1. Lesson 2-2 Weather Elements Part 1 of 2

2. Lesson Overview Types of air masses and fronts
Factors that impact air masses
Fronts
Wind and Atmospheric Motion
High- and low-pressure systems
Terrain factors that affect weather
Normal weather patterns in U.S.
Chapter 2, Lesson 2

4. Types of Air Masses and Fronts
Weather refers to temperature, moisture, wind velocity, visibility, and barometric pressure
Air mass
Large body of air (usually 1,000+ miles across
Generally same temperature and moisture content within the entire mass
Fronts are the boundary between two different air masses
Chapter 2, Lesson 2
Courtesy of NASA

5. Air Masses Air masses are classified based on their source region
Polar air mass (P) is cold
Tropical air mass (T) is warm
Maritime air mass (m) forms over water and is humid
Continental air mass (c) forms over land and is dry
Possible Types:
mP mT
cP cT A

6. Air Mass Classification

7. Air Mass Classification
Aviators and meteorologists in the U.S. are mainly concerned with air masses originating from two sources
Masses that move south and east from Polar Regions
Masses that move north and east from Tropical Regions (mainly the Gulf of Mexico)

8. Temperature Classification of Air Masses
Based upon its temperature in relation to the surface over which it passes
A cold air mass is cooler than the Earth’s surface over which it is moving
Tends to create updrafts with this air mass
A warm air mass is warmer than the Earth’s surface over which it is moving
Typically won’t exhibit updrafts

9. Air Mass Characteristics
Outside factors affecting air mass characteristics include:
Source Region
Surface qualities of terrain air mass passes over
The seasons (fall, winter, spring, summer)
As an air mass moves away from its source, its temp and pressure traits change due to the surface it passes over.
Can become warmer or colder
Can absorb or lose moisture
Can be lifted up by mountains or subside into valleys
Cold air masses move quicker than warm air masses
Weather generally depends on the nature of a prevailing air mass or the interaction of two or more air masses

10. Air Masses -- Quick Review
Where do air masses originate?
In a source region
How do they form? How do they “adopt” the temperature and moisture traits of their source region?
By remaining over an area long enough
What type of terrain makes an ideal source region?
Flat, uniform areas where temperature and moisture don’t change rapidly

11. Air Masses -- Quick Review
What is the letter designation for a moist polar air mass? mP
As an air mass moves away from its source, what causes its traits to change?
The surface the air mass passes over
What traits can change?
Can become warmer or colder
Can become wetter or drier

12. Air Masses -- Quick Review
What is the process called when an air mass drifts away from its source region and adopts the temp and moisture trais of the surface over which it passes?
Modification
How do you define a cold air mass?
An air mass that is cooler than the surface it passes over
What happens when two different air masses meet?
Fronts develop and weather changes

13. Fronts Fronts are the boundaries between two different air masses
They always signal a change in weather
As fronts move along the surface, one air mass is displacing the other air mass
Four types based on temperature of advancing air compared with temperature of the air it is replacing
Cold Front => cold air mass replaces warmer air mass
Warm Front => warm air mass replaces colder air mass
Chapter 2, Lesson 2
Reproduced from NASA

14. Types of Fronts Warm Cold Stationary Occluded Chapter 2, Lesson 2
Courtesy of NASA

15. Warm Front Usually connected to a low-pressure cell
Usually moves slowly in east-northeast direction
If warm air mass did not move, the front would dissipate
The warm front slips upward over cool air and forms a wedge
Rises slowly which delays condensation

16. Warm Front As warm front advances, it replaces colder air
Before a warm front arrives, you’ll likely see:
High-level cirrus clouds, layered stratus clouds, and fog
As the front passes over you’ll see:
Stratus clouds and some drizzle and fog
After it passes, you’ll see:
Stratocumulus and nimbostratus clouds and rain showers

17. Warm Front
Question: Why is fog so common in warm fronts as they pass over and past your location?
After front passes, there is a rise in temperatures, general clearing and change in wind direction
In winter a warm front causes icing conditions at low altitudes
In northern latitudes snow may also be produced

18. Cold Front Cold fronts generally move from northwest to southeast.
Can travel very far south
Amount of energy depends on amount of cold air and how high the pressure is
As the cold air mass catches up to the warmer air mass, cold air forces the warm air upward

19. Cold Front
As cold air pushes the warm air upwards, the cold air replaces the warm air near the surface
Before the cold front arrives, you’ll see:
Cirrus, cirrocumulus, cirrostratus; clouds get lower as front gets closer
As the front passes over:
Cumulus or cumulonimbus clouds form => rain, hail, wind, etc
After the front passes:
Clouds dissipate, clear skies, cooler weather

20. Cold Front In western states: In southern and eastern states:
The most noticeable change is shift of wind, temperature, and blowing dust
In southern and eastern states:
Generally lots more moisture, larger areas of cloud development, and more problems for aviators
Test Question: Why are there more cumulonimbus build-ups and tornado activity associated with cold fronts than warm fronts?

21. Cold Front
Along the cold front there will be a low-pressure cell where weather is worst
Almost any type of clouds can be found near the cell
Squall lines are common ahead of these fast moving fronts
Poor visibility, low ceilings and rain in summer
Freezing rain and snow in winter

22. Stationary Front
Forms between two air masses of relatively equal strength
When air masses stop, a stationary front develops
Weather can be bad for aviation along the front
About every form of weather can be found

23. Stationary Front
Toward the trailing edge of the any front there will be a stationary-front condition
The trailing edge stationary fronts are a great distance from the parent cell
As the distance become greater the front no longer exists.

24. Occluded Front
Forms when fast–moving cold front catches up with slow–moving warm front
There is a tendency for a horizontal bend or wave to occur along the front
After a frontal bend starts, cold air moves ahead of a warm front

25. Occluded Front Warm Front Occlusion: Cold Front Occlusion:
Air mass overtaking the warm front is not as cool as the cold air in front of the warm front, and rides over the cold air mass while lifting the warm air
Cold Front Occlusion:
Air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses

26. Occluded Front
A wide variety of weather can be found along an occluded front
Thunderstorms possible, but usually their passage is associated with a drying of the air mass

27. Next…. Types of air masses and fronts Factors that impact air masses
High- and low-pressure systems
Terrain factors that affect weather
Chapter 2, Lesson 2