1. Lecture  14
Air Mass
Fronts
1

2. Air Masses

3. Weather map 11/02/2009
Weather strongly depends on where
an air mass comes from!!!

4. What is an air mass?
An air mass is a large body of air
with similar temperature and humidity
An air mass is named by its
birthplace (source region)
4

5. Must be dominated by light wind
(or non at all)
Must have an
extensive, uniform surface
5

6. air mass name
By two letters with two properties

7. air mass name
By two letters with two properties
cA
cP
cT
mP
mT

8. 8

9. 9

10. Why cold cP passage causes
clear weather?

11. The upper air flow on the map below would bring ____
air masses into the Pacific Northwest and ____ air
masses into the eastern United States.
a.
b.
c.
d.
mT, mP
mP, cP
mT, cP
mP, cT
11

12. Air masses of North America
mP/west coast
12

13. What would happen as cP
passes over great lakes?
13

14. Lake-effect snow
is produced in the winter when cold, arctic winds move across
long warmer lake water, and is deposited on the lee shores.
Figure 1, p. 207

15. What would happen as two air masses meet?
15

16. Fronts

17. Atmospheric Fronts
Boundary, transition zone between two different air masses
frontal boundary can be 1-100 km wide
cP
17

18. Cold Fronts
Cold air replaces warm
Much steeper than warm fronts
Advances faster than warm front
More violent weather –cumulonimbus clouds
Short, sharp showers
mT
cP
18

19. Weather associated with cold front passing
Weather
Before Passing
While Passing
After Passing
Winds
south-southwest
gusty; shifting
west-northwest
Temp.
warm
sudden drop
steadily dropping
Dew
Point
high; remains steady
sharp drop
lowering
Pressure
falling steadily
minimum, then sharp rise
rising steadily
Precip.
short period of showers
heavy rains, sometimes with
hail, thunder and lightning
showers then
clearing

20. Warm Fronts
Warm air replaces cold
Gentle slope
Covers a wide area with its weather
Stratus clouds get lower as front approaches
Drizzle or steady rain
cP
mT
20

21. mT
cP

22. Weather associated with warm front passing
Winds
Temperature
Dew Point
Precipitation
Before Passing
south-southeast
cool-cold, slow
warming
While Passing
variable
steady rise
steady rise
After Passing
south-southwest
warmer, then
steady
steady
light-to-moderate rain,
snow, sleet, or drizzle
rise, then steady
drizzle or none
usually none,
sometimes light
rain or showers
Why?

23. Warm Fronts
Warm air replaces cold
Gentle slope
Covers a wide area with its weather
Stratus clouds get lower as front approaches
Drizzle or steady rain
cP
mT
23

24. Stationary Fronts
 no moving
weather: clear, partly cloudy, cloudy, light precipitation
24

25. Occluded Fronts
 separate two cold air masses
Lifts the warm air off the ground
Weather is complex
Cold occluded front– cold front lifted warm front
Warm occluded front- cold front rises up and over warm front
25

26. Catches up
26

27. Catches up
27

28. Atmospheric Fronts
Boundary, transition zone between two different air masses
frontal boundary can be 1-100 km wide
cP
28

29. The diagram below represents a side view of a ____
occluded front with the coldest air located at position
____.
a.
b.
c.
d.
cold type, B
warm type, B
cold type, A
warm type, A
29

30. Review Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
What is an air mass?
What and where are good source regions for air masses?
How are air masses classified?
What are the characteristics of the air masses that commonly affect weather over the U.S.?
How can air masses be modified?
What are the fundamental, physical processes involved with generating lake effect snows?
What is a front?
What are the types of fronts? How are they analyzed (what symbol) on a surface weather
map?
What are the characteristics of each kind of front?
What is observed during frontal passage for each kind of front?
Why is the weather at LA much drier than at Atlanta?
What are the two jet streams and how are they created?
What is the Walker circulation?
What is El Nino? How often does it occur?
How do pressure, precipitation, SSTs and ocean currents change in the equatorial pacific
during an El Nino event?
30