Chapter: The Atmosphere in Motion Table of Contents Section 3: Air Masses and FrontsAir Masses and...

Chapter: The Atmosphere in Motion

Table of ContentsTable of Contents

Section 3: Air Masses and Fronts

Section 1: The Atmosphere

Section 2: Earth’s Weather

• The atmosphere—the layer of gases surrounding Earth—provides Earth with all the gases necessary to support life.

Investigating Air

The AtmosphereThe Atmosphere

11

Investigating Air


11

• He weighed a flask, then injected air into it and weighed it again.

• Galileo Galilei (1564-1642), an Italian astronomer and physicist, suspected that airwas more than just empty space.

• Galileo observed that the flask weighed more after injecting the air.

Investigating Air


11

• He concluded that air must have weight and therefore must contain matter.

• Today scientists know that air stores and releases heat and holds moisture.

Investigating Air


11

• Because it has weight, air can exert pressure.

• Because it is composed of matter and has mass, the atmosphere is subject to the pull of gravity.

Composition of the Atmosphere


11

• The atmosphere is composed of a mixture of gases, liquid water, and microscopic particles of solids and other liquids.

• This graph shows the gases found in the atmosphere.

Gases


11

• Nitrogen (N2) is the most abundant gas—it makes up about 78 percent of the atmosphere.

• Oxygen (O2), the gas necessary for human life, makes up about 21 percent.

Gases


11

• Water vapor (H2O) makes up from 0.0 to 4.0 percent of the atmosphere and is critical to weather.

• Of the trace gases, two have important roleswithin the atmosphere.

• Carbon dioxide is needed for plants to make food.

Gases


11

• Carbon dioxide in the atmosphere absorbs heat and emits it back toward Earth’s surface, helping keep Earth warm.

• Solids such as dust, salt, and pollen and tiny liquid droplets such as acids in the atmosphere are called aerosols (AR uh sahlz).

Aerosols


11

• Salt enters the atmosphere when wind blows across the oceans.

• Pollen enters the atmosphere when it is released by plants.

Aerosols


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• Volcanoes add many aerosols to the atmosphere.

• Human activities also release aerosols into the air.

• The atmosphere is divided into layers based on temperature changes that occur with altitude.

Layers of the Atmosphere


11

• The white temperature scale shows temperatures in the thermosphere and exosphere.

• The troposphere (TROH puh sfihr) is the atmospheric layer closest to Earth’s surface.

Troposphere


11

• It extends upward to about 10 km.

• The troposphere contains about three-fourths of the matter in Earth’s entire atmosphere and nearly all of its clouds and weather.

• About 50 percent of the Sun’s energy passes through the troposphere and reaches Earth’s surface.

Troposphere


11

• Temperatures in the troposphere are usually warmest near the surface and tend to cool as altitude increases.

• The stratosphere extends from about 10 km to about 50 km above Earth’s surface.

Stratosphere


11

• Most atmospheric ozone is contained in the stratosphere.

• Without the ozone in this layer, too much radiation would reach Earth’s surface, causing health problems for plants and animals.

• Above the stratosphere is the mesosphere (ME zuh sfihr).

Upper Layers


11

• This layer extends from approximately 50 km to 85 km above Earth’s surface.

• The thermosphere (THUR muh sfihr) is above the mesosphere.

Upper Layers


11

• Temperatures increase rapidly in this layer to more than 1700°C.

• The thermosphere layer filters out harmful X rays and gamma rays from the Sun.

Upper Layers


11

• A part of the thermosphere and mesosphere is called the ionosphere (i AH nuh sfihr).

• This layer of ions is useful because it can reflect AM radio waves making long distance communication possible.

Upper Layers


11

• The outermost layer of the atmosphere is the exosphere.

• It extends outward to where space begins and contains few atoms.

• No clear boundary separates the exosphere form space.

Earth’s Water


11

• Earth’s surface is about 70 percent water.

• As the table shows, water exists as solid snow or ice in glaciers.

• In oceans, lakes and rivers water exists as a liquid and in the atmosphere it exists as gaseous vapor.

The Water Cycle


11

• Earth’s water is in constant motion in a never-ending process called the water cycle.

The Water Cycle


11

• Water on Earth’s surface—in oceans, lakes, rivers, and streams—absorbs energy and stores it as heat.

