction of wind. It is important to understand each of these driving forces. If we are to
understand weather systems and learn to forecast weather, we must first understand the
atmosphere’s basic rules of motion.
The pressure gradient force (PGF) is the driving force that starts wind blowing. Pressure
gradient is defined as the change in pressure across a given distance. An area with high air
pressure tends to have many molecules piling up (convergence) near the surface as a result of
sinking air. An area with low pressure often has molecules moving away (divergence) from the
surface. This creates an imbalance in the atmosphere: there are many molecules near the surface
at the high pressure, and fewer molecules near the surface at the low pressure. In an attempt to
regain balance, it is necessary to move some of the ‘excess’ molecules (high pressure) to the area
of fewer molecules (low pressure). The PGF accomplishes this by causing air molecules to
move from the high-pressure area towards the low-pressure area. This movement of air
molecules is called wind. You can summarize the pressure gradient force with one simple
sentence: Wind will blow from high to low.
1020 mb
H
1000 mb
Wind
L
Figure 4.2: The pressure gradient force causes wind to blow from areas of high pressure (convergence) to
areas of low pressure (divergence).
Once wind is moving, its speed is determined by the strength of the PGF and the presence of
friction. A big pressure gradient (drastic change in pressure over a fixed distance) will cause
high-speed winds, while a low pressure gradient (slight change in pressure over a fixed distance)
will result in slow-moving breezes. Regardless of the initial wind speed, air molecules
3
Exercise 4, Part 1
experience friction if they are flowing over the surface of the Earth. Friction acts opposite to the
movement of the wind, causing it to slow down. For this reason, winds near the surface are
much slower than winds in the upper atmosphere, which are far above the surface and experience
little friction to slow them down.
The PGF and friction would be enough to explain wind movement if the Earth were not rotating.
However, the rotation of the planet causes wind direction to alter from the original course
determined by the PGF. Wind is much like a missile fired at a moving target. Once launched,
the missile will fly straight and miss the target if the target moves. Air molecules flowing over
the Earth are unaffected by the Earth rotating under them. As a result, winds are deflected to the
right of the original wind direction in the Northern Hemisphere and to the left of the original
wind direction in the Southern Hemisphere. This is known as the Coriolis effect. The Coriolis
effect is greatest at high latitudes and is non-existent directly over the equator. In addition, the
Coriolis effect increases as wind speed increases, so its effects are weakest near the surface and
greatest in the upper atmosphere.
WIND DIRECTION
Once wind is blowing as dictated by the Coriolis effect and the PGF, it is always named for the
direction it is blowing from. For example, a wind blowing from the north to the south is a
northerly wind, while a wind blowing from the east to the west is called an easterly wind.
DRIVING FORCES EXPERIMENTS
In this part of the exercise, you will use your knowledge and creativity to examine two driving
forces of wind: the pressure gradient force and the Coriolis Effect.
1. CREATING WIND
1a. In your own words, explain the pressure gradient force and how it creates wind.
1b. List at three ways that you could CREATE high pressure or sinking air. Describe the
supplies would you need to accomplish each.
4
Exercise 4, Part 1
1c. List at least three ways that you could CREATE low pressure or rising air. Describe
the supplies you would need to accomplish each.
1d. Consider the supplies provided by your instructor. You will need to use these
supplies to create wind. In order to accomplish this, you will create a zone of high pressure and
a zone of low pressure. When they are side-by-side, a breeze should begin to blow from the high
towards the low. You can easily demonstrate that a breeze is blowing by holding a length of
tissue or crepe paper in between your high and low pressure zones. (You may need to place your
model inside a box to prevent outside air motion from interfering.) When you have successfully
created a breeze that will move the tissue, show your demonstration to your instructor. NOTE:
To receive credit for this part of the exercise, your instructor must view your demonstration and
sign below.
5
Exercise 4, Part 1
Line 1
A
B
Line 2
A
B
A
6
Exercise 4, Part 1
2. THE CORIOLIS EFFECT
Cut out the circle above and attach it to a turntable with a piece of tape.
2a. Find the dashed lines labeled “Line 1” and “Line 2” on the circle you have taped to
the turntable. Assume the center of the circle is the North Pole and the outer perimeter is the
Equator. Complete the following experiment:
•
One group member should place the tip of a colored pencil at point A on Line 1
(you should hold the pencil such that it writes, but does not tear the paper as the
page moves). Another group member rotates the turntable in a clockwise
direction. A line will be drawn on the paper, stop drawing when the line is a total
of three inches. Repeat this process for Line 2, keeping the speed of the colored
pencil and the speed of the turntable exactly the same as before.
