10/7/2008
Weather Fronts and Weather Charts
Air Masses that affect Minnesota
Jet Stream, Air Masses and weather in Minnesota
Components of Fronts
Clouds associated with Cold Fronts
Clouds associated with Warm Fronts
Occluded Fronts
Fig. 11-2, p. 287
1
10/7/2008
Jet Stream, Air Masses and winter in
Minnesota
• Continental Polar(cP) Maritime Polar (mP)
• Cold, Dry Wet, relatively warm
• Surface weather map for 7 A.M., EST, December 24,
1983. Solid lines are isobars. Areas shaded green represent
precipitation. An extremely cold arctic air mass covers
nearly 90 percent of the United States.
2
10/7/2008
The Jet Stream and direction of storms
• High Pressure systems tend to move Southeast
• Low pressure systems tend to move Northeast
• High Pressure systems tend to move Southeast
• Low pressure systems tend to move Northeast
3
10/7/2008
Typical vertical temperature summer
and winter
• Areas shaded purple show regions that
experience heavy lake-effect snows.
4
10/7/2008
lake effect snows and early winter conditions
• The formation of lake-effect snows. Cold, dry air crossing
the lake gains moisture and warmth from the water. The
more buoyant air now rises, forming clouds that deposit
large quantities of snow on the lake’s leeward shores.
Review of weather conditions for Low
and High Pressure Systems
• For High Pressure
Clockwise rotation
• Fair weather
• For Low Pressure
Counter Clockwise
rotation
• Stormy weather
5
10/7/2008
What happens when the
wind comes from the east
with gulf moisture?
Big snow in Denver area:
some places received 5 ft. of snow.
6
10/7/2008
Warm and Cold Fronts
• Warm air advancing from the
south rose up over cold air
retreating northward on the east
side of the low center. The
y
boundary between the two is a
warm front as marked in red.
• On the southwest side of the low
center, dense cold air advancing
from the north lifted the warm air,
forming a cold front marked in
blue.
• The similarity between battling
i d h b l f h d i
air and the battles fought during
World War I inspired the use of
the word front to describe the
battle lines of advancing and
retreating air masses. The figure
at the right shows the Western
Front in Belgium in 1918.
Components of Fronts
• Blue - Cold Front, usually dry air, fast moving.
• Red - Warm Front, usually wet air, slow moving.
Front,
• Purple - Occluded Front Cold Front overrunning
a Warm Front.
7
10/7/2008
Cold Fronts
Clouds associated with Cold Fronts
• 1st: Ci – Cirrus Clouds
• 2nd Cs - Cirrostratus Clouds
• Finally: Cb – Cumulonimbus Clouds (thunderstorms)
• After cold front passes Cu – Cumulus Clouds
8
10/7/2008
• A closer look at the surface weather associated with the cold front
situated in the southeastern United States
• Dark green-shaded area represents rain;
• White-shaded area represents snow.)
Summary of weather conditions
associated with Cold Fronts
9
10/7/2008
• Warm air overrides Warm Fronts
cold air near
surface.
• Approach of front
more gradual than
a cold front.
• Warm fronts move
slower than cold
fronts.
• Possibilities for
severe weather not
as likely with warm
fronts as with cold
fronts.
10
10/7/2008
• Surface weather
associated with a
typical warm front.
• Green-shaded area
rain
represents rain,
• Pink-shaded area
represents freezing
rain and sleet;
• White-shaded area
represents snow.)
Clouds associated with Warm Fronts
• 1st: Ci – Cirrus Clouds
• 2nd Cs - Cirrostratus Clouds
• 3rd As & St – Altostratus and Stratus Clouds
• Finally: Sc or Cb – StratoCumulus or
possibly Cumulonimbus Clouds
11
10/7/2008
Summary of weather conditions
associated with Warm Fronts
Cold Front March 19 – 20, 2004
12
10/7/2008
Development of Thunderstorm ahead of
cold Front 3/20/04
Weather charts and Dew Points (DP)
• Ahead of cold front: Air warm and wet: High DP.
• Behind cold front: Air dry, cool: Low DP
• Dew point: measure of amount of moisture in
atmosphere.
13
10/7/2008
Occluded Fronts
• Cold Fronts move faster than Warm Fronts.
• A Cold Front that overruns a Warm Front is an
Occluded Front.
14
10/7/2008
Review of weather chart
information including fronts
Four types of thunderstorms
• Single Cell
• Multi Cell
• Squall line Cells
• Super Cell
15
10/7/2008
Single Cell storms
• Single cell storms form when the
atmosphere is unstable. Since cold
,
air is denser than warm air, the
atmosphere is stable when cold air is
at the surface and warm air is aloft.
