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Atmospheric Science 101 Midterm #2
May 20, 2002
100 Total Points
1. Geostrophic Wind
The figure below represents a common pattern for the 500 mb surface height in the Northern
A. Mark with an "X" the location where the wind is strongest (4).
B. Draw double arrows (==>) for wind velocity at locations A and B (remember that the length
of the arrows must be proportional to the speed of the wind.) (4)
C. Draw and label single arrows ( ) for the forces acting at location A. (4)
2. Rain Shadow
Orographic lifting results in precipitation on the windward side of mountain ranges.
A. Air flowing towards the mountain is forced up and over the mountain. The air is cloud free
at the start of the ascent, but somewhere on the way to the top a cloud forms. Describe how
the temperature of the air changes going up the windward side. (5)
The unsaturated air cools at the dry adiabatic lapse rate as it rises up the windward side of the
mountain, until it cools to its dew point temperature. At this point the air reaches saturation
(100% relative humidity). As the air continues to rise, it now cools at the moist adiabatic lapse
rate because water vapor is condensing and releasing its latent heat.
B. If it rains on the way up the windward side of the mountain, how does this affect the
temperature of the air as it passes down the leeward side? How does the temperature at the
base on the leeward side compare to the temperature at the base of the windward side?
Explain. (7)
On the way down, the air is warmed by compression. If all of the liquid is rained out of the
cloud, the air will warm at the dry adiabatic rate all the way down the leeward side. Therefore it
will be warmer at the base on the leeward side than it was at the base on the windward side.
3. Coriolis Force.
On the diagram of the earth shown below, rockets B and G are flying due west, and all other
rockets are flying due east. Each rocket is flying at the speed written above the arrow showing its
direction of flight.
A. On the diagram, draw an arrow for each rocket indicating the direction of the Coriolis force
on the rocket (or no arrow if there is no Coriolis force). (3)
B. Of rockets G, E, and C, which is experiencing the strongest Coriolis force? Explain. (4)
G. It is at the highest latitude and CF = 2Ω(sinφ)*v, where φ is latitude.
C. Of rockets D, E, and F, which is experiencing the strongest Coriolis force? Explain. (4)
F. It is moving the fastest, and all three are at the same latitude. CF = 2Ω(sinφ)*v, where v is
4. Sea Breezes
A. Consider a coastal region at dawn on a summer day. At this time, the surface pressure is the
same over the land and the ocean. Think about how the pressures will change throughout the
day. For a time in the mid-afternoon, draw the High and Low pressure centers aloft and at
the surface and how they relate to the resulting horizontal and vertical circulation aloft and
at the surface. Explain how this pressure/circulation scenario develops, and how the areas of
High and Low pressure are related to the circulation. (12)
As the land heats up faster than the water during the day, the atmosphere above the land will be
warmer. By the ideal gas law, we know that temperature is inversely proportional to density – a
warmer atmosphere will expand and take up more space.
Horizontal pressure gradients determine winds. At a given height in the atmosphere, the
pressure will be higher over the land. Aloft, the wind will be off-shore. Mass is being moved
from the land to the water. Pressure is the weight of the atmosphere above you. The water has
more atmosphere above it than the land, so the surface pressure will be high on the water and
low on the land. The resulting surface wind is on-shore.
Air will rise over the land as the atmosphere expands. Air will sink over the water to complete
the circulation cell.
This circulation is the same as for the sea breeze when we discussed it earlier in the quarter.
This time, however, we were looking for a more sophisticated answer.
4. Precipitation
A. Briefly describe how cloud droplets (< 20µm) form from water vapor. (7)
Water vapor condenses onto Cloud Condensation Nuclei (CCN), small particles which are
attractive to vapor. This allows condensation to occur at lower vapor pressures than otherwise.
Larger droplets grow at the expense of smaller droplets due to flatter surfaces (i.e., larger
drops) requiring a lower vapor pressure to be in equilibrium than more curved surfaces (i.e.,
smaller drops), a process known as the curvature effect.
B. What types of clouds will produce larger raindrops? State what droplet growth mechanism
plays a role and how it is related to your answer. Explain your answer. (5)
Deep clouds with strong updrafts (i.e., cumulonimbus) will produce larger droplets due to the
collision and coalescence mechanism. Large droplets fall at a faster rate than smaller ones. As
the body of drops descends, larger ones will overtake (collide) and absorb (coalesce) with the
smaller ones. If a cloud is deep, then the drops will fall a longer distance surrounded by
droplets, and if the cloud has strong updrafts, the drops may be swept back up to fall through
the cloud repeatedly.