***Show your results to your instructor BEFORE continuing.***
2b. Compare the path of the colored pencil for Lines 1 and 2. Which line showed the
greatest deflection?
2c. Based on your results, describe the relationship between latitude and the strength of
the Coriolis effect.
7
Exercise 4, Part 1
2d. Find the dashed line labeled “Line 3” on the paper circle. Complete the following
experiment:
•
One group member will use a colored pencil (different that the color used in the
last exercise. A second group member will rotate the turntable. A third group
member will use a watch to count off the seconds.
One group member will serve as a timer and count three seconds out loud (“start,
one, two, three”). When the timer says “start”, the spinner will begin to rotate the
turntable and the drawer will draw the colored pencil toward themselves, starting
at point A and moving two inches towards the outside of the circle. The colored
pencil should take the full three seconds to travel two inches. (Adjust the speed of
the colored pencil so that it will reach two inches as the timer says “three”.)
Repeat the experiment with another colored pencil, and let the colored pencil
travel four inches in three seconds. (Again, adjust the speed of the colored pencil
so that it will reach the four-inch mark as the timer says “three”. It will be
moving faster than before.)
***Show your results to your instructor BEFORE continuing.***
2e. Which line (first or second) ENDED furthest from Line 3?
2f. Based on your results, what can you say about the relationship between wind speed
and the strength of the Coriolis Effect? Remember the relationship between Coriolis and the
speed of the rotating planet (i.e. Jupiter example in lecture).
NOTE: For studying purposes, each group member should copy the colored pencil lines onto
their own circles in their lab exercises. Staple/tape the cut-out circle back onto the lab exercise it
was cut from.
8
Exercise 4, Part 1
PRACTICE WITH PGF AND CORIOLIS
In each of the following boxes, draw a dashed arrow that shows the wind direction according to
the PGF only. Then, draw a solid arrow showing the wind direction that results from the PGF
and the Coriolis force. (Remember, wind deflects to the right of its original direction in the
Northern Hemisphere, and to the left in the Southern Hemisphere.) HINT: Make sure you are
deflecting the wind to the right/left of the original wind direction, and not to YOUR right or left.
EXAMPLE:
Northern Hemisphere
Northern Hemisphere
Southern Hemisphere
H
H
L
L
L
Northern Hemisphere
H
Southern Hemisphere
Southern Hemisphere
H
L
H L
H
L
Southern Hemisphere
Northern Hemisphere
Southern Hemisphere
H
H
H
L
L
Northern Hemisphere
L
Southern Hemisphere
L
Northern Hemisphere
L
H
L
H
9
H
Exercise 4, Part 1
GLOBAL WIND PATTERNS
•
To the left of the globe, label each pressure belt with its name. To the right of the
globe, draw arrows demonstrating rising or sinking air at each of the pressure
belts. Then, draw arrows completing the circulation cells.
Inside the globe, draw several arrows between each set of pressure belts showing
the surface wind direction. Wind can start at any point along a line of high
pressure and will blow directly north or south towards the low pressure. Don’t
forget to include the appropriate Coriolis deflection.
Label each wind belt with the final wind direction. (Remember, wind is named
for where it’s blowing from.)
•
•
90°N
HIGH
60°N
LOW
30°N
HIGH
0°N
LOW
30°S
HIGH
60°S
LOW
HIGH
90°S
10
Exercise 4, Part 2
EXERCISE 4, PART 2: CLIMATE CONTROLS
AND LOCAL WINDS
CLIMATE CONTROLS
The term weather refers to the condition of the atmosphere at any given moment, and
encompasses the study and forecasting of individual storm systems. Climate is the long term
average of the weather at a given location. For example, a weather forecast might tell you to
expect 10 cm of rain from a passing storm, while a climate study would reveal that a location
receives an average of 150 cm of rain during the month of January.