• However, during the summer when
the hot Sun heats the ground or a
cold front comes from the north the
atmosphere becomes unstable and a
develop
thunderstorm may develop.
• A single cell storm is usually short
lived, lasting for less than an hour
after becoming strong enough to
produce lightning.
Squall line
• Squall line storms are formed
as an organized line or lines of
multicell storms often with a
t f t Th ft f
gust front. They often form
from convective updrafts
generated by strong cold fronts
such as the cold front shown in
the figure at the right.
• The squall line is propelled by
its own outflow, which
reinforces continuous
development of updrafts along
the leading edge. Heavy rain
and hail may occur in squall
lines. Damaging winds can also
occur.
16
10/7/2008
The freezing level in the three stages
of a thunderstorm
• Ice particles carried aloft are charged positively which can produce
lightning.
• Hail is produced when rain drops get caught in the updrafts and then
downdrafts.
dow d a ts.
Multicell
• Multicell storms are groups of
cells in different stages of
p
development which have merged g
into a larger system.
• The cloud becomes divided into
updraft and downdraft regions
separated by a gust front. The
gust front may extend for several
miles ahead of the storm,
bringing with it increases in wind
speed and atmospheric pressure,
decreases in temperature, and
shifts in wind direction.
17
10/7/2008
Super Cell
• Supercell storms are large, severe steady-state
storms which form when the wind changes speed
("wind shear )
or direction with height ( wind shear") producing a
separate downdraft from the updraft (i.e.,
precipitation is not falling down through the
updraft) and contain a strong, rotating updraft (a
"mesocyclone").
• These are the most damaging type of thunderstorm,
g g yp ,
and may produce heavy rain, hail, damaging
outflow winds and tornadoes.
Supercells
• In large storms strong updrafts of warm air and
strong downdrafts of cold air.
18
10/7/2008
Satellite images of the
development of a thunderstorm
The March 1993 Superstorm
19
10/7/2008
March 1993 superstorm
movement of low pressure with Fronts
March 1993 superstorm
20
10/7/2008
1. The origin of cP and cA air masses
that enter the United States is:
76%
Northern Sib i
• a. N th Siberia
• b. Northern Atlantic
Ocean
• c. Antarctica 14%
3% 7%
d.
• d Northern Canada
and Alaska
a
ia
an
t ic
..
r
d.
be
ce
rc
an
Si
O
a
nt
ic
a
n
ad
A
er
nt
c.
rth
n
tla
Ca
A
o
n
. N
rn
r
he
he
a
t
t
or
or
. N
. N
d
b
2. Compared to an mP air mass,
mT air is ____.
79%
• A. warmer and drier
• B. warmer and
moister
• C.colder and drier 17%
• D.colder and moister 3% 0%
r
r
r
er
r ie
ste
rie
st
d
d
oi
oi
nd
d
m
m
an
a
nd
nd
er
er
a
a
ld
m
er
er
.co
ar
m
ld
. w
.co
C
ar
A
. w
D
B
21
10/7/2008
3. The greatest contrast in both temperature
and moisture will occur along the boundary
separating which air masses?
a.
• a cP and cT in 52%
summer
• b. mP and mT in
winter
• c. cP and mT in 17% 17%
summer 7% 7%
• d. cA and mT in winter
• e. mP and mT in
er
er
er
er
r
te
m
m
m
summer
nt
in
m
m
m
wi
w
su
su
su
n
n
T i
in
T i
n
in
T i
T
m
m
cT
m
m
d
nd
d
an
nd
nd
an
a
P
A
a
a
P
P
. m
. c
cP
. c
. m
d
a
c.
b
e
4. Record breaking low temperatures
are associated with which air mass?
57%
• a. mT
• b. mP
• c. cP
25%
• d. cT
11%
7%
T
P
T
cP
. c
. m
. m
c.
d
a
b
22
10/7/2008
5. Lake-effect snows are best developed
around the Great Lakes during:
66%
• a. early spring when moist,
tropical air moves over the
frozen lakes
• b. late fall and early winter
when cold, dry polar air moves
over the relatively warm water
• c. late fall and early winter 14% 10% 10%
when moist, polar air sweeps in
,p p
from the east
• d. middle winter when the
unseasonably warm air mass
...
. ..
...
...
oi
in
in
n
m
w
w
he
n
moves over the cold water
ly
rly
r w
he
r
ea
ea
g w
te
nd
nd
in
rin
w
l a
a
ll
e
sp
l
fa
fa
dl
r ly
id
te
te
. m
a
a
la
. e
. l
c.
d
b
a
6. Generally, the greatest lake effect snow fall
will be on the ____ shore of the Great Lakes.
