Metr 201 (502) Final Exam     Name: Solution

December 8, 2000

20 questions, equally weighted, 2 hours

 

1)      The plot below shows how temperature (red line) and dew point (green line) varied over Oakland, CA yesterday.  On this plot, temperature and dew point increase towards the right on the x (horizontal) axis.  The y (vertical) axis is just pressure, which you can think of as height since pressure decreases with height.  As a reference, a pressure of 500 mbar corresponds to a height of about 5 km.  At about what pressure is the tropopause (division between the troposphere and stratosphere) found?  (hint: Think of this as just one of the temperature profile plots used in your text to describe the different layers of the atmosphere).  Also, at about what pressure would you expect to find clouds?

 

 

 

 

 

Temperature decreases with height in the troposphere and increases with height in the stratosphere.  You probably remember that the division between these two layers is at about 11 km.  Since this problem gives you pressure rather than height, it forces you to use the temperature profile to determine where the tropopause is.  The reference height (5 km) was given just so you would know that the tropopause should be higher in the atmosphere than 500 mbar (assuming you remembered it was around 11 km).  From the figure, you can see that the transition from a positive lapse rate to a negative lapse rate occurs at about 220 mbar, and this is where the tropopause is.

The second part of the question simply required that you remember that clouds form when temperature is equal to dew point.  From the curves on the figure it is clear that this occurs at around 650 mbar.

 

2)      Now that we have made it to winter, you probably have a better appreciation for what radiational cooling is.  Explain how cloud cover and wind speed impact the temperature near the ground at night (i.e., how they affect radiational cooling).

 

Radiational cooling occurs because the Earth emits radiation more efficiently than the atmosphere, so it cools faster.  When this happens, the cold Earth cools down the air closest to the ground.  When clouds are present that absorb some of the radiation emitted by the Earth and re-radiate it back to the ground.  This keeps the ground warmer than it would be were the sky clear, which reduces radiational cooling.  Wind doesn’t influence the emission of radiation by the surface, but it does mix the cold layer of air nearest the ground with warmer air above, which limits the effect of radiational cooling.

 

3)      How is it that the water vapor pressure near the poles is much less than that near the equator, but the relative humidity is almost the same?

 

Relative humidity describes the amount of water in the air relative to the maximum amount the air can hold.  In other words, it describes how close the air is to saturation.  Since warm air can hold much more water vapor than cold air, the water vapor pressure near the cold poles can be much lower than at the warm equator and still result in the same relative humidity.

 

4)      The figure below was taken straight from your text.  Label each of the cloud types indicated by letters:

 

a) cirrocumulus

b) cirrostratus

c) cirrus

d) altocumulus

e) altostratus

f) nimbostratus

g) stratus

h) stratocumulus

i) fair weather cumulus

j) cumulonimbus

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


5)      Assume that the dew point of the air over College Station remains constant on a summer day.  The figure below shows how temperature varies during this summer day.  Add to this figure a curve that represents how you would expect relative humidity to change.

 

As discussed above, relative humidity describes the amount of water vapor in the air relative to the amount of water vapor the air can hold.  In this case, the amount of water vapor stays the same (since the dew point is constant), but the amount of water vapor the air can hold increases during the day as the temperature increases.  So the relative humidity will be highest at the coldest time of the day, and lowest at the warmest time of the day.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


6)      As an unsaturated air parcel rises in the atmosphere, how does the dew point of the air inside the parcel change?

 

As an unsaturated air parcel rises in the atmosphere both its pressure and temperature decrease.  Since dew point only describes how much water vapor is in the air, it only varies with temperature when the air is saturated, which is not the case here.  However, as the pressure of the air parcel changes, so does the water vapor pressure.  With a rising parcel this means that water vapor pressure decreases, which means that dew point also decreases.  You might recall that in general the rate of this decrease is about 2 oC/km, which is significantly lower than either the dry or moist adiabatic lapse rates. 

 

7)      Name four types of frozen precipitation, and for each indicate whether you would expect the air temperature near the ground to be above or below freezing.

 

The most common types and expected ground level temperature are:

 

            Snow – below freezing

            Freezing rain – below freezing

            Sleet – below freezing

            Hail – above freezing

 

8)      Why don’t winds close to the surface always move parallel to the isobars?

 

Far above the surface, winds move parallel to isobars since there is a balance between the pressure gradient force in one direction and the Coriolis force in the other.  Near the ground, friction also influences the wind, which acts to slow it down.  When this happens, the Coriolis force decreases since it is directly related to wind speed.  So now the pressure gradient force is larger than the Coriolis force, so the winds no longer move parallel to the isobars, but instead cross them at some angle that depends on how strong the frictional force is.

 

9)      Rank the following temperatures from coldest to hottest:  230 oF, 230 oC, 230 K

 

I intended these values to be obvious enough so that you wouldn’t have to calculate the values, but in case you did:

 

oC = K – 273  → 230 K = (230 – 273) oC = -43 oC

oC = (oF – 32)*5/9 → 230 oF = (230 – 32)*5/9 = 110 oC

(you probably could have gotten close to this by just remembering that water boils at 100 oC or 212 oF)

 

So –43 < 110 < 230 oC  or 230 K < 230 oF < 230 oC

 

10)  Say the atmospheric environmental lapse rate just before sunrise is exactly equal to the dry adiabatic lapse rate.  Then as the day goes on, the temperature of the air well above the surface stays the same, while the temperature of the air just above the ground heats up.  When this occurs, will the atmosphere be absolutely stable, absolutely unstable, or conditionally unstable?

