Chapter 25: Weather Answers to Chapter 25 Review Questions

1 Weather is the state of the atmosphere at a particular time and place; climate is the general
pattern of
weather over a long period of years.

2 Atmospheric pressure; temperature; wind; precipitation; cloudiness; and humidity.

3 Rays of the Sun strike the poles at a steep angle with the result that solar radiation is less
intense than
nearer the equator, resulting in lowered temperature.

4 The tilt of Earth’s axis produces the differences in solar intensity; this gives rise to seasons
and affects the length of daylight.

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1 Radiation emitted from Earth has a lower frequency than radiation emitted by the Sun.
2 Molecules of atmosphere absorb solar or terrestrial radiation which energizes them and sends
them aloft.
3 Atmospheric pressure decreases with altitude.
4 The troposphere.
5 Temperature decreases steadily as density decreases. Fewer atmospheric molecules to trap
terrestrial
radiation.

6 Uneven heating of Earth’s surface.

7 Warm air is characterized by low density and low air pressure, while cool air is characterized
by high density and high pressure.
8 Local differences in surface heating give rise to small-scale convection cells and pressure
gradients, and these create small-scale local winds which are changeable. Planet-scale temperature
differences produce much larger convection cells and pressure gradients that give rise to prevailing
winds, which are on a global scale and are relatively permanent.
9 An ocean current is a stream of water that moves relative to the larger ocean.
10 The Gulf Stream flows northward along the North American Coast warming Norway and
Great Britain.
11 Increases.
12 It condenses.
13 Air rises, expands, and cools. As the air cools, water molecules move slower and condensation
occurs. If there are larger and slower-moving particles or ions present in the air, water vapor condenses
on these particles to create a cloud.
14 (a) They do sink. (b) However, as they fall, they are carried upward by rising air currents
(updrafts).
15 A cold front develops as a colder, denser air mass advances into and displaces a stationary
warm air mass; a warm front is air that moves into territory that had been occupied by a cold air mass.
16 They are associated with day-to-day weather variation.
17 (a) A hurricane is a cyclone—an area of low pressure that winds flow around and they occur
in the warm moist conditions of the tropics; (b) latent heat released by large amounts of condensing
water in warm, moist air.
18 A tornado is a rotating column of air extending from a thunderstorm to the ground.
19 (a) There is a strong observed correlation between the rise of carbon dioxide emissions from
human activities and average global temperature increases. (b) When trees are cut down, the carbon
dioxide they would have absorbed is released into the atmosphere.
20 Disagree—climatologists cannot predict the future climate in this much detail.

Answers to Chapter 25 Multiple-Choice Questions

1b, 2d, 3b, 4a, 5a, 6c, 7b, 8d, 9a
Answers to Chapter 25 Integrated Science Concepts
Chemistry: The Atmospheric “Ozone Hole”
1 Good ozone is the O3 molecule when it is present in the stratosphere; bad ozone is O3 found in
the airwe breathe.
2 CFCs release chlorine atoms high up in the stratosphere, where they react with and destroy
ozone
molecules. Further, the chlorine atom catalyzes the ozone-destroying reaction so that one of them can
destroy 100,000 or more ozone molecules.

Physics: The Coriolis Effect

1. The Coriolis influence winds as well as surface currents by causing them to rotate with respect to
the
Earth from left to right (as viewed from above the Earth at the North Pole).

2. (a) A cyclone moves counterclockwise in the Northern hemisphere. (b) An anticyclone moves
counter
clockwise in the Southern hemisphere. Cyclones and anticyclones both rotate in the directions they
do
because of the Coriolis effect.

Physics: The Greenhouse Effect

1 Whitewash is sometimes applied to greenhouses to better reflect light, thus reducing the
amount of
incoming solar radiation and subsequent indoor temperature.

2 Without the greenhouse effect, the Earth’s average temperature would be a frigid 18¡C
otherwise. An intensified greenhouse effect would be a bad thing, because it could lead to global
warming.

