Earth Science
Chapter 17
Section 1
A. Types of Air Masses:
An Air Mass is a large body of air in which temperature,
humidity and pressure are uniform throughout.
Air Masses are, like winds, named according to their place of
origin.
The characteristics of an air mass depend on the region of the
earth over which it originates.
Areas of origination for air masses: Poles, Tropics, continents,
and large bodies of water.
Types of Air Masses
Polar air masses originate in cold regions such as northern
Canada, the north Atlantic or Pacific.
Those originating over land are called Continental Polar air
masses (cP) Characteristics - Cold and Dry
Those originating over ocean areas such as the north Atlantic
or Pacific are called Maritime Polar (mP)
Characteristics - Cold and Moist
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Tropical air masses originate in the tropics, or areas such as
Texas, deserts of the southwest and northern Mexico, Gulf of
Mexico, Caribbean, Pacific off, the tip of California, and
Middle Atlantic Ocean.
Those originating over land such as Texas are called
Continental Tropical air masses (cT).
Characteristics - Warm and Dry
Those originating over oceans such as the Gulf of Mexico are
called Maritime Tropical (mT) Characteristics- Warm and
Moist
B. Types of Fronts:
There are four types of fronts.
1. Warm Front
2. Cold Front
3. Stationary Front
4. Occluded Front
Warm Fronts:
- The surface where a moving warm air mass meets a cold air
mass is called a warm front.
- The advancing warm air mass pushes the colder air ahead
of it. Since warm air is lighter than cold air, the warm air
climbs up over the colder air mass.
- A warm front is marked by cirrus and then stratiform
clouds. It brings light steady rain for a few days.
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Cold Fronts:
- The boundary or surface where an advancing cold air mass
meets and pushes a warm air mass in front of it is called a
cold front.
- Since cold air is heavier than warm air, cold air moves
faster than warm air.
- The cold air pushes under the warm air causing it to rise
vertically.
- The convection currents which result, form cumuliform
clouds.
Cumulus clouds, followed by cumulonimbus clouds (thunder
clouds) form just behind the cold front, are responsible for
heavy rains which immediately follow the passing of a cold
front.
Stationary Front:
- When two unlike air masses face one another and there is
little air movement, this is known as a stationary front.
- The type of weather that exists at this front is similar to that
of a warm front.
- Eventually, either one or both of the air masses begins to
move resulting in a warm front of a cold front.
Occluded Front:
- Since cold air moves faster than warm air, in time the cold
front will catch up to, overtake and merge with the warm
front. When this occurs an occluded front is formed.
- Warm front type weather appears first then cold front type
weather follows.
- Eventually, the rising warm air cools and one large cold air
mass is formed.
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C. Cyclones and the Anticyclones:
A swirling center of low air pressure is called a cyclone.
Winds spiral counterclockwise inward toward the center of
the system.
The lowest pressure is found at the center of the system.
Clouds and precipitation, storms and warm air are
characteristic cyclones.
A swirling center of high air pressure is called an anticyclone.
Winds spiral clockwise outward away from the center of the
system.
The highest pressure is found at the center of the system.
Clear sunny skies and cooler air are characteristic of
anticyclones.
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