5. Clouds (9)
Low clouds that have a large horizontal extent are called STRATUS. CIRRUS clouds are high
clouds made of ice crystals. CUMULONIMBUS clouds have large vertical extent and can
produce severe weather such as hail and tornadoes. The small, light, fluffy kinds of clouds seen
on sunny days and that children like to draw are called CUMULUS. On a typical Seattle Winter
day, the clouds are wide, featureless and produce drizzle. This type of cloud is called
NIMBOSTRATUS, and is usually found (LOW / mid-level / high) in the troposphere. Cap
clouds are found over MOUNTAIN TOPS. You would expect the winds to be (STRONG /
weak) underneath a cap cloud. The presence of stratus denotes that the atmosphere is (MORE /
less) stable than if cumulus clouds were present.
6. Stability
The graph below shows a simplified but typical temperature profile on the British Columbia
coast in February. You can assume that the dry adiabatic lapse rate is –10°C/km, and that the
moist adiabatic lapse rate is –6°C/km. (11)
-21°C at 4 km
Layer B
2-4 km
Height (km)
-17°C at 2 km
Layer A
0-2 km
-1°C at 0 km
Temperature (°C)
A. In layer A (0-2 km), the profile is (circle one):
absolutely stable
conditionally stable
absolutely unstable
B. In layer B (2-4 km), the profile is (circle one):
absolutely stable
conditionally stable
absolutely unstable
C. Which type of cloud would most likely occur in layer A? Circle one and explain your
.Cumulus: if RH is high enough at the surface, the air will rise and cool at the dry adiabatic
lapse rate until it reaches its dew point, then it will rise and cool at the moist adiabatic lapse
rate, which will cool the parcel more slowly than the surrounding environment cools. If the
temperature of the parcel gets warmer than the temperature of the environment, a cumulus cloud
can form.
Stratus could form if the air is not lifted strongly (as in cumulus), and the relative humidity is
high enough at the surface.
MULTIPLE CHOICE Indicate the single best answer. There are 7 questions, each worth 3 points.
1. Under normal atmospheric conditions, when you point to a star on the horizon, the actual
position of the star is
a. slightly higher.
b. slightly lower.
c. slightly to the left.
d. exactly where you see it.
2. During the Bergeron process (cold clouds) of precipitation formation,
a. only ice crystals need be present in a cloud.
b. the cloud must be cumuliform.
c. the cloud must be stratus.
d. ice crystals grow larger at the expense of the surrounding liquid cloud droplets.
3. If the atmosphere is in hydrostatic balance, then this means that the weight of the atmosphere due
to gravity is balanced by
a. solar heating at the ground.
b. the vertical pressure gradient.
c. the Coriolis force.
d. the geostrophic wind.
4. An unsaturated parcel of air moves over a body of water and water evaporates into the air parcel.
The temperature of the air remains the same. The dew point temperature of the air parcel will
__________, the mixing ratio will __________, and the relative humidity will __________.
a. increase, increase, increase
b. decrease, decrease, increase
c. increase, decrease, increase
d. decrease, decrease, decrease
5. If the air molecules scattered all wavelengths of visible light equally, the mid-day sky would be
a. black.
b. white.
c. dark blue.
d. green.
6. Radiation fog is due to
a. cold dry air moving over warmer water.
b. warm moist air moving over colder water.
c. nighttime infrared cooling of moist air at the surface.
d. warm moist air moving over cold land.
7. On a typical day, the relative humidity will be highest at
a. noon.
b. midnight.
c. sunrise.
d. sunset.
Extra Credit (4 points)
During Ernest Shackleton’s last expedition to Antarctica, on May 8, 1915, seven days after the sun
had set for the winter, he saw the sun reappear. Explain how this event – called the Novaya Zemlya
Effect – can occur.
Shackelton saw the sun rise seven days after it set because of refraction. Refraction causes the path
of the incoming sun to be bent towards the dense air. In this case, the sun had set for the winter
(because the Earth's rotation about the sun, plus tilt of rotation axis about the solar plane).
However, weather conditions and the lack of sunlight days later must have created a cold enough
air mass that, seven days after the sun set, refraction had become great enough to bend the light
over the true horizon to Ernest's ship.
Extra Credit (2 points)
When you flush your toilet, do you expect the Coriolis force to be a significant factor in determining
the direction of the water as it drains? Explain.
The time it takes for water to drain from a toilet is measured in seconds. The time it takes for the
Earth's rotational effects to become significant is many hours (on the order of a day). Hence,
rotational effects (Coriolis Effects) are not significant to understand how water drains from a toilet.
[If you had a toilet the size of the Kingdome and a drain the size of basketball, you would probably
have to deal with rotation.]