1. A cold front moving in brings stormy weather
A cold front moving in brings stormy weather. The arrow shows the direction that the front is moving.

2. Air Masses
How do air masses form?
An air mass forms when a large body of air becomes fairly stationary over a region of Earth’s surface or as air moves over a large, uniform region like an ocean.

3. Air Masses
An air mass is a large body of air that has fairly uniform physical properties, such as temperature and moisture content, at any given altitude.

4. Air Masses
Air masses can cover large portions of continents. The air is strongly influenced by the properties of the region.
The air above the Gulf of Mexico contains more water vapor than the air above northwestern Canada.
Each type of air mass reflects the characteristics of the region where it formed.

5. Air Masses
An air mass that forms over water, for example, contains more water vapor than one that forms over land.
An air mass that forms over the Arctic is much colder than one that forms over the tropics.

6. Air Masses
Air masses are classified according to whether they form over water or land, and the latitude where they form.
A maritime air mass forms over water.
A continental air mass forms over land.
A polar air mass forms north of 50° north latitude or south of 50° south latitude.
A tropical air mass is an air mass that originates in the tropics.

7. Air Masses
The air masses that most affect weather in North America include maritime polar air masses, continental polar air masses, maritime tropical air masses, and continental tropical air masses.

8. Air Masses
Air masses that form in different regions affect North American weather.

9. Fronts
What are the four types of fronts?
There are four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.

10. Air masses move with the circulation of air throughout the world.
Fronts
Air masses move with the circulation of air throughout the world.
As an air mass moves, it tends to retain the properties of the region where it formed.
When different air masses meet, they ordinarily don’t mix. Instead, a boundary forms between them.
The sharply defined boundary that forms when two unlike air masses meet is called a front.

11. Fronts
For example, when a continental polar air mass moves southeast and collides with a maritime tropical air mass, a front forms between the two.
Clouds and precipitation often accompany fronts.

12. A cold front occurs when a cold air mass overtakes a warm air mass.
Fronts
Cold Fronts
A cold front occurs when a cold air mass overtakes a warm air mass.
Cold air is denser than warm air, so the cold air mass remains close to the ground and moves underneath the warm air mass.
The cold air mass forces the warm air mass to rise.
Cold fronts usually move quickly and are relatively steep, so the warm air mass is often lifted rapidly.

13. Cold air mass sinks under the warm air mass.
Fronts
A cold front forms when a cold air mass overtakes a warm air mass.
Rising warm air
Direction of front
Precipitation
Cold air mass sinks under the warm air mass.

14. Fronts
The rapid rise of warm air often produces cumulus and cumulonimbus clouds.
Cold fronts often bring strong winds, severe thunderstorms, and large amounts of precipitation.
The slope of a cold front is steep, and it travels quickly, so the severe weather usually lasts for only a short time.
After the front passes, skies clear and temperatures drop.

15. A warm front occurs when a warm air mass overtakes a cold air mass.
Fronts
Warm Fronts
A warm front occurs when a warm air mass overtakes a cold air mass.
The cold air is denser, so it remains close to the ground.
The advancing warm air mass rises up over the cold air mass.
A warm front has a gentle slope because the advancing warm air mass gradually moves up and over the cold air mass.

16. A warm front occurs when a warm air mass overtakes a cold air mass.
Fronts
A warm front occurs when a warm air mass overtakes a cold air mass.
Precipitation
Direction of front
Rising warm air
Clouds form as rising moist, warm air condenses.
Cold air mass sinks under the warm air mass.

17. As the warm air rises, stratus clouds form.
Fronts
As the warm air rises, stratus clouds form.
If the warm air mass contains a large amount of moisture, there will be a steady rain.
Occasionally heavy showers or thunderstorms occur.
Once the front has passed, the warm air mass settles in, the skies are mostly clear, and temperatures rise.

18. Fronts Stationary Fronts
Sometimes when two air masses converge, neither air mass overruns the other.
A front forms and either stays where it formed or moves slowly.
When two unlike air masses have formed a boundary but neither is moving, the front is called a stationary front.
Stationary fronts often result in clouds and steady rain or snow for several days.

19. Fronts Occluded Fronts
An occluded front forms when a warm air mass is caught between two cooler air masses.
The colder air masses force the warm air mass to rise, trapping it between the two cold air masses.
Water vapor typically condenses from the warm air.
Occluded fronts usually bring cloudy skies and precipitation.

20. Low- and High-Pressure Systems
What types of weather are associated with cyclones and anticyclones?
Cyclones are associated with clouds, precipitation, and stormy weather.
The weather associated with an anticyclone includes clear skies, very little precipitation, and generally calm conditions.

21. Low- and High-Pressure Systems
Air masses are part of larger weather systems.
Weather systems are organized around either a center of high air pressure or a center of low air pressure, and may include more than one type of air mass.

22. Low- and High-Pressure Systems
Cyclones
A weather system with a center of low air pressure is called a cyclone, or sometimes simply a low.
As the warm air at the center of a cyclone rises, pressure decreases.
Because air moves from high-pressure areas to low-pressure areas, air spirals in toward the center of a cyclone.