Answers to Chapter 25 Exercises

1. In what direction does a sea breeze blow? Does it blow in the day or at night? What causes a sea
breeze?
A sea breeze blows from the sea toward the shore and occurs mostly during the day. The
reason is that land cools off faster than the ocean, so during the day, cooler, high-pressure
air blows toward the land. At night, the cooler, high-pressure air forms over land, so it flows
toward the area of lower pressure out to sea.
2. What are Hadley cells? Why are they important?
Hadley cells are the pairs of convection cells that comprise the prevailing winds.
Winds as well as precipitation and air pressure vary according to the flow described
by these convection cells.
3. What slows low-altitude winds relative to winds high in the troposphere?
Friction.
4. What kind of weather is associated with an approaching cold front? With an approaching warm
front?
As a cold front approaches, cirrus clouds typically form, wind shifts direction, and
temperature and air pressure drop. As a warm front approaches, cirrus clouds may form
then thicken so that the sky becomes overcast. Winds usually pick up and snow or rain may
fall.
5. After a day of skiing in the Rocky Mountains, you decide to go indoors and get a cup of hot cocoa.
As
you enter the ski lodge, your eyeglasses fog up. Why?
 The change in environment from cold to warm. As we leave the cold outdoors the warm air
inside comes into contact with the cold surface of the eyeglasses. As the air touching the
glasses cools to its dew point, water vapor condenses onto the eyeglasses. Notice the
similarity to Exercise 13—same physics, different situation.
6. Use what you learned about air pressure and barometers to analyze and explain how a straw works.
An old-fashioned mercury barometer works in a way similar to a straw: as the weight of the
atmosphere pushes the mercury in a dish down, the mercury rises in a low-pressure glass
column. Similarly, when one sucks on a straw, one reduces the pressure in the straw and this
allows the weight of the atmosphere to press the liquid up into the straw.
7. In some places, temperature inversions (areas where the air above is warmer than the air below) are
common. Local air pollution can then become a serious problem. Why is that?
The layer of cold air only allows minimal convection currents to occur.
8. Identify the clouds shown in the photo. How are they formed? What cloud group do they belong
to?
These clouds are cumulus clouds, which are clouds of vertical development. Like other
clouds, these formed from water vapor in rising air that expanded, cooled, and
condensed.
9. The Earth is closest to the Sun in January, but January is cold in the Northern Hemisphere. Explain.
Seasonal temperatures are caused by solar intensity, solar radiation per area. In the
winter in the Northern Hemisphere, the tilt of Earth’s axis leads to solar radiation at the
widest angle, reducing solar intensity to a minimum.
10. During a summer visit to Cancun, Mexico, you stay in an air-conditioned room. Getting ready to
leave your room for the beach, you put on your sunglasses. The minute you step outside, your
sunglasses fog up. Why?
The change in environment from cold to warm. As you leave the air-conditioned room, the
warm air outside comes into contact with the cold surface of the sunglasses. During contact,
the cold surface cools the air by conduction, and the warm air’s ability to hold water vapor
decreases. As the air cools to its dew point, water vapor condenses onto the sunglasses.
11. Air is warmed and rises at the equator and then cools and sinks at the poles, as shown in the
figure below. Is this an accurate picture of the global circulation of air? Explain why or
why not.
It is not accurate. Atmospheric circulation is broken up into six convection cells due to the
influence of the Coriolis effect.
The low cloud cover acts as an insulation blanket inhibiting the outflow of terrestrial
radiation.
12. As an air mass moves first upslope and then downslope over a mountain, what happens to
the air’s moisture and heat content?
As an air mass is pushed upward over a mountain, the rising air cools, and if the air is
humid, clouds form and precipitation occurs. As the air mass moves down the other side of
the mountain (the leeward slope), it warms. This descending air is dry, because most of its
moisture was removed in the form of clouds and precipitation on the windward (upslope)
side of the mountain.
13. Why are condensation and saturation more likely to occur on a cold day than on a warm day?
Because cool air has slower moving molecules, and warm air can hold more water vapor than
cold air.
14. How do fronts cause clouds and precipitation?
When two air masses make contact, differences in temperature, moisture, and pressure can
cause one air mass to ride over the other, forming clouds and causing precipitation.
15. Why does the surface temperature of the ground increase on a calm, clear night as low cloud
cover moves overhead?
The low cloud cover acts as an insulation blanket inhibiting the outflow of terrestrial
radiation.
16. Why does dew form on the ground during clear, calm summer nights?
On a clear summer night, the ground cools as it radiates away heat absorbed during daytime
and this has a cooling effect on surrounding air. As air temperature lowers, relative
humidity increases. If leaves of grass or other surfaces cool below the cooled air’s dew point,
dew will form as the humid air comes into contact with them.
17. Why does warm, moist air blowing over cold water result in fog?
Warm air is able to hold more water vapor before becoming saturated than can cold air. As
warm moist air blows over cold water, it cools, which causes the water vapor to condense into
tiny droplets of fog.
18. Explain why your ears pop when you ascend to higher altitudes.
The air pressure at higher altitudes is less than at the surface. Time is required for your
body to adjust to this new pressure, so the air inside your body pushes outward more than
the atmosphere pushes inward, producing that popping feeling.
19. What role does the Sun play in ocean currents?
The Sun heats the ocean unevenly; equatorial waters are warmed more than parts of the
ocean nearer the poles. Currents redistribute heat so that it is dispersed more evenly.
20. How does the ocean influence weather on land?
The ocean acts to (1) moderate the temperature of coastal lands; and (2) provide a reservoir
for
atmospheric moisture.

