WEST VISAYAS STATE UNIVERSITY
LA PAZ, ILOILO CITY
(Effective Alternative Secondary Education)
Little Rose G. Santiago
MA.ED Physical Science
Prof. Vilma Templora
Module in Air
What this module is about
An air mass is an extremely large body of air whose properties of temperature and
moisture content (humidity) are similar in any horizontal direction. Air masses can cover
hundreds of thousands of square miles. Areas in which air masses originate are called source
regions. Because the atmosphere is heated chiefly from below and gains its moisture by
evaporation from earth’s surface, the nature of the source region largely determines the initial
characteristics of an air mass.
This module is all about air masses, their source region, properties, classification and
modification. Knowledge of an air masses in atmosphere can help you answer questions like
Where do air masses from? What factors determine the nature and degree of uniformity of an air
mass? These two basic questions are closely related because the site where an air mass forms
vitally affects the properties that characterize it.
To make the discussion easy for you, the module is divided into three lessons:
Lesson 1- What is an Air Mass and its Source Region?
Lesson 2- How are Air Masses Modified?
Lesson 3- What are the Properties of an Air Masses?
What you are expected to learn
After going through this module, you should be able to:
1. classify air masses;
2. relate the properties of air masses to their behavior;
3. discuss the air masses modification and its source region
How to learn from this module
Here are some pointers to remember as you go over this module.
1. Read and follow the instructions carefully.
2. Answer the pre-test first before reading the content of the module.
3. Take down notes and record points for clarification.
4. Always aim to get at least 70% of the total number of items given.
5. Be sure to answer the post test at the end of the module.
What to do before (Pre-test)
Take the pre- test before proceeding in the lessons. Check your answers against the answer key at the
end of the module.
I. Multiple Choice. Choose the letter of the best answer. Write the chosen letter on a separate sheet of
1. Which of the following statements is most plausible?
a. In winter, cP source regions have higher temperatures than mT source regions.
b. In summer, mP source regions have higher temperatures than cT source regions.
c. In winter, cA source regions have lower temperatures than cP source regions.
d. In summer, mT source regions have lower temperatures than mP source regions.
e. They are all equally plausible.
2. Compared to an mP air mass, mT air is ____.
a. warmer and drier
b. warmer and moister
c. colder and drier
d. colder and moister
3. One would expect a cP air mass to be:
a. cold and dry.
b. cold and moist.
c. warm and dry.
d. warm and moist.
4. The air mass with the highest actual water vapor content is ____.
5. Along the boundary between continental polar and maritime tropical air masses, ____ is often found.
a. a large area of calm (extremely light wind)
b. intense heat and drought
c. widespread precipitation and storminess
d. both a and c
6. A mT air mass would be best described as
a. cold and wet
b. warm and dry
c. warm and wet
d. cold and dry
7. Which of the following is an appropriate classification for the nature of the surface of the
d. a and b
8. A good source region for a continental air masses would be:
a. the Southern Indian Ocean
b. the Hawaiian Island
c. the Canadian Prairie
d. lake Ontario
9. Continental polar air masses are typically associated with what kind of winter weather?
a. clear skies and warm temperatures
b. sunny but cold
c. cool, rainy condition
d. Hazy, hot and humid, with scattered afternoon
10. The earth’s dessert regions serve as source regions for _____ air masses.
a. continental tropical ( cT)
b. continental polar (cP)
c. maritime polar (mP)
d. continental Arctic (cA)
What is an Air Mass?
In the middle latitudes, most weather patterns are the result of the movements of large
bodies of air, called air masses. An air massis a large body of air, usually 1600 kilometers or more across
and perhaps several kilometers thick, which is characterized by homogeneous physical properties (in
particular, temperature and moisture content) at any given altitude.
A region under the influence of an air mass will probably experience generally constant weather
conditions, a situation referred to as air mass weather.
What you will do
To develop an understanding of air masses.
In this activity, you will learn about air masses and demonstrate how differences in their
density cause warm and cold fronts. You will learn how source regions give rise to the names of air
masses and how these names can be combined to describe the humidity and temperature of the source
1-cup (250-ml) measuring cup
blue food coloring
one 20-ounce (600-ml) clear plastic bottle
1 cup (250 ml) of liquid cooking oil
Fill the measuring cup with water.
Add three drops of food coloring to the water and stir.
Pour the water into the bottle.
Fill the measuring cup with oil.
Tilt the bottle and slowly pour the oil into the bottle (see Figure 27.1)
Based from the behaviour of the particles of one substance, what have you observed the movement
of oil into the bottle?
The oil moves across the top of the blue water.
An air mass is a large body of air with about the same temperature and humidity throughout. Air masses
form when air stays over a region long enough to take on the temperature and humidity characteristics of
that region. It takes a week or more for an air mass to form.
The density of air masses varies with the temperature and humidity of the air. Warm air masses are less
dense than cold air masses, and humid air masses are less dense than dry air masses. When air masses
with different densities meet, the two masses do not mix. As with oil and water, a distinct boundary forms
between the air masses. In the experiment, the oil represented a warm air mass and the colored water a
cold air mass. As with the oil and water, warm, less dense air moves over cold, denser air.
Vilhelm Bjerknes (1862-1951), a Norwegian physicist and meteorologist, coined the term front to
describe the boundary between warm and cold air masses. The leading edge of a warm air mass
advancing into a region occupied by a cold air mass is called a warm front. Acold front occurs when a
cold air mass advances into a region occupied by a warm air mass. If the boundary between the cold and
warm air masses doesn't move, it is called a stationary front. The boundary where a cold air mass meets
a cool air mass under a warm air mass is called an occluded front. At a front, the weather is usually
unsettled and stormy, and precipitation is common.
