Earth & Space
Chapter 11 – Movement in the Atmosphere
Lesson 1 – Air Masses
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Lesson 1 Objectives
1.
Understand how air mass source regions affect the
characteristics of air masses.
2.
Describe how fronts form between air masses.
3.
Describe weather typically associated with each type of front.
4.
Explain how fronts can cause precipitation to occur.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses

Weather changes because air is always moving.

Air Mass – a huge body of air that has somewhat uniform
temperature and humidity and covers hundreds or
thousands of square kilometers of the earth’s surface and
can reach heights of several kilometers.
 Weather is typically uniform throughout an air mass.
 Air masses typically do not mix with other air masses.
 Precipitation and changing weather occur where air
masses meet.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Sources of Air Masses

Source Region – A large area of land or water with
uniform humidity and temperature where air masses
form by slowly moving over and taking on the
temperature and humidity of that area.
 Source regions must not be windy.
 Air masses need to remain in contact with their
source region for a period of time and only form
in relatively still air.
 Air masses are named after their source regions.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Air Mass Naming – By Humidity

Continental (c) – Air masses that form over land – dry air.

Maritime (m) – Air masses that form over water – humid air.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Air Mass Naming – By Temperature

Arctic (A) – Air masses from extremely cold areas.

Polar (P) – Air masses from cold or cool areas.

Tropical (T) – Air masses from warm or hot areas.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Types of Air Masses

Continental Arctic (cA) – dry and
very cold.
 Form over the Arctic Circle in the
winter only.
 Bring extremely cold air down to
Alaska and North Western
Canada.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Types of Air Masses

Continental Polar (cP) – dry and
cool/cold.
 Form over Canada and move
southward over the central and
eastern states.
 Summer – Bring clear, cool weather.
 Winter – bring cold spells that last for
weeks. “Polar Vortex”
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Types of Air Masses

Continental Tropical (cT) – dry
and warm/hot.
 Form over Mexico and move
northeast over the central
states.
 Summer & Winter – bring
dry, warm weather.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Types of Air Masses

Maritime Polar (mP) – humid and
cool/cold.
 Form over the North Atlantic or North
Pacific.
 Bring humid and cool air down over
the northwest.
 Results in a large amount of
precipitation.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Types of Air Masses

Maritime Tropical (mT) – humid and
warm/hot.
 Form over the tropical Pacific, tropical
Atlantic, and Caribbean.
 Summer – Bring hot, muggy weather
to the southeastern US
 Winter – Bring mild and cloudy
weather.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Meteorological Air Mass Naming

Air masses are named by their temperature
compared to the temperature of the ground
over which they move.
 Warm air masses – warmer than the
surfaces they pass over, tropical.
 Cold air masses – colder than the
surfaces they pass over, polar.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Weather Fronts

Where two air masses meet they do not
usually mix unless they have similar
temperature and humidity.

A front is formed when two air masses meet
and do not mix.
 Cooler, denser air moves under the warmer,
less dense, air like a wedge.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Weather Fronts

Typically fronts will move as one air mass
moves away and the other air mass
replaces it.

Stationary Front – formed when the
boundary between the air masses does
not move.
 Will stay in place until a stronger,
larger, or faster moving air mass
moves into the area and pushes the
stationary front out of the way.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Weather Fronts

Moving fronts:
 Warm Front – form when warmer air is
replacing cooler air.
 Occurs when a warm air mass overtakes a
slower moving cold air mass.
 The warm air rises over the thin wedge of
cold air, and cools to its dew point.
 Large systems of clouds form and typically
bring long, steady rain to a large area.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Weather Fronts
 Moving fronts:
 Cold Front – form when colder air replaces
warmer air.
 Often advance faster than warm fronts.
 Have a steeper slope that forms due to
friction with the ground slowing the air
movement near the surface.
 If the cold front moves faster than 20 mph it
forces the warmer air to rise quickly and often
brings heavy showers or violent
thunderstorms that only last a short time.
 Squall Line – a line of violent thunderstorms
that may occur just before a cold front.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses
Weather Fronts

Moving fronts:
 Occluded Front – form when a warm air mass is
trapped between two cold air masses and the
warm air mass loses all contact with the ground.
 The rising of the warm air typically causes
precipitation that lasts for a few days.
 Often occur in the fall and winter in North
America as fresh, cold polar air masses move
into the mid latitudes and overtake stalled cool
air masses that are slowly making their way
eastward.
Ch. 11 – Movement in the Atmosphere
11.1 – Air Masses