Unit 2: Surface Processes and the Hydrosphere
Lesson 4: Air Masses and Fronts
(Heath Earth Science – Pg. 542-555)
Today’s Objectives Describe the function of the
hydrologic cycle, including: Identify the characteristics of the main
kinds of air masses (eg., continental polar) and the resulting weather patterns
Air Masses Before we begin, lets check out some videos!
Weather fronts song Air masses video
So, what IS an air mass? An air mass is a huge body of air that is
characterized by a similar temperature and moisture throughout.
Usually 1500 kilometers or more across and perhaps several kilometers thick
Located in the lower troposphere
Formation of Air Masses Air masses are so big, just two or three air
masses can cover an entire continent How do they form? When a parcel of air maintains the same
position for days or even weeks over a large, uniform surface, such as a cold ocean, or a warm area of land If a body of air stays over the South China Sea for
several days, it would become warm and moist If a body of air stays over Heilongjiang province for
several days, it would become cold and dry Air masses originate in parts of the world where
winds are light, such as in the polar and subtropical high pressure belts where air is sinking
Types of Air Masses Maritime Tropical (mT)
Originates over tropical ocean areas Warm and moist air
Continental Tropical (cT) Originates over tropical land areas Warm and dry air
Maritime Polar (mP) Originates over cold ocean areas Cold and moist air
Continental Polar (cP) Originates over cold land areas Cold and dry air
Continental Arctic (cA) Originates over arctic regions Very cold, very dry air
Weather in Air Masses Air masses are so large, they may take several
days to pass a given place, for example: When mP air comes in from the northern ocean, it
brings cool, humid weather mT air masses bring mild, humid weather in
winter, or hot, humid weather in summer Air masses will change as they move
southward or northward over different surfaces Dry air masses become moist over moist ground Moist air masses become dry over dry ground
The faster an air mass moves, the less it is affected by the surface it crosses, therefore, the faster moving air masses bring more extreme weather changes
Weather Fronts The boundary where two or more air masses
meet is called a front Because different air masses have different
temperatures and humidities, the front is where weather changes
An approaching front means a change in weather The greater the difference between air masses,
the greater the change in weather On weather maps, fronts are drawn in
regions of great change in temperature and wind direction
Weather Fronts Weather fronts video Quick Quiz!
What are the following weather fronts called, and what kind of weather is found at each one?
Kinds of Fronts
1) Warm Fronts
Warm air is pushing ahead, and displacing colder air
Warm air is less dense than cold air, so it slowly rises above the cold air
As it rises, the air cools, and the moisture condenses into clouds, and moderate precipitation
Cloud types: nimbostratus, altostratus, cirrostratus, cirrus
Weather: Increasing temperature, light winds and moderate precipitation
Warm Front
Kinds of Fronts
2) Cold Fronts
Cold air is pushing ahead and displacing warm air Cold air is more dense than warm air, so it causes
the warm air to rapidly rise above the cold air Cold fronts move more quickly and are steeper
than warm fronts, so they bring a more rapid, violent change in weather
Cloud types: cumulonimbus Weather: Decreasing temperature and
heavy precipitation and wind, often thunderstorms and/or hailstorms
Cold Front
Kinds of Fronts
3) Stationary Fronts
When air flow on both sides of front is neither toward the cold air mass or warm air mass, but almost parallel to the front, the front does not move across the surface
This is called a stationary front Warm air may gently rise over cold air causing
light precipitation May eventually become a warm or cold front
Cloud types: stratus, altostratus, nimbostratus
Weather: Light precipitation, large temperature difference across the front
Stationary Front
Kinds of Fronts
4) Occluded Fronts
When a faster moving cold front overtakes a slower moving warm front, the result is an occluded front
The cold air wedges the warm front upward, a new front is created between the advancing cold air and the cool air that the warm front was gliding over
The wedge of warm air rises quickly and causes heavy rain initially, that gradually diminishes
Cloud types: cumulonimbus, nimbostratus, variable
Weather: highly variable, but usually moderate to high precipitation, cooling temperatures
Occluded Front
Cloud Types
Cloud Types
• Nimbus is Latin for rainstorm, so Cumulonimbus and Nimbostratus are precipitating cloud types
Homework Topic Questions, pg. 546, #1-4 Topic Questions, pg. 549, #6-8 Topic Questions, pg. 551, #9-12 Review (matching), pg. 554-555
Group Project – Due October 12 You will be presenting your projects on
Saturday Remember – Your presentation should be
between 10-15 minutes long Each group member must have a speaking
role in the presentation!