Try New Approaches
Model a cold front produced by the movement of a cold air mass into a region occupied by a
warm air mass. Do this by repeating the experiment, but place the oil in the bottle first, then slowly
pour in the colored water.
Does the volume of the air masses affect the results? Repeat the original experiment twice, first
using 11/2 cups (375 ml) of water and 1/2 cup (125 ml) of oil, then using 1/2cup (125 ml) of water and
11/2 cups (375 ml) of oil.
Self Test 1.1
Now that you are through with the first lesson, try to answer the following and see for yourself how
much you learned.
Matching Type. Match the items in column A with the type of fronts in column B.
1. Occluded front
2. Stationary front
4. air mass
5. cold front
A. boundary between the cold and warm air masses doesn't move
B. boundary where a cold air mass meets a cool air mass under a warm air mass
C. boundary between warm and cold air masses
D. cold air mass advances into a region occupied by a warm air mass
E. large body of air with about the same temperature and humidity
How are Air Masses Modified?
Areas in which air masses originate are called air mass source regions.
An ideal source region must meet two criteria. First, it must be an extensive and physically uniform area.
The second criterion is that the area is characterized by a general stagnation of atmospheric circulation so
that air will stay over the region long enough to come to some measure of equilibrium with the surface.
The classification of an air mass depends on:
(1) the latitude of the source region, and
(2) the nature of the surface in the area of origin—ocean or continent.
A source region might be an ocean, a large forest, a dessert or open grasslands. Source regions
must be large and have similar, or uniform, traits throughout.
Abbreviations for Air Mass Source Regions
•Air masses are identified by two-letter codes.
•With reference to latitude (temperature), air masses are placed into one of three categories:
A lowercase letter (m, for maritime or c continental ) is placed in front of the uppercase letter to
designate the nature of the surfaces and the humidity characteristics of the air mass.
Using this classification, the following air masses are identified: cA, cP, cT, mT, cA, cP, cT, mT, and mP
mP. Note that there is not a mA source region.
Air Mass Modifications
Once an air mass moves from its source region, it not only modifies the weather of the area it is travelling
over, but it is also gradually modified by the surface over which it is moving.
•Modifications can result from:
•temperature differences between an air mass and the surface,
•vertical movements induced by cyclones and anticyclones, or
The day-to-day weather we experience depends on the temperature, stability, and moisture content
of the air mass we are experiencing. Continental polar(cP)and continental arctic(cA)air masses are, as
their classification implies, cold and dry. Although cP air masses are not, as a rule, associated with heavy
precipitation, those that cross the Great Lakes during late autumn and winter sometimes bring lake-effect
snows to the leeward shores. Maritime polarair masses (mP) form over oceans at high latitudes and are
cool to cold and humid. The weather associated with an invasion of mP air from the Atlantic into an area
east of the Appalachians and north of Cape Hatteras is known as a nor'easter . Maritime tropical(mT)
air masses affecting North America most often originate over the warm waters of the Gulf of Mexico, the
Caribbean Sea, or the adjacent western Atlantic Ocean. As expected, mT air masses are warm to hot and
they are humid. During winter, when cP air dominates the central and eastern United States, mT air only
occasionally enters this part of the country. However, during the summer, mT air masses from the Gulf,
Caribbean, and adjacent Atlantic cover a much wider area of the continent and are present for a greater
period. The mT air masses from the Gulf-Caribbean-Atlantic source region are also the source of much (if
not most) of the precipitation received in the eastern two-thirds of the United States. Isohyets, lines
drawn on a map that connect places having equal rainfall, illustrate the greatest rainfall in the Gulf region
and a decrease in precipitation with increasing distance from the mT source region. Hot and dry
continental tropical(cT) air is produced only in the summer in northern interior Mexico and adjacent
parts of the arid southwestern United States
Air Mass Move
As air masses leave their source region and begin moving to other areas, they bring with them the
conditions that are common in the source region such as Canada will be cold, especially if it forms during
the winter. As this air mass begins to move towards the south, it brings with it the cold temperature that
are common in the northern parts of Canada. When their happens, southern parts of the continent
experience much cooler than they are used to.
Lesson 3 u What are the Properties of an Air Masses?
The abbreviations used to classify air masses
use the following letters: c, m, T, P, and A.
For each letter, describe its property:
Self Test 3.1
Now that you are through with the second lesson, try to answer the following and see for yourself how
much you learned.
What type of air mass
would form over each
area? Please label
Multiple Choice. Choose the letter of the best answer. Write the chosen letter on a separate sheet of paper.
1. What causes changes in weather?
a. air masses
b. the air gets more humid
c. water evaporates
d. clouds form
2. The air mass with the highest actual water vapour content is ____.
3. The characteristics of an airmass which formed over the Gulf of Mexico would probably be
a. warm and humid
b. cool and humid
c. warm and dry
c. cool and dry
4. An air mass originating over the North Pacific Ocean would most like be
a. maritime tropical
b. maritime polar
c. continental tropical
d. continental polar
5. On a weather map, an air mass that is very warm and dry would be labelled
6. A mT air mass would most likely originate over which type of earth surface?
a. warm and dry
b. cold and moist
c. warm and moist
d. cold and dry
7. Compared to a maritime tropical air mass, a maritime polar air mass has
a. lower temperature and less water vapour
b. lower temperature and more water vapour
c. higher temperature and more water vapour
d. higher temperature and less water vapour
8. Which true properties characterize an air mass
a. temperature and pressure
b. moisture and pressure
c. temperature and moisture
d. temperature and location
9. What type of front forms when the surface position of the front does not move?
10. The boundary that separates different air masses is called as ___.
Key to Answers
Frederick K. Lutgens, Edward J. Tarbuck (2013).The Atmosphere: An Introduction to Meteorology (12 ed.)