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Interactive Textbook 31 Understanding Weather

SECTION

2 Air Masses and FrontsUnderstanding Weather

Name Class Date

CHAPTER 2

After you read this section, you should be able to answer these questions:

• How is an air mass different from a front?

• How do fronts affect weather?

What Are Air Masses?Have you ever been caught outside when it suddenly

started to rain? What causes such an abrupt change in the weather? Changes in weather are caused by the move-ment of bodies of air called air masses. An air mass is a very large volume of air that has a certain temperature and moisture content.

There are many types of air masses. Scientists classify air masses by the water content and temperature of the air. These features depend on where the air mass forms. The area over which an air mass forms is called a source region. One source region is the Gulf of Mexico. Air masses that form over this source region are wet and warm.

Each type of air mass forms over a certain source region. On maps, meteorologists use two-letter symbols to represent different air masses. The first letter indicates the water content of the air mass. The second letter indi-cates its temperature. The figure below shows the main air masses that affect North America.

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BEFORE YOU READ National Science Education StandardsES 1j

STUDY TIPSummarize As you read, make a chart comparing the four kinds of fronts. In your chart, describe how each kind of front forms and what kind of weather it can cause.

READING CHECK

1. Identify How do scientists classify air masses?

TAKE A LOOK 2. Apply Concepts Describe the temperature and moisture content of a cT air mass.

NAT_IT_SCI_C02_NWEA_S02_031 31 8/2/06 10:34:59 AM



SECTION 2

Name Class Date

Air Masses and Fronts continued

COLD AIR MASSESMost of the cold winter weather in the United States

comes from three polar air masses. Continental polar (cP) air masses form over northern Canada. They bring extremely cold winter weather. In the summer, cP air masses can bring cool, dry weather.

Maritime polar (mP) air masses form over the North Pacific Ocean. They are cool and very wet. They bring rain and snow to the Pacific Coast in winter. They bring fog in the summer.

Maritime polar air masses also form over the North Atlantic Ocean. They bring cool, cloudy weather and precipitation to New England.

WARM AIR MASSESFour warm air masses influence the weather in the

United States. Maritime tropical (mT) air masses form over warm areas in the Pacific Ocean, the Gulf of Mexico, and the Atlantic Ocean. They move across the East Coast and into the Midwest. In summer they bring heat, humid-ity, hurricanes, and thunderstorms to these areas.

Continental tropical air masses (cT) form over deserts and move northward. They bring clear, dry, hot weather in the summer.

Air mass How it affects weather

cP from northern Canada

mP from the North Pacific Ocean

mT from the Gulf of Mexico

cT from the deserts

What Are Fronts?The place where two or more air masses meet is called

a front. When air masses meet, the less dense air mass rises over the denser air mass. Warm air is less dense than cold air. Therefore, a warm air mass will generally rise above a cold air mass. There are four main kinds of fronts: cold fronts, warm fronts, occluded fronts, and stationary fronts.

READING CHECK

3. Identify What is the source region for cP air masses?

Critical Thinking4. Infer Why don’t warm air masses form over the North Atlantic or Pacific oceans?

TAKE A LOOK 5. Identify Fill in the blank spaces in the table.

READING CHECK

6. Define What is a front?




SECTION 2

Name Class Date


COLD FRONTS

A cold front forms when a cold air mass moves under a warm air mass. The cold air pushes the warm air mass up. The cold air mass replaces the warm air mass. Cold fronts can move quickly and bring heavy precipitation. When a cold front has passed, the weather is usually cooler. This is because a cold, dry air mass moves in behind the cold front.

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A cold front forms when a cold air mass pushes a warmer air mass away. The front moves in the direc-tion that the cold air mass is moving.

WARM FRONTSA warm front forms when a warm air mass moves in

over a cold air mass that is leaving an area. The warm air replaces the cold air as the cold air moves away. Warm fronts can bring light rain. They are followed by clear, warm weather.

Warm air mass

Cold air mass

Movement of front

Warm front

A warm front forms when a warm air mass moves in and replaces a cold air mass. The front moves in the direction the warm air mass is moving.

TAKE A LOOK 7. Describe What happens to the warm air mass at a cold front?

READING CHECK

8. Define What is a warm front?




SECTION 2

Name Class Date


OCCLUDED FRONTSAn occluded front forms when a warm air mass is caught

between two cold air masses. Occluded fronts bring cool temperatures and large amounts of rain and snow.

