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ATS 351, Spring 2010
Lab #10
Winds, Fronts, and Cyclogenesis - 60 points
Please show your work for calculations.
Question #1: Pressure
(5 points) Using the ideal gas law, please calculate what is referred to as the “standard” sea level
pressure of the Earth. Values needed: average global temperature = 288K (15C); constant (R) =
287 J/kg/K; average sea level air density = 1.226 kg/m3. Please give pressure in millibars (what
you calculate will be in Pascals).
Ideal Gas Law: p = RT
p = 1.226 kg/m3 * 287 J/kg/K * 288 K
p = 101,336.26 Pa
p = 101,336.26 Pa * (1 mb/ 100 Pa)
p = 1013.36 mb
Question #2: Coriolis Force
a) (10 points) The equation for the Coriolis Force is:
Coriolis = 2 Ω sin (φ)
where Ω = the Earth’s angular rate of spin, 7.27x10-5 s-1
and φ = latitude in radians (To convert from degrees to radians, simply multiply your
degree by П and then divide by 180...and make sure your calculator mode is set to radians!)
Using the equation above, calculate the magnitude of the Coriolis force at:
a) the equator
b) the North Pole
c) Fort Collins (latitude ~ 40°)
Using you answers, please explain how the Coriolis Force changes with latitude.
a = 0 = 0.0 radians; b = 90 = 1.57 radians, c = 40 = 0.70 radians,
CFa= 2 * 7.27x10-5 s-1 * sin(0.0 rads)
CFa= 0.0 s-1
CFb = 2 * 7.27x10-5 s-1 * sin(1.5 rads)
CFb = 1.45x10-4 s-1
CFc = 2 * 7.27x10-5 s-1* sin(0.70 rads)
CFc = 9.37x10-5 s-1
The Coriolis force increases as you move further away from the equator. It is greatest at the poles
and zero at the equator.
Question #3: Drawing Forces using Vectors
a) (5 points) Assuming geostrophic and frictionless flow, please draw vectors for the pressure
gradient and coriolis forces for the Northern Hemisphere.
L
1000 mb
PGF
1004 mb
Coriolis
H
1008 mb
b) (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)
LPGF
Cor
H
Cor
PGF
Low pressure: the PGF vector is pointing towards the center of the low and the Coriolis
force is directed to the right of the wind movement. Since centripetal force is pulling inward,
the net force is going to be directed inwards towards the low and the PGF vector is
therefore larger.
High pressure: the PGF vector is pointing from high to low pressure (outward) while the
Coriolis force is directed to the right of the wind movement (inward). Since centripetal force
is again pulling inward, the net force will be towards the center of the high and the Coriolis
vector is larger in this case.
Question #4: Pressure Gradient Force and Wind Movement
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.
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
Question #5: 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). During 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.
Question #6: Fronts
(7 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
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 boundary. Label the cold front on the above map using the appropriate symbol.
Above – cold front line is blue with triangles pointed towards warmer air
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. Label the warm front
on the above map using the appropriate symbol.
Above – warm front line is red with half circles pointed away from warmer air.