21. Why is it important that mountain climbers wear sunglasses and sunblock even when
temperatures are below freezing?
At high altitudes, there is a higher concentration of UV radiation due to a decrease in the
concentration of UV-absorbing atmospheric gases.
1 Why does the East Coast of the United States experience wider seasonal variation than the
West Coast, even though both areas have oceans along their margins?
2 The aneroid barometer, which measures atmospheric pressure, is at the heart of the altimeters
(devices that measure altitude) used in modern aviation. What’s the connection between a barometer
and an altimeter?

Because pressure decreases in a regular way with altitude, a device for measuring pressure
can be used to measure altitude by recalibrating the scale.
24. What does convection in Earth’s atmosphere produce? What does convection in the Earth’s
mantle produce?
Wind; plate tectonics.
25. What would be a good state for you to live in if you like extreme temperatures? If you like a
moderate climate? Explain why your preferred area has the weather patterns it has.
People who prefer definite seasons prefer inland areas, while those who like more
moderate climates would prefer to live in coastal areas, due to the moderating effect of
large bodies of water on climate (due to water’s high specific heat capacity).
26. While hiking, you survey the sky from a mountaintop. The sky is blue everywhere except for
a puffy cloud directly over a mountain. Explain how orographic lifting caused this cloud to
form.
 This cloud apparently formed though orographic lifting. This means that a parcel of
warm, moist air moved up the mountain’s upward-sloping side. As the air lifted, it
cooled and water vapor in it condensed to form a cloud.
27. How might you expect vegetation on the leeward side of a mountain to differ from the vegetation
on the windward side?
The leeward side of a mountain has a much drier climate than the windward side, so leeward
vegetation would consist of plants adapted for dry climates—cacti and succulents, rather
than ferns and conifers, for example.

Answers to Chapter 25 Problems

1. Consider a house at sea level that has 2000 square feet of floor area. What is the total force that
the air inside this house exerts upward on the ceiling?
2 2
Standard atmospheric pressure 14.7 lb/in . Convert this to units of feet: (14.7 lb/in ) (144
2 2 2
in ft ), and we have atmospheric pressure 2100 lb/ft . Knowing that pressure is defined as
force per area, we have F P A. So the total force that the air inside the house exerts
2 2 6
upward on the ceiling is (2100 lb/ft ) (2000 ft ) 4.2 10 lb of force.
2. If a tornado passed next to the front of the house, the pressure there could easily drop by 15% in
less than a second. Calculate the net force on the front door of a closed house if the outside
pressure suddenly dropped by 15%.

The air inside the house is at standard atmospheric pressure; from Table 25.1 this is 14.7
2
lbs. per square inch. The outside air is at 85% of this pressure or (0.85)(14.7 lbs./in ) 12.5
2 2 2
lbs./in . Thus, the net pressure pushing the door outward (14.7 lbs./in ) (12.5 lbs./in ) 2.2
2 2
lbs./in . The area of the door is 6.5 feet by 3 feet or (78 in)(36 in) 2,808 in . To find the
outward force on the door, we use the relation Force Pressure Area. We have: Force (2.2
2 2
lbs./in )(2,808 in ) 6178 lbs. Employing significant figures, the outward force on the door
due to the tornado is 6200 lbs. It’s easy to see why the door will be flung far in the storm.
3
3. At 50¡C, the maximum amount of water vapor in the air is 9 g/m . If the relative humidity is 40%,
3
what is the mass of water vapor in 1 m of air?
Relative humidity [(water vapor content)/(water vapor capacity)] 100%. Rearrange to
3
solve for the mass of water in 1 m of air: water vapor content [(relative humidity)(water
3
vapor capacity)]/100%. Then: water vapor content at 50¼C [(40%)(9 g/m )]/100% 3.6
3
g/m .