Warm air mass

Cold air mass Cold air mass

Warm air mass

Movement of front

Occluded front

Occluded front

An occluded front forms when a warm air mass is trapped between two cold air masses. The cold air masses move together and push the warm air out of the way.

STATIONARY FRONTA stationary front forms when a cold air mass and

a warm air mass move toward each other. Neither air mass has enough energy to push the other out of the way. Therefore, the two air masses remain in the same place. Stationary fronts cause many days of cloudy, wet weather.

A stationary front forms when air masses stay in one place.

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TAKE A LOOK 9. Describe What happens to the warm air mass in an occluded front?

TAKE A LOOK 10. Infer What do you think is the reason that stationary fronts bring many days of the same weather?




SECTION 2

Name Class Date


How Does Air Pressure Affect Weather?Remember that air produces pressure. However, air

pressure is not always the same everywhere. Areas with different pressures can cause changes in the weather. These areas may have lower or higher air pressure than their surroundings.

A cyclone is an area of the atmosphere that has lower pressure than the surrounding air. The air in the cyclone rises. As the air rises, it cools. Clouds can form and may cause rainy or stormy weather.

An anticyclone is an area of the atmosphere that has higher pressure than the surrounding air. Air in anticyclones sinks and gets warmer. Its relative humidity decreases. This warm, sinking air can bring dry, clear weather.

Cyclones and anticyclones can affect each other. Air moving out from the center of an anticyclone moves toward areas of low pressure. This movement can form a cyclone. The figure below shows how cyclones and anti-cyclones can affect each other.

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Anticyclone Cyclone

Critical Thinking11. Compare Give two differences between cyclones and anticyclones.

TAKE A LOOK 12. Identify In which direction does air move: from a cyclone to an anticyclone, or from an anticyclone to a cyclone?




Name Class Date

Section 2 Review NSES NSES ES 1j

SECTION VOCABULARY

air mass a large body of air throughout which temperature and moisture content are similar

anticyclone the rotation of air around a high pressure center in the direction opposite to Earth’s rotation.

cyclone an area in the atmosphere that has lower pressure than the surrounding areas and has winds that spiral toward the center

front the boundary between air masses of different densities and usually different temperatures

1. Identify Relationships How are fronts and air masses related?

2. Compare Fill in the table to describe cyclones and anticyclones.

Name

Compared to surrounding air pressure, the pressure in the middle is...

What does the air inside it do?

What kind of weather does it cause?

cyclone ...lower than surrounding pressure.

anticyclone

sinks and warms

3. List What are four kinds of fronts?

4. Identify What are the source regions for the mT air masses that affect weather in the United States?

5. Describe What kind of air mass causes hot, clear, dry summer weather in the United States?


2. Pollutant Source Negative effects

Solutions

CFCs air con-ditioners and aerosol sprays

holes in the ozone layer

ban on CFCs

Ozone vehicle exhaust that reacts with air

lung damage

driving limits, vehicle emission controls

Sulfur dioxide

burning of fossil fuels

acid rain less fossil fuel use

3. Answers include: soil, water, plants, animals, buildings

4. The thinning ozone layer lets harmful UV radiation reach Earth. UV can cause skin cancer.

Chapter 2 Understanding WeatherSECTION 1 WATER IN THE AIR 1. Weather is a description of what is going on

in the atmosphere at a certain time and place.

2. condensation

3. Warmer air can hold more water vapor.

4. about 30 g/cm3

5. 10 g/m3 23 g/m3 0.43

0.43 × 100 43% relative humidity

6. Humidity is the absolute amount of water vapor in the air. Relative humidity is a measure of how saturated with water vapor the air is.

7. wet-bulb and dry-bulb thermometers

8. about 87%

9. Water vapor condenses into liquid water.

10. The glasses are as cold as the air outside. They absorb heat from the air inside. As a result, the air inside cools below its dew point. Water condenses on the glasses.

11. water vapor in the air

12. by shape and by altitude

13. A nimbostratus cloud is producing precipitation.

14. water that falls to Earth from clouds

15. They get bigger.

16. several ice crystals joined together

17. when it is too heavy to be carried by updrafts

Review 1. Possible answer: Water can condense from

air that is below its dew point.

2. the sun

3. Air rises and cools. When it cools below its dew point, water vapor condenses to form liquid water droplets or ice crystals. These droplets or ice crystals form a cloud.

4. Sleet forms when liquid water freezes in clouds and falls to the ground as ice. Snow forms when water vapor turns directly into a solid.

