ATS 351, Spring 2010
Week 12, Lab #10
Winds, Fronts, & Mid-Latitude Cyclones – 61 points
Question #1: Drawing Forces using Vectors (21 points)
a) (4 points) Assuming geostrophic and frictionless flow, please draw vectors for
the pressure gradient and coriolis forces for the Northern Hemisphere.
b) (2 points) Are geostrophic winds a good approximation for surface winds?
Please explain why or why not.
No, geostrophic winds are not valid at the surface because isobars are rarely
straight there and wind is not at a constant speed. Also, friction has a strong
influence on the winds at the surface. Geostrophic wind is just the balance
between the PGF and Coriolis, with no effect of friction included.
c) (10 points) Given the pictures below of low and high pressure, please draw the
direction of the flow and the force vectors associated with areas of low and high
pressure. Please give a brief discussion on your picture. (Hint: you will only be
utilizing 3 out of the 4 forces at this point)
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d) (5 points) Please draw the vectors for PGF, Coriolis force, and friction that are
associated with surface wind.
L
1000 mb
PGF
Friction
Sfc
Cor
H
2
1004 mb
Question #2: Pressure Gradient Force and Wind Movement (13 points)
A sea-level pressure is shown below where isobars are drawn every 4mb (Northern
Hemisphere). Answer the following questions.
(a) (2 pts) Place a dashed line through the ridge and a solid line through the trough.
(shown below)
(b) (2 pts) What would be the wind direction at point A and at point B?
A: winds from the southeast
B: winds from the west
(c) (5 pts) Where would the stronger wind be blowing, at point A or B? Explain.
B: the pressure gradient is tighter at point B which indicates stronger winds.
(d) (4 pts) Compute the pressure gradient between points 1 and 2 and between points
3 and 4.
PG between 1 and 2: (1004-996mb)/1000km = 8mb/1000km = 0.008mb/km
PG between 3 and 4: (1012-1000mb)/1000km = 12mb/1000km =0.012mb/km
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Question #3: Air Masses (8 points)
What type of air mass (cP/cA, mP, mT, or cT) would be responsible for the
weather conditions listed as follows?
a) Hot and dry weather cT
b) Heavy snow showers and low temperatures at Buffalo, NY mP
c) Hot, muggy summer weather in the Midwest and the East mT
d) Record low winter temperatures in South Dakota cP
e) Daily afternoon thunderstorms along the Gulf Coast mT
f) Heavy snow showers along the western slope of the Rockies mP
g) Refreshing, dry breezes after a long summer hot spell on the Central Plains cP
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h) Warm and moist weather mT
i) Persistent cold, damp weather with drizzle along the East Coast mP
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Question #4: Mid-latitude Cyclones (10 points)
Describe the process of how a mid-latitude cyclone develops, becomes mature,
and then becomes occluded (drawing pictures will help explain). Which stage the storm
system is most intense? What makes the storm system die? How long do these systems
usually last?
(This is very detailed, do no expect as much from the students – pictures with some
explanation is good)
It begins as a stationary front that is part of the polar front. This sets up cyclonic
wind shear and a wavelike kink forms on the front (2nd figure). This figure shows a
newly formed wave with a cold front pushing southward and a warm front moving
northward. Steered by the winds aloft, the system typically moves east or northeastward
and gradually becomes a fully developed open wave in 12 to 24 hours (3rd figure). As the
open wave moves eastward, its central pressure continues to decrease and winds blow
more vigorously (4th figure). The faster-moving cold front eventually overtakes the
slower moving warm front and the system becomes occluded (5th figure). When an
occlusion forms, the storm is usually most intense (with clouds and precipitation covering
a large area). The center of the intense storm system gradually dissipates, because cold
air now lies on both sides of the occluded front. Without the supply of energy provided
by the rising warm, moist air, the old storm system dies out and gradually disappears (6th
figure). The entire life cycle of a wave cyclone can last from a few days to over a week.
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Question #5: Fronts (9 points)
Use the SLP map shown below to answer the following questions about the
pictured mid-latitude cyclone.
a) Using the isobar analysis (red lines, mb), does the center of the above cyclone have a
relatively high or low-pressure center compared to the surrounding environment?
Relatively low pressure
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b) A cold front is the boundary between the cold, dry cP air and warm, moist mT air.
There is also usually a clear wind shift (change in wind direction) and a significant (~
10°F) temperature change at this boundaryIf there is a cold front associated with the midlatitude cyclone, label this cold front on the above map using the appropriate symbol.
Above – cold front line is blue with triangles pointed towards warmer ai
c) A warm front is the boundary between the warm, moist mT air and the cooler,
moist mP air. There is usually a slight wind shift at this boundary as well. If there
is a warm front associated with the mid-latitude cyclone, label this warm front on
the above map using the appropriate symbol.
Above – warm front line is red with half circles pointed away from
warmer air.
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