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Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(1 of 10)
Further Reading: Chapter 08 of the text book
Outline
- air masses
- air masses of the world and of N. America
- fronts: warm, cold and occluded
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(2 of 10)
Introduction
•
Previously,
– We discussed the general circulation of the atmosphere and oceans
– We looked at surface winds and pressure patterns as well as upper-air winds
– We also discussed surface circulations and thermohaline circulations of the deep ocean
– An overarching theme for all of this discussion was the role these circulations played in
transporting energy from low latitudes to high latitudes
•
Today,
– We begin our discussion of phenomena that affect temperature, rainfall, etc. on a day
to day basis, i.e. weather
– We will define characteristics of air masses involved in producing weather
– We will also look at the characteristics of fronts
– Throughout, we will have to keep in mind all of the principles we’ve learned before
• Atmospheric humidity
• Instability
• Dynamic forces
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(3 of 10)
Air Masses
• Weather Systems: Recurring or common circulation patterns and their associated weather
- Example: traveling low pressure centers (cyclones) where inspiraling air that brings warm
moist air in contact with cold dry air to results in clouds and precipitation
- 1 km (Tornado) to 100s km (anti-cyclones)
- may last from hours to weeks
- weather systems are associated with movements of air masses
•
Air Masses
– Large bodies of air with (more or less) uniform properties (temperature and moisture)
– Air masses can be very large: ~ 1000’s of km across
– Properties reflect properties of the source region
- hot and moist air masses develop over tropical oceans
- cold and dry air masses develop over continental polar locations
– Described bases upon
• Latitude (defines temperature)
• Surface type (defines moisture content)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(4 of 10)
Classification of Air Masses
Name
Arctic/Antarctic
Polar
Tropical
Equatorial
Maritime
Continental
• Ex.:
Symbol
A/AA
P
T
E
m
c
cA - Continental Arctic
mP - maritime Polar
Source Region
Arctic Ocean
50-60N
20-35N
<10N
Oceans
Continents
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Air Masses of the World
- The maritime tropical airmass (mT) and maritime
equatorial air mass (mE) originate over warm oceans
- high specific humidity
- warm (20+C)
- capable of producing heavy precip
- The continental tropical airmass (cT) originates
over the subtropical deserts of the continents
- substantial water vapor
- low relative humidity
- Polar air masses (mP, cP) originate in the subarctic
latitude zone
- mP has low water vapor (moderate precip)
- cP originates over N. America and Eurasia
(low humidity and very cold)
- The continental arctic (cA) and antarctic (cAA)
air masses are extremely cold (-45C) and have little or
no water vapor
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(5 of 10)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(6 of 10)
Air Masses of North America
– cP: cold, stable air originating from high
pressures in N. Canada
– mP: cool, moist air, usually unstable
originating from Aleutian low; region
of cyclone formation; heavy precip
in coastal ranges
– mT: warm moist unstable air originating
from the Gulf of Mexico; good for
producing thunderstorms
– cT: hot dry air from Mexico; impacts
southern AZ and CA
mP
cP
mP
cT
mT
mT
mT
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Fronts
•
Implicit in all of our discussion is that air masses
can move and still retain their identity
– May result in the collision of two air masses
with very different properties
– The zone of contact is called a front
– A front is an interface between air masses
with different temperatures and moisture
content
– This figure shows the polar front of which
you learned in the past
– Here the front is between cold air to the north
and warm air to the south
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(7 of 10)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(8 of 10)
Warm Front
– Warm air is less dense than cool air
– The warm air rises over the cool air
– As it does so, it goes through “adiabatic cooling” which allows moisture to condense
into clouds and rain
– Typically there is gradual lifting leading to relatively mild precipitation
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(9 of 10)
Cold Front
– Warm air is forced to flow over the cooler air
– As it does so, it goes through “adiabatic cooling” which allows moisture to condense
into clouds and rain
– For these fronts, the boundary tends to be sharp with strong instability, leading to
heavy precipitation
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 22: Air Masses and Fronts
Mar-23-05
(10 of 10)
Occluded Front
–
–
–
–
–
Similar to cold front in that warmer air is forced to rise over cold air
Cold fronts move along the ground faster than warm fronts
Thus, a cold front can overtake a warm front
The warm air mass may be completely lifted off the ground
For these fronts you may find hail and sleet -> as rain from the warm air falls through
the cold air below it, it freezes and forms hail