The Water Cycle


11

• When water has enough heat energy, it changes from liquid water into water vapor in a process called evaporation.

The Water Cycle


11

• Water vapor then enters the atmosphere. • Evaporation occurs from all bodies of water,

no matter how large or small.

The Water Cycle


11

• Water also is transferred into the atmosphere from plant leaves in a process called transpiration.

The Water Cycle


11

• Eventually, the water molecules change back into droplets of liquid water. This process is called condensation.

The Water Cycle


11

• Eventually, these droplets become large enough to be visible, forming a cloud.

• If the water droplets continue to grow, they become too large to remain suspended in the atmosphere and fall to Earth as precipitation.

Section CheckSection Check

11Question 1

Identify the layers of the atmosphere and briefly explain their characteristics.

11Section CheckSection Check

AnswerTroposphere: weather occurs in this layer; Stratosphere: contains atmospheric ozone, absorbs ultraviolet radiation from the Sun; Mesosphere: contains little ozone and therefore absorbs very little radiation; Thermosphere: filters out harmful X-rays and gamma rays from the Sun; Ionosphere: reflects AM radio waves making long-distance communication possible; Exosphere: extends into space


Question 2How does an erupting volcano affect Earth’s atmosphere?


AnswerWhen a volcano erupts, it releases tiny particles of ash or aerosols into the atmosphere. This ash creates a blanket over Earth’s surface that reflects solar energy and influences weather and climate.


Question 3What is the process called when water changes from a liquid into a vapor?

A. condensationB. evaporationC. precipitationD. transpiration


AnswerThe answer is B. Evaporation occurs when water has enough heat energy to change from a liquid into water vapor

Weather

• Weather describes the current condition of the atmosphere.

• Factors of weather include temperature, cloud cover, wind speed, wind direction, humidity, and air pressure.

Earth’s WeatherEarth’s Weather

22

Temperature

• Temperature is a measure of the average movement of molecules.

• The faster they’re moving, the higher the temperature is.


22

Energy Transfer

• The transfer of energy that results when molecules collide is called conduction.

• It is conduction that transfers heat from Earth’s surface to those molecules in the air that are in contact with it.


22

Energy Transfer

• Rising air cools as it gets higher.

• If it becomes cooler than the surrounding air, it will sink.


22

• The process of warm air rising and cool air sinking is called convection.

• It is the main way heat is transferred throughout the atmosphere.

Atmospheric Pressure• Air pressure decreases with altitude in the

atmosphere.


22

• This is because as you go higher, the weight of the atmosphere above you is less.

Atmospheric Pressure

• Temperature and pressure are related.


22

• When air is heated, its molecules move faster, and the air expands. This makes the air less dense, which is why heated air gets moved upward.

Atmospheric Pressure

• Less dense air also exerts less pressure on anything below it, creating lower pressure.


22

• Therefore, rising air generally means lower pressure and sinking air means higher pressure.

Humidity

• The amount of water vapor in the atmosphere is called humidity.


22

• The graph shows how temperature affects how much moisture can be present in the air.

Humidity

• When air is holding as much water vapor as it can, it is said to be saturated and condensation can occur.


22

• The temperature at which this takes place is called the dew point.

Relative Humidity

• Relative humidity is a measure of the amount of water vapor that is present compared to the amount that could be held at a specific temperature.


22

• When air is holding all of the water vapor it can at a particular temperature, it has 100 percent relative humidity.

Clouds

• Clouds form when air rises, cools to its dew point, and becomes saturated.


22

• If the temperature is not too cool, the clouds will be made of small drops of water.

• If the temperature is cold enough, clouds can consist of small ice crystals.

Low Clouds

• The low-cloud group consists of clouds that form about 2,000 m or less in altitude.


22

• These clouds include the cumulus (KYEW myuh lus), layered stratus (STRA tus), and nimbostratus (nihm boh STRA tus) cloud.

Low Clouds


22

• Fog is a type of stratus cloud that is in contact with the ground.

Middle Clouds

• Clouds that form between about 2, 000 m and 8,000 m are known as the middle-cloud group.


22

• Their names often have the prefix alto- in front of them, such as altocumulus and altostratus.

Middle Clouds


22

• Sometimes they contain enough moisture to produce light precipitation.

High and Vertical Clouds

• Some clouds occur in air that is so cold they are made up entirely of ice crystals.


