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

3. Air Masses and Fronts 3
A. Air Masses
1. Weather changes quickly when a different air mass enters an area.
2. An air mass is a large body of air that develops over a particular region of Earth’s surface.

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B. Types of Air Masses
1. A mass of air that remains over a region for a few days picks up the characteristics of that area.
2. Six major air masses affect North America.

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C. Fronts
1. Where air masses of different temperatures meet, a boundary between them, called a front, is created.
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C. Fronts
2. Air does not mix along a front. Because cold air is more dense, it sinks beneath warm air.
3. The warm air is pushed upward and wind develop. Fronts usually bring a change in temperature as they pass, and they always bring a change in wind direction.
4. There are four kinds of fronts – cold, warm, stationary and occluded.

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D. Cold Fronts
1. When a cold air mass advances and pushes under a warm air mass, the warm air is forced to rise.
2. The boundary is known as a cold front.
3. A narrow band of violent storms can result from a cold front.

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E. Warm Fronts
1. If warm air is advancing into a region of colder air, a warm front is formed.
2. As the warm air mass moves upward, it cools. Water vapor condenses and precipitation occurs over a wide area.

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F. Stationary Fronts
1. A stationary front is a front where a warm air mass and a cold air mass meet but neither advances.
2. Cloudiness and precipitation occur along the front.

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G. Occluded Fronts
An occluded front forms when a fast-moving cold front overtakes a slower warm front.
All types of occluded fronts can produce cloudy weather with precipitation.

11. H. High- and Low-Pressure Centers
Air Masses and Fronts 3
H. High- and Low-Pressure Centers
In areas where pressure is high, air sinks.
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.

12. H. High- and Low-Pressure Centers
Air Masses and Fronts 3
H. High- and Low-Pressure Centers
4. As air flows into a low-pressure center, it rises and cools.
5. Eventually, the air reaches its dew point and the water vapor condenses, forming clouds and precipitation.
6. Air circulates in a counterclockwise direction in the northern hemisphere in a low-pressure center.

13. I. Severe Weather—Thunderstorms
Air Masses and Fronts 3
I. Severe Weather—Thunderstorms
1. Thunderstorms develop from cumulonimbus clouds.
2. Falling droplets collide with other droplets and grow bigger.
3. As these larger
droplets fall, they cool the surrounding air, creating downdrafts that spread out at the surface.

14. I. Severe Weather—Thunderstorms
Air Masses and Fronts 3
I. Severe Weather—Thunderstorms
4. These are the strong winds associated with thunderstorms.
5. Lightning and thunder also are created in cumulonimbus clouds.

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

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J. Tornadoes

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K. Hurricanes
1. A hurricane is a large storm that begins as an area of low pressure over tropical oceans.
2. The Coriolis effect causes winds to rotate counterclockwise around the center of the storm.

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K. Hurricanes
3. As the storm moves, it pulls in moisture. The heat energy from the moist air is converted to wind.
4. The sustained winds in a hurricane can reach 250 km/h (155 mph)with gusts up to 300 km/h (186 mph).
5. When a hurricane passes over land, high winds, tornadoes, heavy rains, and storm surge pound the affected region.

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K. Hurricanes
6. 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.

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

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M. Watches and Warnings
Watches and warnings are issued for severe thunderstorms, tornadoes, tropical storms, hurricanes, blizzards, and floods.
Local radio and television stations announce watches and warnings, along with NOAA (NOH ah) Weather Radio.

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M. Watches and Warnings
3. During a watch, stay tuned to a radio or television station and have a plan of action in case a warning is issued.
4. If the National Weather Service does issue a warning, take immediate action to protect yourself.

23. Section Check 3
Question 1
Where is a front most likely to occur?

24. Section Check 3
Question 2
Explain how lightning forms.

25. Section Check 3
Question 3
If conditions are right for severe weather to develop in a particular area, what will the National Weather Service issue?
A. an advisory
B. Doppler radar
C. a warning
D. a watch

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27. End of Chapter Summary File