5. Name Altitude Shape Precipitation?

Cirro-stratus high layered no

Alto-cumulus

middle puffy no

Nimbo-stratus

low layered yes

Cumulo-nimbus

low to middle

puffy yes

SECTION 2 AIR MASSES AND FRONTS 1. by water content and temperature of the air

2. dry, warm

3. northern Canada

4. Warm air masses form over warm water or land, and the North Atlantic and Pacific are cold.

5. Air mass How it affects weather

cP from northern Canada

very cold winter weather and cool, dry summer weather

mP from the North Pacific Ocean

rain and snow in the Pacific in the winter, fog in the summer

mT from the Gulf of Mexico

heat, humidity, hurricanes, thunderstorms in East Coast and Midwest

cT from the deserts clear, dry, hot weather in the summer

6. a place where two or more air masses meet

7. It rises because the cold air pushes it up.

8. a place where a warm air mass moves over a cold air mass

9. The cold air masses push it out of the way.

10. Stationary fronts do not move, so the weather they bring stays in one place.

11. Cyclones have lower pressures than sur-rounding areas, but anticyclones have higher pressures. Cyclones bring rainy or stormy weather, but anticyclones bring dry, clear weather.

12. anticyclone to cyclone


Interactive Textbook Answer Key 57 Weather and Climate

I Weather and Climate Answer Key continued

NA_ITB_SC_FM_AK_057 57 8/9/06 12:24:43 PM

Review 1. A front is a place where two air masses meet.

2. First row, from left to right: rises and cools; cloudy, rainy, and stormy

Second row, from left to right: ... higher than surrounding pressure; dry and clear

3. cold, warm, occluded, stationary

4. Pacific Ocean, Atlantic Ocean, Gulf of Mexico

5. cT

SECTION 3 SEVERE WEATHER 1. It must contain enough water vapor to form

clouds.

2. Electricity travels between an area with a positive charge and an area with a negative charge.

3. sound produced by vibrations in expanding air that was superheated by lightning

4. The center of the tornado has low pressure.

5. when it touches the ground

6. 74 mph to 112 mph

7. Ocean water is too cold there.

8. the Coriolis effect

9. the center of the hurricane that contains warm, fairly calm air with low pressure

10. a rise in sea level because of a storm

11. to learn where the storm is and whether it is getting worse

Review 1. Lightning superheats the air to produce

thunder.

2. They can produce strong winds, tornadoes, hail, or flash floods.

3. in the United States

4. Cold, dry air from Canada meets warm, moist air from the Tropics, producing thunderstorms.

5. Solar energy causes water to evaporate. As it rises, cools, and condenses, it releases the energy. Thunderstorms form, which can combine to form hurricanes.

6. when they move over cooler water or land

7. If you are outside, stay away from tall objects that can attract lightning. If you are in the open, crouch down. Stay away from water.

SECTION 4 FORECASTING THE WEATHER 1. information on atmospheric conditions

2. The balloons transmit the data using radio signals.

3. Possible answer: Wind direction can indicate which way a weather system is moving.

4. a device that measures wind speed

5. Locate weather systems; determine how much precipitation is being produced.

6. NWS, NOAA

7. 27°F

8. No, because the air temperature is above the dew point (temperature).

9. The three Hs on the map should be circled.

Review 1. An anemometer measures wind speed; wind-

socks and wind vanes measure only wind direction.

2. temperature, pressure, relative humidity

3. tracking storms, imaging weather systems, measuring wind speed/humidity/temperature from different altitudes

4. Station A has higher air temperatures and higher wind speeds.

5. Station A: southwest

Station B: north

Chapter 3 ClimateSECTION 1 WHAT IS CLIMATE? 1. temperature and precipitation

2. The sun’s energy is focused on a small area.

3. The equator receives about the same amount of sunlight all year round.

4. differences in air pressure

5. west to east

6. Temperature decreases as elevation increases.

7. The air cools as it rises and drops below its dew point.

8. Water absorbs and releases heat more slowly than land, so the water helps to stabi-lize the temperatures on land.

9. The cool water absorbs heat from the air, causing the temperature of the air to decrease.

10. a warm current

11. latitude

12. tropical zone

13. The climate of an area affects the organisms that live there.

14. along the equator


Interactive Textbook Answer Key 58 Weather and Climate

I Weather and Climate Answer Key continued

NA_ITB_SC_FM_AK_058 58 8/9/06 12:24:44 PM

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CHAPTER SECTION 2 Air Masses and Fronts - Lab RatKOSlabratkos.weebly.com/uploads/2/3/7/4/23741170/interactive_textbook... · Chapter 2 Understanding Weather SECTION 1 WATER IN THE