22

• These are known as the high-cloud group.

• They include cirrus (SIHR us) clouds and cumulonimbus (kyew myuh loh MIHM bus).

High and Vertical Clouds


22

• Cumulonimbus clouds create the heaviest precipitation of all.

Precipitation

• When drops of water or crystals of ice become too large to be suspended in a cloud, they fall as precipitation.


22

• The type of precipitation that falls depends on the temperature of the atmosphere.

• For example, if air aloft is above freezing while air near Earth’s is below freezing, freezing rain might occur.

Wind

• Typically, air moves from high-pressure areas toward low-pressure areas.


22

• Because pressure and temperature are directly related, wind can be thought of simply as air moving form one temperature or pressure area to another.

Wind

• The greater the difference in temperature or pressure between two areas, the stronger the winds that blow between them will be.


22

• Wind speed is measured by and instrument called an anemometer (an uh MAH muh tur).

• The fastest wind speed ever measure was 371 km/h.

Global Air Circulation• In any given year, the Sun’s rays strike Earth

at a higher angle near the equator than near the poles.


22

• Warm air flows toward the poles from the tropics and cold air flows toward the equator form the poles.

Global Air Circulation

• Because Earth rotates, this moving air is deflected to the right in the northern hemisphere and to the left south of the equator.


22

• This is known as the Coriolis (kor ee OH lus) effect.

Surface Winds• Farther from the equator, at about 30° latitude,

the air is somewhat cooler.


22

• They blow in a general east-to-west direction.

• As this air flows, it is turned by the Corioliseffect, creating steady winds called the trade winds.

Westerlies and Easterlies • Major wind cells also are located between

30° and 60° latitude north and south of the equator.


22

• They blow from the west and are called the prevailing westerlies.

Westerlies and Easterlies

• Many of Earth’s major weather systems form along these boundaries, so these regions are known for frequent storms.


22

Westerlies and Easterlies • Near the poles, cold, dense air sinks and flows

away from the poles.


22

• As the cold air flows away from the poles, itis turned by the Coriolis effect. These winds, the polar easterlies, blow from the east.

Jet Streams

• Jet streams are like giant rivers of air.


22

• They blow near the top of the troposphere from west to east at the northern and southern boundaries of the prevailing westerlies.

• Their positions in latitude and altitude change from day to day and from season to season.

Other Winds

• Slight differences in pressure create gentle breezes.


22

• Great differences create strong winds.

• The strongest winds occur when air ruches into the center of the low pressure. This can cause severe weather like tornadoes and hurricanes.


22Question 1

How are temperature and pressure related?


22Answer

When air is heated, its molecules move faster and the air expands. This makes the air less dense and it rises. As this less dense air rises and exerts less pressure on anything below it. Cooled air becomes denser and sinks as the molecules slow down and move closer together. This cool air exerts more pressure on anything below it.


22Question 2

Which winds blow in an east-to-west direction and are formed by cool, sinking air?


22

A. polar easterliesB. prevailing westerliesC. surface windsD. trade winds


22Answer

The answer is D. Cooler air sinks toward the equator and is turned by the Coriolis effect creating trade winds.


22Question 3

Which are responsible for producing heavy precipitation and thunderstorms?

A. altostratus cloudsB. cirrus cloudsC. cumulonimbus cloudsD. nimbostratus clouds


22Answer

The answer is C. Cumulonimbus clouds have high vertical development and produce thunder, lightning, and heavy rain.

Air Masses

• Weather changes quickly when a different air mass enters an area.

• An air mass is a large body of air that develops over a particular region of Earth’s surface.

Air Masses and FrontsAir Masses and Fronts

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Types of Air Masses• A mass of air that

remains over a region for a few days acquires the characteristics of the area over which it occurs.

• Six major air masses affect North America.


33

Fronts• Where air masses of different temperatures

meet, a boundary between them, called a front, is created.


33

Click image to view movie.

Cold Fronts• When a cold air mass advances and pushes

under a warm air mass, the warm air is forced to rise.


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• The boundary is known as a cold front.

• A narrow band of violent storms can result from a cold front.

Warm Fronts

• If warm air is advancing into a region of colder air, a warm front is formed.


33

• As the warm air mass moves upward, it cools. Water vapor condenses and precipitation occurs over a wide area.

Stationary Fronts

• A stationary front is a front where a warm air mass and a cold air mass meet but neither advances.