While there are many complex components of the atmosphere that create weather systems, there
are only four basic controls for climate. The first of these is latitude. As you have already
learned in previous lab exercises, latitude directly dictates how much solar radiation a location
receives, including both the duration and intensity of radiation. This in turn means that latitude
is a principle control of air temperature. High latitudes receive highly variable inputs of
radiation. In the winter, days are short and the insolation receipt is low, so air temperatures are
very cold. In the summer, days are longer and insolation receipt is much higher, leading to
warmer temperatures. As a result, high latitudes have higher annual ranges in air temperature
(the difference between the warmest month and the coldest month). On the other hand, low
latitudes receive consistently intense radiation throughout the year and have warm average air
temperatures year-round.
The second climate control is related to land-sea differences. Specific heat is a property of a
substance that controls how easily that substance changes temperature as it absorbs energy.
Water has a very high specific heat. It can absorb enormous amounts of energy and exhibit very
little change in temperature. As a result, the oceans tend to vary in temperature only slightly
over the course of a year. Land has a low specific heat. It changes temperature quickly in
response to comparatively low amounts of energy absorption. In the summer, land heats up very
easily, while the ocean remains the same temperature and is therefore cooler. In the winter, land
rapidly depletes stored heat energy and becomes very cold, while the ocean still remains the
same temperature and is warmer than the land. This principle is known as continentality, and it
has a dramatic impact on climate. Locations near oceans or other large bodies of water have
moderated air temperatures over the course of a year. The water keeps them cool in the summer
and warm in the winter, producing a low annual temperature range. Continental locations that
are located inland away from the influences of water have much higher annual temperature
ranges: they become very hot in the summer and very cold in the winter.
Proximity to the ocean has a second important ramification for climate. All oceans experience
surface circulations that are driven by prevailing winds. These circulations serve the important
purpose of distributing excess heat energy from the equatorial latitudes to areas of energy deficits
in the high latitudes. In the Northern Hemisphere, the oceans circulate in a clockwise direction.
In the Southern Hemisphere, they circulate in a counter-clockwise direction. This means that in
both hemispheres, the warm water from the equator is moving pole-ward along the eastern coasts
of the continents, and the cold water from the poles is flowing equator-ward along the western
coasts. Air temperatures in coastal communities are influenced accordingly. The warm Gulf
Stream that flows along the East Coast of the United States means warm ocean water to surf in
and warmer air temperatures. On the West Coast, frigid waters move south from the poles and
11
Exercise 4, Part 2
result in cooler air temperatures and much colder waters – surfers usually have to wear
bodysuits!
Altitude is the third climate control. Locations at high altitudes have much less of the
atmosphere above them. As a result, the insulating affects of the atmosphere are reduced and
cooler air temperatures are experienced.
Cloud cover is the fourth climate control. Clouds play a complex role in regulating surface air
temperatures, and it can sometimes be complicated to sort out. Clouds have two functions
related to radiation in the atmosphere. First, they reflect incoming shortwave radiation from the
sun. This means that less insolation is received at the surface, and air temperatures are
consequently lower. Second, they absorb and reradiate longwave radiation emitted by the
Earth’s surface. Because they absorb heat energy that the surface has lost and radiate it back
towards the surface, clouds are able to maintain warmer surface air temperatures than might
occur during clear skies. You must examine each setting carefully to determine which of these
cloud functions is the most influential on climate.
Wind can also sometimes be considered a climate control. The prevailing wind direction will
determine what type of weather can move into an area. If the ocean is upwind, the location will
likely receive plenty of moisture and experience temperatures moderated by the water. If a large
continent is upwind, most of the air masses that move over a location will be very hot and dry in
the summer and cold and dry in the winter.
LOCAL AND REGIONAL WINDS
High and low pressure belts exist in a fixed pattern on a global scale, based on either thermal
properties related to latitude or mechanical operations of the atmosphere’s circulation system.
However, high and low pressure systems can exist on a much smaller scale, induced by either
weather systems or localized features of the topography. Here, we will discuss several types of
winds that occur only in certain locations and require certain topographical features to be
present.
A land-sea breeze occurs along the coast of any significant body of water. During the day, the
land grows much warmer than the adjacent water, and a thermal low develops over the land. The
wind blows from the relative high pressure over the cool water towards the low pressure over the
land, and an onshore breeze develops. At night, the situation reverses. The land cools off while
the water maintains its temperature. The air over land cools and begins to sink, causing a
thermal high to develop. An offshore breeze develops as the wind blows from the high over the
land towards the relative low over the water.
