Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor
AIR MASSES AND STORMS STUDY GUIDE
Assigned reading: Elemental Geosystems, Christopherson, Chapter 5, Pages 170-191
Web Learning Module: Air Masses and Storms
- The changing weather of the mid-latitudes is caused by different
air masses. An air mass is a large body of air extending hundreds
of miles horizontally with uniform temperature and moisture conditions.
The properties of an air mass are picked up from the land and ocean surfaces
over which it has rested. These source regions are usually areas
of high pressure causing the air mass to move outward to regions of lower
- Air masses are classified according to their source region and latitude.
Tropical (T) air masses originate in the tropics. Polar (P) air masses
originate in high latitudes, and Arctic (A) and Antarctic air masses over
the North and South Pole. They are described as continental (c) if
they form on a continent and maritime (m) if they form over the ocean.
Thus, a mT air mass is warm and moist and a cP air mass is dry and cold.
- A front is a zone of contact between two different air masses.
There are four types of fronts. A stationary front occurs when cold
and warm air masses meet but neither is advancing. Cold fronts occur
where cold air is advancing and gaining ground over warm air. Warm
fronts occur where warm air is advancing. Occluded fronts occur as
mid-latitude storms gradually dissolve.
- Mid-latitude cyclonic storms are low pressure centers that develop
along the polar front where warm and cold air masses meet. According
to wave cyclone theory, they progress through a series of stages.
They travel from west to east at about 20-40 miles per hour. They
may be up to 600-1000 miles in width.
- In a mature cyclone, warm fronts form when warm air masses are forced
to rise over a cold air mass along a low angle slope. Stratiform
clouds and steady rain or drizzle occur along a wide area in advance of
the warm front.
- Cold front weather is more violent. Cold air is pushed beneath
the rising warm air producing short, sharp showers and cumuliform clouds.
Winds change quickly after the passage of the cold front. Cold, clear
weather and anticyclones follow.
- As the cyclone moves east, the cold front overtakes the warm front
forcing the warm air up. This produces an occluded front that may
be cloudy and rainy.
- Violent storms such as hurricanes, tornadoes, and thunderstorms
are similar because they are all low pressure centers and depend upon the
latent heat of condensation for energy.
- Tropical cyclones are also known as cyclones (Indian Ocean), hurricanes
(Atlantic Ocean), or typhoons (Pacific Ocean). Hurricanes develop
over warm ocean water (80°F) in latitudes 7°-15° N and S of
the equator in late summer or early fall. They do not develop at
the equator, as the coriolis effect does not work at the equator.
Hurricanes (100-300 miles wide) move westward in the Trade Winds Belt.
They later come under the influence of the Westerlies and move eastward.
Hurricanes are well-developed low pressure systems with strong
convectional air movement that forms a wall of clouds and spiraling winds.
In the center of the hurricane is an eye that is noted for its calm, clear
- Damage from hurricanes comes from high winds (75-125 miles per hour), storm
wave surges, and flash floods. Hurricanes tend to weaken over land
where the water supply is less. They also weaken over cold waters where
there is less evaporation taking place.
- Tornadoes are the most violent of storms and often form in advance
of a cold front. They occur frequently in spring and early summer
in the Great Plains and the Middle Western United States. They are
smaller in size and shorter in duration than hurricanes. Tornadoes
have very high winds (250-500 miles per hour), cumulonimbus clouds, heavy rains,
or hail. They may have a diameter of a quarter of a mile and travel
from SW to NE.
- Destruction from tornadoes is the result of high wind velocity,
explosion resulting from pressure differences in buildings, and the strong
updraft of air within the funnel.
- Storms are an important mechanism for transferring heat around
the earth. In addition to the exchanges of warm and cold air that
occur during storms, latent heat, and ocean currents contribute to global
heat transfer between the low and high latitudes.
- Define air mass and source region.
- Describe how air masses are classified and the symbols used.
- Describe the four kind of fronts and the symbols used to show them on weather maps.
- Describe a mid-latitude cyclone in terms of size, direction, and
speed of travel.
- Describe the weather associated with warm fronts.
- Describe the weather associated with cold fronts.
- Describe an occluded front.
- Name two features that violent storms have in common.
- Describe the hurricane in terms of location of occurrence, direction
of travel, size, and weather.
- Explain the danger from hurricanes. How do they die out?
- Describe tornadoes. When and where do they occur? Describe
their size, duration, and path of travel.
- Explain the damage from tornadoes.
- What role do storms play in the global environment?
In essay form, with the help of diagrams, compare hurricanes with tornadoes.
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Created by Rita D. Haberlin, Geography Professor
Peralta Community College District
333 East Eighth Street
Oakland, CA 94606
This Geography Site Is Maintained By Patricia A. Kulda
Last Update July 14, 2010