1. Ch 9: Air Masses and Fronts

2. Introduction
Air masses are distinguished by uniform temperature and humidity characteristics
They affect vast areas
Fronts are boundaries between unlike air masses
Fronts are spatially limited
They are inherently linked to mid-latitude cyclones

3. Formation of Air Masses
Surface energy and moisture exchanges imprinted in temperature, pressure, and moisture characteristics of overlying air
near surface conditions control in large part the condition of atmosphere
exchanges – vary through space and time

4. Source Regions
areas in which air masses form  requires days for temp/moisture imprinting
high/low latitudes  require large regions of land or water and uniform topography
air takes on temperature and humidity characteristics of the surface below
air masses classified by moisture and temperature characteristics of source region
moisture: continental (dry) v. maritime (marine) – c or m
temp: tropical (warm), polar (cold), arctic (very cold) – T, P or A
Thus, 5 catagories of air masses: cT, cP, cA, mT, and mP (no mA)

5. North American air masses and air mass source regionsmP mP cP
Continental
Tropical cT mT mT mT

6. Once formed, air masses migrate
Upon movement, air masses displace residual air over locations they overtake and change the temperature and humidity of those locations
In addition, the air masses themselves
change from surface influences

7. Continental Polar (cP) and Continental Arctic (cA) Air Masses
Winter: originate over high-latitude land masses (northern Canada, Siberia)
Low solar angle, short days, high albedo  cooling of over-lying air  inversions and highly stable conditions
very cold and dry  limited cloud formation, bright and sunny
Summer: warmer with higher moisture content  fair weather cumulus develop
Continental Arctic (cA)
extremely cold and dry conditions: low temperature  limited vapor content
boundary between cA and cP  Arctic Front cP
cT or mT cA

8. Migrations of cP air  induce colder, drier
conditions over affected areas
As air migrates it warms and moisture capacity
increases  stability decreases

9. Maritime Polar (mP) Air Masses
similar to cP but warmer and higher moisture content
forms over N. Pacific/Atlantic
Example: cP flows out of Asia, over warm Japan current  adds heat/moisture and becomes mP
Affects northwestern U.S. and Canada year round
California coast of the US  mP air affects
regions mainly during winter
process is different along east Coast: 
cyclones cause winds to approach coast from the northeast (nor’easters)
associated with cold and heavy snowfall

10. Example of Nor’easter
Counterclockwise flow around east coast low pressure brings cold moist air to east coast from the northeast

11. Continental Tropical (cT) Air Masses
Summer phenomena – hot, low-latitude areas  SW US, northern Mexico
little available moisture, high temp  hot, dry air masses
steep lapse rates, unstable conditions due to intense surface heating
However, limited cloud formation
due to low moisture content
thunderstorms may occur:
if unstable layer is deep
if orographic lifting occurs
Continental
Tropical cT

12. Maritime Tropical (mT) Air Masses
Form over low latitude oceans: Gulf of Mexico, tropical Atlantic, Pacific
very warm, humid  unstable
Huge influence on southeastern US
Migration inland heating of air mass from ground surface increases lapse rates  increases instability  intense precip (thundershowers)  localized uplift
moisture content: reduced in northward
direction  Miami v. Chicago
Arizona monsoon 
mT from Gulf of California in
late summer

13. Fronts
separate air masses  movement of fronts leads to changes in temperature and humidity as one air mass is replaced by another
changes in temp  lead to uplift and precip
four types of fronts:
cold  cold advancing on warm
warm  warm advancing on cold
stationary  air masses not advancing
occluded  does not separate tropical
from polar/arctic, boundary between two
polar air masses

14. Cold Fronts cold air displaces warm air move at speeds of 0-50 km/hr
results in heavy, short-lived precipitation events and rapid temperature drops
Sometimes get extreme precipitation due to extensive vertical lifting
warm air ahead of the front is forced aloft  clouds form
Frontal development about a
low pressure system
The vertical displacement of air along a cold front boundary

15. The sharp cold front boundary brings narrow bands of short-lived precipitation

16. Warm Fronts occur when warm air displaces colder air
overrunning – warm air gently rides over cold, dense air
leads to gradual progression of cloud types  stratus, nimbostratus, altostratus, cirrostratus, cirrus
From viewer’s perspective: cirrus is seen first, clouds continue to thicken and become lower as warm front approaches (usually from south or southwest)
zone of contact is less steep than cold fronts  greater horizontal extent
longer, less intense periods of rain (uplift not as dramatic)
precipitation falls through cold air mass  may getfrontal fog, or sleet/freezing rain

17. Warm frontal precipitation is less intense and spread over a broader region than cold front precipitation

18. Stationary Fronts
contact zones sometimes are stalled  relatively ‘fixed’ in position
classification is subjective in terms of speed  difficult to establish contact zone precisely
Fronts may slowly migrate and warmer air is displaced above colder

19. Occluded Fronts
Occluded front (occlusion): closure  faster moving cold air mass cuts off warm air from ground
separate cold/warm air masses BUT at surface, cold air masses merge
cold-type v. warm-type occlusion
cold-type occlusion: eastern half of the continent  cP air meets mP
warm-type occlusion: western edges of continent  mP advances on cP

20. Occlusion
sequence

21. Alternative Mechanisms: Occluded Fronts
Some occlusions form when the surface low
elongates and moves away from the junction of
the cold and warm fronts
Some occlusions occur when the intersection
of the cold and warm fronts slides along
the warm front

22. Drylines
Fronts can be based on temp and/or density differences of air masses
Humidity (moisture content) affects density  humid air < dense (H20 lower molecular weight than N2 and O2)
Dryline: boundary between humid and dry air  Great Plains of NA (spring/summer)  severe storms
Dew points: east side (humid) is greater than west (dry air) 
 lead to tornadoes and severe thunderstorms
A dryline over Texas

23. Concepts Understand air masses Understand fronts How they are formed
5 main types
Understand fronts
4 types and their characteristics
Precipitation associated with warm and cold fronts
drylines

24. Next Class: Hand back quizzes Read chapter 10 for lecture
Lecture may cover severe weather too (no reading req’d)