A similar type of wind occurs between mountains and valleys. During the day, the sun shines
down into the valley, warming the ground and the air over it. This causes the air to rise up the
slopes as a valley breeze. At night, the air cools off rapidly at the higher elevations. This highdensity, cold air is then pulled down the slope by gravity, forming a mountain breeze that flows
down into the valley. Any location surrounded by mountains will experience some degree of this
mountain valley circulation. When a large region is elevated, vast amounts of cooled air can
develop and flow down the slope in the form of a katabatic wind. The Santa Ana winds flow
down out of the Great Basin of the western United States, warming and drying as they descend.
12
Exercise 4, Part 2
They continue to gain speed as they are channeled through the many mountain valleys of the
southwest. Unlike the refreshing mountain breezes, the Santa Ana winds are fast, dry, and hot,
bringing little relief to the desert climate they cross.
Monsoonal winds are regional winds that result from seasonal shifts in the pressure regime over
a location. The most significant example of monsoonal winds occurs in India. During the
winter, the subtropical high migrates south. (Recall that the solar declination is at the Tropic of
Cancer in the Southern Hemisphere. The most direct radiation occurs there, causing the ITCZ
and its rising air to move south of the Equator. Other pressure belts respond similarly.) At this
time, India is located in the belt of the northeasterly tradewinds, with the subtropical high just to
the north. This means that the air blows from the northeast, off of the Asian continent.
However, in the Northern Hemisphere summer, the solar declination is located across the central
portion of India. The ITCZ migrates to the northern portion of India. Now the country is in
control of the southeasterly trade winds, which bring warm moisture inland from the Indian
Ocean. Combined with the rising air of the ITCZ, this switch in the wind direction brings a
tremendous amount of rainfall.
EXPERIMENT
Fill one dish with room-temperature sand and a second dish with room-temperature water.
Measure the precise temperature of both and record it in the table below. Place the two dishes
side-by-side under a heat lamp so that they are receiving equal amounts of radiation. Using a
thermometer, measure the temperature of the sand and water at five-minute intervals for a halfhour and record your results in the table below. Then, plot your data on the graph below.
Follow all proper graphing conventions.
TIME
SAND TEMPERATURE
INITIAL
5 MINUTES
10 MINUTES
15 MINUTES
20 MINUTES
25 MINUTES
30 MINUTES
13
WATER TEMPERATURE
Exercise 4, Part 2
1. a. What was the total temperature change for the sand?
b. What was the total temperature change for the water?
2. Which substance began to respond to the heat first?
14
Exercise 4, Part 2
3. Which substance experienced the most rapid change in temperature?
4. Which substance experienced the greatest total change in temperature? Why?
5. What would you expect to happen if you were to chill the sand and water rather than
heating them? Why?
CASE STUDY: PHOENIX, ARIZONA
1. Consult an atlas or topographic map to answer the following questions.
a. Give the latitude of Phoenix, Arizona.
b. Describe the topography to the north of Phoenix. Name any significant mountain
ranges.
c. Describe the topography to the south of Phoenix. Name any significant mountain
ranges.
15
Exercise 4, Part 2
d. Describe the topography to the east of Phoenix. Name any significant mountain
ranges.
e. Describe the topography to the west of Phoenix. Name any significant mountain
ranges.
f. Based on what you found in the surrounding areas, how would you describe the
topographic location of the Phoenix metropolitan area?
6. a. Where is Phoenix located according to the subtropical high (south, north, directly
under)?
b. What does this mean for wind in the Phoenix area? Does this agree with what you
have observed about winds in Phoenix?
c. How does the subtropical high influence the amount of clouds in the Phoenix sky?
16
Exercise 4, Part 2
3. Which type of local wind is Phoenix more likely to experience: the land-sea breeze, or the
mountain valley breeze? Why?
4. Given your answer to #3, what happens to air pollution during the day in Phoenix? What
happens to it at night? What implications does this have for air quality in the Phoenix
metropolitan area?
5. What happens to the latitude of the subtropical high in the Northern Hemisphere summer?
What are the results of this for Arizona?
17
Exercise 4, Part 2
6. In the space below, write a brief essay discussing how each of the climate controls affects the
climate of Phoenix, Arizona. Use the information you gathered above and your knowledge of
climate controls to explain why Phoenix has a desert climate. If a climate control is not relevant
to Phoenix, explain why not.
18

Purchase answer to see full
attachment