Transcript No Slide Title

Outline for Lecture 16
(Chapter 7)
Recap
(Chapter 8)
Air Masses
Source Regions
Classifying Air Masses
Air Mass Modification
Properties of North American Air Masses
(Chapter 9)
Fronts
4/03/01
4/1/03
Zonal Precipitation Patterns are Related
to Pressure Patterns
H
L
H
L
H
L
H
The Subtropical belts are
the most complicated.
Intertropical Convergence Zone
p. 405 of your book
Subtropical High Pressure Cells
Subtropical high pressure cells have different characteristics with strong
subsidence (which leads to dry regions) on the eastern side of these systems.
Locations of Great Deserts
H
p. 406 of the book
Glance back at Ch. 15
p. 395 of your book
What is an air mass?
Air Mass – A large body
of air, usually 1600 km
or more across, that is
characterized by homogenous
physical properties
(temperature and
moisture) at any
given latitude.
A cold Canadian air mass moving southward bringing very cold air
Source Regions
Two things are critical for an area to be a source region.
• The region must be an extensive area that is physically uniform.
• The region must be characterized by a general stagnation of
circulation.
H
Locations dominated by slow moving or stationary anticyclones
are ideal areas for source regions.
Average Surface Pressure Systems and Associated Circulation
January
Siberian High, Azores High
Aleutian Low, Icelandic Low
Average Surface Pressure Systems and Associated Circulation
July
Bermuda High
mP
cA
mP
cP
Your book has
a MUCH better
figure
cT
mT
mT
Classifying Air Masses
Air masses are classified by:
the surface over which it formed and
its latitude
Air masses are identified by two-letter codes.
The first designates the surface of the region:
c continental
m maritime
The second designates latitude/temperature:
P Polar
A Arctic
T Tropical
Air Masses
cA
cP
cT
mT
mP
continental Arctic
continental Polar
continental Tropical
maritime Tropical
maritime Polar
Note: no maritime Arctic!
General Properties
Continental
• Drier
• More extreme: the hot
is hotter and the cold
is colder
Maritime
• WET
• Smaller temperature
range—hey, it’s over
water!
Continental Arctic Air Masses (cA)
Forms over continents at high latitude.
VERY cold and VERY dry.
Source: far north, like Greenland and the Arctic Basin
cA
cP
cA
“Arctic Express”
VERY cold
VERY dry
A cold snap
Continental Polar Air Masses (cP)
Forms over continents at high latitude. Cold and dry,
but not as cold as the continental arctic in the winter time.
Source: interior Canada and Alaska, anywhere in the interior
north of 50º latitude and covered with snow
cA is just cP with an attitude problem.
p. 222 of the
Book has a
Better figure
cP/cA
cP
Maritime Polar Air Masses (mP)
Forms over oceans at high latitude. Cool to cold, and moist,
but not as cold as the continental polar in the winter time.
Unstable and stormy in winter.
Two regions are important sources for mP air that influences
North America: the North Pacific and the Northwestern
Atlantic.
nor’easter
mP
mP
Rain, snow,
clouds
Maritime Polar Air Masses (mP)
In summer, mP has different characteristics—it is generally cool,
wet and stable. So we have no summer nor’easters.
mP
mP
Maritime Tropical Air Masses (mT)
Copious moisture is transported from the tropics to the mid
latitudes when mT air masses move northward over land.
The mT air mass over the Gulf of Mexico and Western Atlantic
is important to weather over the eastern half of the nation.
Moist, warm,
and stable
mT
mT
Moist, hot,
unstable (on the
western side of a
subtropical high)
Average annual precipitation for the eastern two thirds of the
United States. Precipitation decreases with distance from the
mT source regions.
Isohyets are lines of
equal precipitation
Continental Tropical Air Masses (cT)
A summertime-only phenomenon in North America. Forms over
Mexico and the desert southwestern U.S. Tends to be hot, dry,
and unstable, though it’s so dry that clouds cannot form.
Tends not to roam much, but if it goes over the central plains, it
will produce drought.
cT
Dryline
HOT dry air
over here
Warm moist air
over here
cT
mT
This leads to differences in lapse rates, and explosive thunderstorm
Development in the afternoon over West Texas in summer.
Monthly Precipitation Data for Tucson Arizona
8
{
North American Monsoon
Precipitation (cm)
7
6
5
Tucson Precip
4
mT
3
2
1
0
j
f
m
a
m
j
j
Month
a
s
o
n
d
Air Mass Modification
After an air mass forms, it normally migrates from the area where
it acquired its distinctive properties to a region with a different
surface characteristics. As the air mass migrates it exchanges
physical properties with the surface below it. The type of exchange
determine cloud types and weather patterns.
Cold air over warm surfaces will result in instability () whereas,
warm air over cooler surfaces will result in more stable conditions
and limited vertical motion ().
Lake Effect Snow
As continental polar air crosses the Great Lakes in winter, it
acquires moisture and is made less stable because of warming
from below. A lake-effect snow shower on the lee side of the
lakes is often the consequences of the air-mass modification.
Differential Heating of Land and Water
In the winter time, warm, moist air moves over much colder
the land which creates instability ().
The snowbelts of the Great Lakes
region are the zones that most frequently
experience lake-effect snowstorms.
Current Air Masses
cP
cA
mP
mP
cP
cT
mT
MRF 850 mb Temperature (°C) and Height (m)
mT
Storm of the century
(extreme example of two air masses interacting)
A very cold
continental
arctic air (cA)
mass collided
with a warm,
moist maritime
tropical (mT) air
mass from the
Gulf of Mexico.
Polar Front over the U.S.
A series of mid-latitude cyclones forming along the polar front
Fronts are boundaries
between air masses.
Fronts
•One air mass is usually
warmer and often contains
more moisture than the
other.
•Along the zone where the
air masses meet, they
exchange moisture and
temperature characteristics.
Warm fronts
Warm air rides over colder, more dense air.
As the air is lifted it is cooled__________-ly
causing moisture in the air to ___________ into clouds.
slope = 1/200
1 mi
0
Slow-moving warm front and stratus clouds
200 mi
Cold fronts
Cold air wedging under warm, moist air.
The forceful lifting of warm, moist air along the cold front releases
________ which adds the ______ of the air to create more lifting.
Most often, the air behind a front is continental polar.
slope = 1/100
1 mi
100 mi
0
Fast moving cold front and cumulonimbus clouds.
Thunderstorm development along a cold front over the Great Plains
Lightning results when very strong vertical lifting creates a static
charge within the cloud.
Stationary Fronts
The air flow on both sides of a front is parallel to the front.
This condition is called a stationary front.
This will serve as a focal point for storm formation.
A rapidly moving cold front can
Occludedovertake
Fronts a warm front.
As the cold air wedges the
warm front upward a new
front, known as an occluded
front, is formed.
There are cold occluded fronts
and warm occluded fronts.
Cold and Warm Occluded Fronts
Air is colder than the air it
is overtaking. This is a cold
occluded front. Cold occluded
fronts are the most common
type of occluded front.
It is also possible for air behind
the advancing cold front to
be warmer than the cold air
it is overtaking. This is a warm
occluded front.
Exam 2
• Will be April 15, as scheduled
• Will cover Chapters 6-10 (mostly)
• Might not include tornadoes if we don’t get
to that