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!