 

If the environmental lapse rate is greater than the dry adiabatic lapse rate, the atmosphere is absolutely unstable.

 

If the environmental lapse rate is less than the moist adiabatic lapse rate, the atmosphere is absolutely stable.

 

If the environmental lapse rate is in between the dry adiabatic and moist adiabatic lapse rates, the atmosphere is conditionally unstable.

 

In this case, the environmental lapse rate was initially the same as the dry adiabatic lapse rate.  Then the lower part of the atmosphere was warmed while the upper part remained at the same temperature.  When this happens the rate of temperature change between the bottom of the atmosphere and the top (environmental lapse rate) has to increase.  So, regardless of how much the environmental lapse rate increases, it must be greater than the dry adiabatic lapse rate since that’s what it initially was.  Therefore, the atmosphere becomes absolutely unstable.

 

11)  The “heat-island” effect describes a thermal circulation pattern that results when the air temperature over a city is higher than surrounding areas.  When this type of circulation pattern occurs, would you expect to have convergence or divergence over the city?

 

Similar to the other thermal circulations we discussed such as sea breezes, the surface wind will blow from cool regions to warm regions (sea to land in the case of a sea breeze).  For this, the city is warm and the surrounding regions are cool, so the winds blow in towards the city.  Since these winds will come from all directions, they must converge over the city.  So the answer is “convergence”.

 

12)  When is the ocean temperature off the coast of Peru warmer – during an El Niño period or a La Niña period?  What causes the change in ocean temperature between the two?

 

During El Niño periods, prevailing winds over the equatorial Pacific switch direction, which “shuts off” the upwelling that typically occurs near the coast of Peru.  Since upwelling brings cold water to the surface, El Niño is associated with warmer than usual sea surface temperatures.

 

13)  On the outline of the Earth below, draw arrows showing the prevailing surface wind directions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


14)   If you are in a plane in the Northern hemisphere, flying towards the southeast, which direction is the Coriolis force “pushing” you?

 

In the Northern hemisphere, the Coriolis force always “pushes” towards the right.  From the diagram below, if you are traveling towards the southeast and are pushed to the right, you are being pushed towards the southwest.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


15)  Describe the temperature and moisture characteristics of each of the four types of air masses discussed in class and in the text.

 

Continental polar (cP) – Cold, dry

Continental tropical (cT) – Hot, dry

Maritime polar (mP) – Cool, moist

Maritime tropical (mT) – Warm, moist

 

16)  Draw a middle latitude cyclone as it would be shown on a weather map (i.e., show the fronts with arrows, semicircles,… and show where the low pressure region is) just after the cold front catches the warm front (i.e., just after the front becomes occluded).  Obviously you don’t have to worry about the red, blue, and purple colors usually used.

 

 

 

 

 

 

 

 

 

 

 

 

 


17)  Approximately how long do each of the following weather phenomena last?

 

·        Ordinary (air mass) thunderstorm  ~ 1 hr

 

·        Tornado  Few minutes to an hour

 

·        Hurricane  Few days to more than a week

 

·        Lightning   Less than a second

 

·        Middle latitude cyclone  Few days

 

·        Bermuda high  Always there

 

 

18)  If a tornado is moving towards the north, on which side (i.e., east, west, north, or south) would you expect to find the highest wind speed (hint:  think about which way the tornado is rotating).

 

The region where overall wind speed will be the greatest is where the winds due to rotation around the center of the tornado are in the same direction as the winds due to the general motion of the tornado itself (in this case towards the north).  In the figure below, the red arrows represent the wind speed and direction resulting from the rotation of the tornado and the green arrows represent the wind speed and rotation due to the overall movement of the system.  From this figure it is clear that the two winds add to one another on the east side of the tornado.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


19) 

S
 
Is air rising or sinking within the eye of a hurricane?  How about in the eye wall?

 

The reason the eye is clear is that air is sinking there.  On the other hand, the reason the heaviest precipitation is found in the eye wall is that the air is rising there.

 

20)  Discuss the two primary factors that lead to a storm surge (raised sea level) as a hurricane approaches land.

 

1.      The low pressure that causes rotation of the hurricane also acts almost like a straw by pulling up on the water below it.  This causes the sea level to rise about 1 cm per mbar of pressure drop (about a half meter for strong hurricanes and up to a meter for the most intense). 

 

2.      As a hurricane approaches the East coast of the U.S., the wind direction nearest land is southerly.  You might recall that the overall motion of the upper 100 or so meters of the ocean is in a direction 90o to the right of the surface wind direction.  Therefore, the water along the East coast is pushed towards the west, or towards land.  This causes the water to “pile up” along the shore, further raising sea level.  This situation is the same regardless of whether the hurricane is approaching the East coast, the West coast, or the Gulf coast.