23. Low- and High-Pressure Systems
Clouds form and precipitation develops.
Because air is converging in a cyclone, air masses collide and fronts develop.
A cyclone generally moves in the direction of global winds, and the weather of the cyclone moves along with it.
Westerly winds typically cause fronts and storms to move across the United States from west to east.

24. Low- and High-Pressure Systems
Winds and heavy rain are common in low- pressure areas, also called cyclones.

25. Low- and High-Pressure Systems
Anticyclones
A weather system with a swirling center of high air pressure is called an anticyclone, or sometimes simply a high.
Winds in an anticyclone circulate clockwise in the Northern Hemisphere.
Because the center of an anticyclone has high pressure, air flows away from that center.

26. Low- and High-Pressure Systems
As air flows away from the center, air from higher in the troposphere descends and replaces it.
When air descends, it warms, and its relative humidity decreases.

27. Low- and High-Pressure Systems
In the Northern Hemisphere, winds in a cyclone blow counterclockwise and converge.
In an anticyclone, winds blow clockwise and diverge.

28. Storms
How do thunderstorms and tornadoes form?
Thunderstorms form when columns of air rise within a cumulonimbus cloud.
A tornado forms when a vertical cylinder of rotating air develops in a thunderstorm.

29. Storms
Thunderstorms
A thunderstorm is a small weather system that includes thunder and lightning.
It is usually accompanied by strong winds and heavy rain or hail.

30. Storms
When a cold front forms, the cold air mass quickly lifts the warm air mass up, and thunderstorms often result.
Thunderstorms can also occur on hot summer days when buoyant air rises as part of a convection current within an air mass.
If the rising air is cooled to the dew point and the convection is strong enough, a thunderstorm can result.

31. Storms
Thunderstorms are associated with tall cumulonimbus clouds where there are strong updrafts of warm air.

32. Storms
One striking feature of a thunderstorm is lightning, which is a sudden electrical discharge in the atmosphere.
Positive charges tend to develop near the top of a cumulonimbus cloud.
Negative charges tend to develop near the base of the cloud.
When the potential difference between the positive and negative charges becomes great enough, a discharge occurs.

33. Storms
Thunder is the sound produced by rapidly expanding air along the path of a lightning discharge.
When lightning occurs, the air around it is heated, expands explosively, and produces sound waves.
You hear thunder after you see lightning because light travels much faster than sound.

34. Storms
Tornadoes
A tornado is a small but intense windstorm that takes the form of a rotating column of air that touches the ground.
Tornadoes most often form at the leading edge of an advancing cold front or along a line of thunderstorms where winds are converging to a center of very low pressure.

35. Storms
A tornado appears as a twisting funnel cloud descending from the bottom of a cumulonimbus cloud.
A tornado’s winds can sometimes be more than 500 kilometers per hour, although they are more typically about 150 kilometers per hour.
A large tornado can cause death and tremendous destruction.

36. Storms
A tornado crosses a farm field.

37. Storms
For a tornado to develop, there must be great differences between the properties of colliding air masses.
In the United States, where most of the world’s tornadoes develop, these storms occur mainly in the Midwest and the South.

38. Storms
Tropical Storms and Hurricanes
Thunderstorms and tornadoes are often associated with cyclones of the middle latitudes.
Cyclones also develop in the tropics, where they can develop into tropical storms or hurricanes.
A hurricane is a large tropical cyclone with winds of at least 119 kilometers per hour (74 miles per hour).

39. A cold front occurs when
Assessment Questions
A cold front occurs when
a cold air mass moves in a uniform direction.
a cold air mass collides with another cold air mass.
a cold air mass converges with a warm air mass.
a cold air mass overtakes a warm air mass.

40. A cold front occurs when
Assessment Questions
A cold front occurs when
a cold air mass moves in a uniform direction.
a cold air mass collides with another cold air mass.
a cold air mass converges with a warm air mass.
a cold air mass overtakes a warm air mass. ANS: D

41. An occluded front is generally accompanied by
Assessment Questions
An occluded front is generally accompanied by
cloudy skies and precipitation.
violent thunderstorms.
cool, dry weather.
warm, dry winds.

42. An occluded front is generally accompanied by
Assessment Questions
An occluded front is generally accompanied by
cloudy skies and precipitation.
violent thunderstorms.
cool, dry weather.
warm, dry winds. ANS: A

43. A weather system associated with high air pressure is called a(n)
Assessment Questions
A weather system associated with high air pressure is called a(n)
occluded front.
warm front.
cyclone.
anticyclone.

44. A weather system associated with high air pressure is called a(n)
Assessment Questions
A weather system associated with high air pressure is called a(n)
occluded front.
warm front.
cyclone.
anticyclone. ANS: D

45. Assessment Questions
A hurricane forms when a vertical cylinder of rotating air develops in a thunderstorm. True False

46. Assessment Questions
A hurricane forms when a vertical cylinder of rotating air develops in a thunderstorm. True False
ANS: F, tornado