1. Water and the Atmosphere – Chapter 4 Lesson 4
Air Masses
Water and the Atmosphere – Chapter 4 Lesson 4

2. Objectives
1. Identify the major air masses that affect the weather in North America and describe how they move.
2. Name the main types of fronts.
3. Explain the type of weather that is associated with cyclones and anticyclones.
We are Covering NGSS
DCI-MS-ESS2.D.1 Weather and climate are influenced by interactions involving sunlight, the ocean, the atmosphere, ice, landforms, and living things. These interactions vary with latitude, altitude, and local and regional geography, all of which can affect oceanic and atm
Keywords: weather, cyclones, anticyclones, air masses, fronts

3. Cyclones and Tornadoes Planet Diary
Misconception: A cyclone is another name for tornado.
Fact: Both cyclones and tornadoes are spinning storm systems.  Both rotate around an area of low pressure.
However, tornadoes cover a much smaller area than cyclones do.  And tornado winds reach much higher speeds.
Evidence: Outside the tropics, cyclones can be 1,000 to 4,000 km across.
Tropical cyclones, which are powerful hurricanes, are smaller, ranging from 100 to 1,000 km across.
But tornadoes are smaller still. Tornado range in size from a few meters to 800 meters across.
Tornado winds are the fastest known winds on Earth. They can reach speeds of 580 km/h, but are usually much slower.
Cyclone winds are strong, but do not move as fast as the fastest tornado winds. Tropical cyclone winds rarely reach more than 320 km/h.
Which kind of storm do you think would cause damage over a larger area, a cyclone or a tornado? Why?
Have you ever seen water swirl down a drain? How is it related to a tornado?

4. What Are the Major Air Masses? Key Concept Summary
An air mass is a huge body of air in the lower atmosphere that has similar temperature, humidity, and air pressure at any given height.   
Four major types of air masses influence the weather in North America: maritime tropical, continental tropical, maritime polar, and continental polar.   
Maritime air masses form over the ocean and can be very humid.
Continental air masses form over land, and are drier than maritime air masses.
Tropical air masses are warm, form in the tropics, and have low air pressure.
Polar air masses are cold, form near the poles, and have high air pressure.
In North America, most air masses move from
west to east.

6. What Are the Major Air Masses? Key Concept Summary
How Air Masses Move 
Prevailing Westerlies are the major belts over the continental US, generally push air masses from west to east.
The jet stream is a band of high-speed wind about 10 kilometers above the surface of Earth that pushes air masses along.
As jet streams generally blow from west to east, air masses are carried along their tracks.
Fronts occur along the boundaries between air masses. Changeable weather develops along fronts.

7. Weather Fronts Interactive Art
A weather front is the area where two air masses with different temperatures and densities collide, but do not mix.
The collision often causes storms and changeable weather.
There are four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.
On the next screens, you will have a chance to explore what happens at each front.
bb ea0e9d2f/16/tier/4fcb3601-e583-38ce-b bce1e35dc7/15/lesson/0951ed4c-acaf a- 1b0d b/15/content/dccc7d83-49c5-35b7-a3b e395196/17

8. What Are the Main Types of Fronts? Key Concept Summary
Colliding air masses can form four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts. 
When a faster cold air mass runs into a slower warm air mass, a cold front forms.
The cold air slides under the warm air.
As the warm air rises, it cools and condenses, often resulting in heavy rain or snow. 
When a faster warm air mass runs into a slower cold air mass, a warm front forms.
The warm air slides up over the cold air, possibly causing light rain or snow.

9. What Are the Main Types of Fronts? Key Concept Summary
When a cold air mass and warm air mass collide, but neither displaces the other, a stationary front occurs.
Water vapor in the warm air condenses into rain, snow, fog, or clouds, lingering for days.   
When a warm air mass is caught between two cooler air masses, the warm air is pushed up and an occluded front forms. (The warm air mass is cut off, or occluded, from the ground.)
Temperatures at the
ground get cooler,
and it may get cloudy
and rain or snow.

10. What Weather Do Cyclones and Anticyclones Bring? Key Concept Summary
A swirling center of low-pressure air is called a cyclone. In the Northern Hemisphere, cyclones spin counterclockwise when viewed from above.
Cyclones and decreasing air pressure are associated with clouds, wind, and precipitation.   
An anticyclone is the opposite of a cyclone.
The descending air in an anticyclone generally causes dry clear weather.

11. Pressure Points Apply It!