72%
• a. northern
• b. southern
• c. eastern
• d.
d western 14%
10%
3%
n
rn
rn
n
er
er
te
he
st
rth
es
ut
ea
. w
o
o
. n
c.
. s
d
a
b
23
10/7/2008
7. The air mass with the highest
actual water vapor content is:
75%
• a. mT
• b. cT
• c. mP
• d. cP
14%
7%
4%
T
T
P
P
. c
. m
m
. c
b
d
c.
a
8. Which air mass forms over
North America only in summer?
57%
1. a. mT
2. b. cT
3. c. mP
4. d. cP
8%
18%
14%
11%
T
P
T
P
. c
. m
m
. c
b
d
c.
a
24
10/7/2008
9. On a weather map, the transition zone
between two air masses with sharply
contrasting properties is marked by:
89%
• a. the letter "H"
• b. the words "air
mass weather"
• c. a front
4% 7%
• d. the letter "L" 0%
nt
"
L"
"H
r "
fro
..
s .
r
tte
a
te
as
et
le
c.
m
e l
he
ir
h
a
. t
. t
s "
d
a
d
or
w
he
. t
b
10. Alternating lines of blue and red
on a surface weather chart indicate
• a. a cold front 82%
• b. a warm front
• c. a stationary front
• d. an occluded front
4% 7% 7%
t
nt
t
nt
on
on
o
f ro
fr
fr
r
y f
d
m
ld
r
de
ar
co
na
w
c lu
tio
. a
. a
oc
a
a
b
st
n
a
. a
c.
d
25
10/7/2008
11. Which of the following is not
correct concerning a cold front?
67%
1. a. a cold front
2. b. a warm front
3. c. a stationary front
19%
4. d. an occluded front 11%
4%
t
nt
t
nt
on
on
o
f ro
fr
fr
r
y f
d
m
ld
r
de
co
ar
na
w
c lu
tio
. a
. a
oc
a
a
b
st
n
a
. a
c.
d
12. Squall lines most often form
ahead of a:
• a. cold front
55%
• b. warm front
• c. cold-type occluded
front 24%
21%
• d. warm-type
l d df t
occluded front 0% 0%
• e. stationary front
t
nt
t
t
on
on
.. .
on
o
d
fr
fr
r
fr
y f
de
m
ld
d
r
o
de
ar
lu
na
. c
c
. w
clu
io
oc
a
at
b
c
pe
e o
t
. s
‐ty
p
e
ty
m
‐
ar
ld
. w
co
d
c.
26
10/7/2008
13. In winter, thunderstorms are
most likely to form along:
• a. cold fronts 81%
• b. warm fronts
• c. stationary fronts
• d. occluded fronts
15%
0% 4%
s
ts
s
s
nt
nt
nt
on
o
fro
ro
fr
fr
f
d
ld
m
ry
de
o
ar
na
. c
. w
lu
io
a
cc
at
b
. o
st
d
c.
14. Which of the following is not
correct concerning a warm front?
48%
• a. it is colored red on a
weather map
• B. it has warm air ahead 30%
(in advance) of it 22%
• c. in winter it is usually
associated with stratiform
clouds 0%
• d. it normally moves
.
...
..
(.
.
a.
more slowly than a cold
or
..
ad
a
m
ly
he
on
al
es
r a
su
d
ov
s u
ai
front
re
m
m
t i
ed
ly
ar
r i
or
al
w
te
m
ol
as
in
or
c
w
h
is
t n
. it
in
. it
. i
B
c.
a
d
27
10/7/2008
15. During the winter as you travel toward a
warm front, the most likely sequence of
weather you would experience is:
79%
• a. snow, f i
freezing
rain, hail, sleet
• b. rain, snow, sleet,
freezing rain
• c. freezing rain, 7% 7% 7%
snow, sleet, rain
i..
.
• d. snow, sleet,
...
e.
.
a
z..
a
le
h
g r
ee
, s
n,
zin
r
ow
i
, f
ra
ee
et
g
sn
freezing rain, rain
zin
le
r
, f
n,
, s
e
et
ai
re
ow
g r
le
, f
, s
n
zin
w
, s
ow
no
in
ee
n
. s
a
fr
. s
. r
a
c.
b
d
16. Occluded fronts may form as:
• a. a cold front
64%
t k f t
overtakes a warm front
• b. a warm front
overtakes a cold front
• c. a cold front 14% 14%
overtakes a squall line 7%
• d. overrunning occurs
s. .
s..
..
along a warm front
.. .
s .
ke
ke
a
ke
rta
rta
rs
rta
u
ve
ve
cc
ve
t o
g o
o
o
nt
on
in
nt
o
nn
fr
fr
fro
ru
m
ld
d
co
ar
r
l
ve
co
w
. a
. o
a
. a
a
d
c.
b
28