2. The Motion of Air Masses
An air mass is a large body of air with fairly uniform temperature and moisture characteristics. Air masses acquire their characteristics from their source regions.
Temperature is indicated using the upper case (for example T for Tropical or P for Polar). Moisture is indicated using the lower case (for example, c for continental (dry) or m for maritime (wet).

3. The Motion of Air Masses
Global air masses and source regions. An idealized continent, producing continental (c ) air masses, is shown at the center. It is surrounded by oceans, producing maritime air masses (m). Tropical (T) and equatorial (E) source regions provide warm or hot air masses, while polar (P), arctic (A), and Antarctic (AA) source regions provide colder air masses of low specific humidity.

4. The Motion of Air Masses
This map of air masses over North America shows how some air masses may move over large distances and may on occasion influence weather over a large proportion of the continent (for example, the cold continental air masses from the north). (Data from U.S. Department of Commerce.)  

5. Cold, Warm, and Occluded Fronts
A given air mass usually has a sharply defined boundary between itself and a neighboring air mass. This boundary is termed a front.
In a cold front, a cold air mass lifts a warm air mass aloft. The upward motion sets off a line of thunderstorms. The front is steep and fast moving.
In a warm front, warm air rides up and over colder air. The front has a shallow gradient and moves slowly. A notch of cloud is cut away in this diagram to show rain falling from the dense stratus clouds.
In an occluded front, a warm front is overtaken by a cold front. The warm air is pushed aloft, and it no longer contacts the ground. Abrupt lifiting by the denser cold air produces precipitation. This is the process that determines the development of wave cyclones.

6. Cold, Warm, and Occluded Fronts
b).
a).
c).
a). A cold front.
b). A warm front.
c). An occluded front.

7. Cyclones and Anticyclones
Cyclones and anticyclones are weather systems that involve masses of air moving in a spiraling motion. In a cyclone, air spirals inward, whereas in anticyclone, air spirals outward. Most types of cyclones and anticyclones are large features that move slowly across the Earth's surface, bringing changes in the weather as they move. These are referred to as traveling cyclones and traveling anticyclones
The circulation of weather systems is determined by their pressure which in turn determines whether air ascends as in the case of low pressure, or descends as in the case of high pressure. Where air rises, condensation and cloud formation results as seen here in the system developing off the west coast of Alaska. In an anticyclone, outspiraling air descends and is warmed adiabatically leading to clear conditions.

8. Cyclones and Anticyclones
Air motion in cyclones and anticyclones.

9. Wave Cyclones
In the middle and high latitudes, the dominant form of weather system is the wave cyclone. The wave cyclone is a large inspiral of air that repeatedly forms, intensifies, and dissolves along the polar front.
*See movie on wave cyclones in the geodiscoveries section of your text’s website.

10. Wave Cyclones
Simplified surface weather maps and cross sections through a wave cyclone. In the open stage (left), cold and warm fronts pivot around the center of the cyclone. In the occluded stage (right), the cold front has overtaken the warm front, and a large pool of warm, moist air has been forced aloft.

11. Examining a Wave Cyclone
Wave cyclones not only develop over time, they also move. Generally mid-latitude wave cyclones track from west to east move from east to west until they encounter west winds aloft near the Tropics of Cancer and Capricorn.

12. Examining a Wave Cyclone
Paths of tropical cyclones and wave cyclones. This world map shows typical paths of tropical cyclones (red) and midlatitude wave cyclones (blue). (Based on data of S. Pettersen, B. Haurwitz, and N. M. Austin, J. Namias, M.J. Rubin, and J-H. Chang.)

13. Tornadoes
A tornado is a small but intense cyclonic vortex in which air spirals at tremendous speed. The dark funnel cloud results from spiraling updrafts from thunderstorms, although the precise details of why some storms result in tornadoes and others do not are still unresolved.

14. How Tornadoes Cause Damage
The high wind speeds in association with the large volumes of debris carried, causes the damage associated with tornadoes.
The scale for tornado intensity is referred to as the Fujita scale or F-scale. Weak tornadoes (0-1) which may result in downed trees include tornadoes with wind speeds up to 181 km/hr. The F-scale ranges up to 5 where wind speeds exceed 500 km/hr and sees houses thrown around by the strong winds.
*See movies on tornadoes in the geodiscoveries section of your text’s website.

15. Occurrence of Tornadoes
The occurrence of tornadoes is associated with thunderstorms spawned by fronts in the midlatitudes of North America. Tornadoes also occur regularly in Australia and occasionally in other mid-latitude locations.
The highest occurrence of tornadoes occurs in parts of Texas, Oklahoma and Florida. This movie clip shows the development of tornadoes over Fort Worth Texas.
*See movie on the occurrence of tornadoes in the geodiscoveries section of your text’s website.

16. Hurricanes
Tropical cyclones are known as hurricanes in the western hemisphere, typhoons in the western Pacific off the coast of Asia, and cyclones in the Indian Ocean.
*See movie on hurricanes in the geodiscoveries section of your text’s website.
Hurricane Gladys from Apollo 7

17. The Eye of the Hurricane
Winds spiral inward in hurricanes due to low pressures at the center. At the center an "eye" develops whereby skies become clear and wind speeds are relatively low. However, outside the eye, tall cumulonimbus clouds develop as the result of the intense circulation with heaviest rainfall and highest winds speeds occurring along the outer edge of the eye (or eye wall). Toward the outer edge of the storm, high cirrus clouds predominate.
Identify these features on this satellite image of hurricane Mitch.

18. The Eye of the Hurricane
Anatomy of a hurricane. In this schematic diagram, cumulonimbus (Cb) clouds in concentric rings rise through dense stratiform clouds. Cirrus clouds (Ci) fringe out ahead of the storm. Width of diagram represents about 1000 km (about 600 mi). Redrawn from NOAA, National Weather Service.
Hurricane Mitch, , NOAA.

19. Mapping a Hurricane
The mapping of hurricanes is important in order to understand the relative location of differing weather conditions. However, tracking hurricanes also sees the use of satellite imagery in order to identify a storm's potential track. For example, hurricanes in the Atlantic track from the east toward the northwest until diverted northeast by westerlies.

20. Mapping a Hurricane
A simplified weather map of a hurricane passing over the western tip of Cuba. Daily locations, beginning on September 3, are shown as circled numerals.
Tracks of typical hurricanes occurring during August. The storms arise in warm tropical waters and move northwest. On entering the region o prevailing westerlies, the storms change direction and move toward the northeast.