33

• Cloudiness and precipitation occur along the front.

Occluded Fronts• An occluded front forms when a fast-moving

cold front overtakes a slower warm front.


33

• All types of occluded fronts can produce cloudy weather with precipitation.

High- and Low-Pressure Centers

• In areas where pressure is high, air sinks.


33

• As it spreads, the Coriolis effect turns the air in a clockwise direction in the northern hemisphere.

• Because the air is sinking, air near a high-pressure center is usually dry with few clouds.

High- and Low-Pressure Centers

• As air flows into a low-pressure center, it rises and cools.


33

• Eventually, the air reaches its dew point and the water vapor condenses, forming clouds and precipitation.

• Air circulates in a counterclockwise direction in the northern hemisphere in a low-pressure center.

Severe Weather—Thunderstorms• Thunderstorms develop from cumulonimbus

clouds.


33

• Falling droplets collide with other droplets and grow bigger.

• As these larger droplets fall, theycool the surrounding air, creating downdrafts that spread out at the surface.

Severe Weather—Thunderstorms• These are the strong winds associated with

thunderstorms.


33

• Lightning and thunder also are created in cumulonimbus clouds.

Tornadoes

• A tornado is a violent, whirling wind that moves in a narrow path over land.


33

• The powerful updrafts into the low pressure in the center of a tornado act like a giant vacuum cleaner, sucking up anything in its path.

Tornadoes


33

Hurricanes• A hurricane is a large storm that begins as an

area of low pressure over tropical oceans.


33

• The Coriolis effect causes winds to rotate counterclockwise around the center of the storm.

Hurricanes

• As the storm moves, it pulls in moisture.


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• The heat energy from the moist air is converted to wind.

• The sustained winds in a hurricane can reach 250 km/h with gusts up to 300 km/h.

Hurricanes

• When a hurricane passes over land, high winds, tornadoes, heavy rains, and storm surge pound the affected region.


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Hurricanes


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• After the storm begins traveling over land, however, it no longer has the warm, moist air to provide it with energy, and it begins losing power.

Weather Safety

• Using technology such as Doppler radar, as well as weather balloons, satellites, and computer, the position and strength of storms are watched constantly.


33

• If the National Weather Service believes conditions are right for severe weather to develop in a particular area, it issues a severe weather watch.

Watches and Warnings

• Watches and warnings are issued for severe thunderstorms, tornadoes, tropical storms, hurricanes, blizzards, and floods.


33

• Local radio and television stations announce watches and warnings, along with NOAA (NOH ah) Weather Radio.

Watches and Warnings

• During a watch, stay tuned to a radio or television station and have a plan of action in case a warning is issued.


33

• If the National Weather Service does issue a warning, take immediate action to protect yourself.


33Question 1

Where is a front most likely to occur?

Fronts occur where air masses of different temperatures meet.

Answer


33Question 2

Explain how lightning forms.


33Answer

Lightning is the flow of energy that occurs between areas of opposite electrical charge. During a storm, the bottom storm cloud has a negative charge while the ground has a positive charge. The negative charge rushes toward the ground and the positive charge rushes toward the cloud. These charges meet in the middle and create lightning.


33Question 3

If conditions are right for severe weather to develop in a particular area, what will the National Weather Service issue?

A. an advisoryB. Doppler radarC. a warningD. a watch


33Answer

The answer is D. A severe weather watch is issued if the conditions are right for severe weather to develop in an area. A severe weather warning is issued if severe weather is already occurring.

Chapter 12 Review 1 of 2

• Atmosphere: layer of gases surrounding Earth.

• Air pressure is due to gravity.• Most of the ozone of the atmosphere is

located in the stratosphere.• The majority of water on Earth can be

found in oceans.• Convection: process of warm air rising and

cool air sinking.

Chapter 12 Review 2 of 2

• Air pressure decreases with altitude in the atmosphere.

• If the amount of water vapor doesn’t change, relative humidity increases as air cools.

• Precipitation can be in the form of rain, freezing rain, sleet, snow, or hail.

• Coriolis effect: moving air is deflected to the right in the northern hemisphere.

• In the northern hemisphere, air circulates in a counterclockwise direction.

Chapter: The Atmosphere in Motion Table of Contents Section 3: Air Masses and FrontsAir Masses and...

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