METR201 Final exam study guide solution

Chapter 1: The Earth and its Atmosphere

1) Arrange the following gases in order of their relative abundance in the atmosphere (highest concentration first).

Carbon dioxide (CO₂), oxygen (O₂), argon (Ar), nitrogen (N₂)

N₂ > O₂ > Ar > CO₂

The amount of water vapor in the air varies significantly, but approximately where would you expect it to fall within that list?

Depending on where and when, water vapor makes up between about 0 and 4% of the air (by volume, or equivalently partial pressure). That puts it either just above argon or just below (either would have been acceptable were this really on the exam).

2) Why do we often refer to ozone (O₃) as either “good” ozone or “bad” ozone depending on what altitude it is present at? In what layer of the atmosphere is ozone considered “good”, and in what layer is it considered “bad”.

Ozone is the air pollutant many cities have had the most trouble controlling during the past few decades. It is a respiratory irritant, and is the main reason your eyes burn when you’re out on a polluted day. So, ozone near the surface (in the troposphere) is “bad”. On the other hand, ozone absorbs ultraviolet radiation, which would be harmful if it reached the Earth’s surface. Since there is much more ozone in the stratosphere than there is in the troposphere, that is where most of the ultraviolet radiation is absorbed. So, ozone up in the stratosphere is “good”.

Chapter 2: Energy: Warming the Earth and the Atmosphere

1) Fill in the blanks in the following statement with the following choices – X-ray, ultraviolet, visible, infrared, microwave, radio wave: The Earth primarily emits infrared radiation, while the sun primarily emits visible radiation.

2) Explain why the concentration of greenhouse gases in the atmosphere is important, and give three examples of important greenhouse gases.

Greenhouse gases are important since they absorb infrared radiation. Since the Earth transfers energy back into space by emitting infrared radiation, greenhouse gases essentially trap this energy in, leading to warming of the Earth / atmosphere. Water vapor is a greenhouse gas, although usually we hear about others like carbon dioxide, nitrous oxide, and methane since they are increasing in concentration.

Chapter 3: Seasonal and Daily Temperatures

1) In College Station, the shortest day of the year (fewest hours of sunlight) occurs around December 21. Why is this the shortest day of the year?

As the Earth orbits the sun, its axis is tilted relative to the plane of its orbit. On December 21 (winter solstice), that tilt causes the Northern hemisphere to be pointed away from the sun more than on any other day of the year. On this day, there are 12 hours of sunlight at the equator, zero above 66 ½ ^oN, and somewhere in between these two extremes everywhere else.

2) The figure below shows how solar insolation (sunlight intensity) varies during a typical day (around this time of year in College Station anyway). Draw on this figure how you would expect temperature to vary (no numbers are necessary) during the day.

Temperature is represented by the dashed line.

Chapter 5: Atmospheric Moisture

1) Describe in words what water vapor pressure is.

The pressure exerted by only the water molecules in the air. It is a useful way of describing how much water there is in the air, and is the numerator when calculating relative humidity.

2) Consider an air parcel with the following properties:

Temperature = 30 ^oC

Dew point = 21 ^oC

Air pressure = 974 mbar

Condensation nuclei concentration = 1000 per cm³

If this parcel remained at the same altitude (and therefore pressure), and began to cool, at what temperature would the parcel saturate, leading to formation of cloud droplets?

The only thing you need to know is the dew point. The parcel will saturate when its temperature reaches the dew point, or at 21 ^oC.

Chapter 6: Condensation: Dew, Fog, and Clouds

1) Why does radiation fog often form only within the lowest few meters of the atmosphere?

Since the ground emits radiation more efficiently than the air, it cools faster, which cools the lowest part of the atmosphere more than the air only a few meters above ground. If this lower layer of air gets cold enough, water may condense and create fog droplets.

2) Stratus, nimbostratus, towering cumulus, and cumulonimbus clouds all produce precipitation. Arrange these cloud types in order of the precipitation rate you would expect from each.

Stratus clouds usually produce drizzle, while cumulonimbus clouds produce heavy downpours during thunderstorms. Nimbostratus clouds usually produce steady precipitation, which can vary in intensity, while towering cumulus clouds often produce showers. So, the lightest precipitation will come from stratus clouds, the heaviest from cumulonimubus clouds, and nimbostratus and towering cumulus fall in between these extremes (either order would receive full credit were this on the exam).

Chapter 7: Stability and Cloud Development

1) Say you take a ski lift from the bottom of a mountain up to the top. The thermometer at the bottom station says it is 2 ^oC, and the thermometer at the top station says it is –15 ^oC (15 below zero). If the base elevation is 2500 m and the summit elevation is 3500 m, is the atmosphere in between stable, unstable, or conditionally unstable?

The atmosphere will always be stable if the environmental lapse rate is less than the moist adiabatic lapse rate, will always be unstable if the environmental lapse rate is greater than the dry adiabatic lapse rate, and will be conditionally unstable between these extremes. The lapse rate in this situation is -(2 – (-15) ^oC)/(3500 – 2500 m) or 17 ^oC/km. Since this lapse rate is greater than the dry adiabatic lapse rate, the atmosphere must be unstable.

2) If the atmosphere were unstable all the way up to the tropopause (division between the troposphere and stratosphere), would you expect to find fair-weather cumulus clouds, towering cumulus clouds, or cumulonimbus clouds?

In general, as long as the atmosphere is unstable, a rising parcel will continue to rise, so the cloud it creates would continue to build until reaching the tropopause. This type of cloud is, of course, called a cumulonimbus cloud.

Chapter 8: Precipitation

1) Name the two processes that produce precipitation-size droplets or crystals. Which process is responsible for most of the precipitation we have in College Station?

i) The collision-coalescence process, and ii) the ice-crystal or Bergeron process. One of the reasons I left this off the exam was that the text obviously doesn’t tell you what process is responsible for precipitation in College Station. From class, though, you probably remember that it is the ice-crystal process, even though the precipitation is rarely frozen when it reaches the ground.

2) What is the basic idea behind seeding a cloud in order to enhance precipitation?

If there are not enough ice nuclei within a cloud to initiate precipitation, adding more can help since these ice nuclei will become ice crystals and fall out as precipitation. However, it’s a tricky business since adding too many can actually inhibit precipitation.

Chapter 9: The Atmosphere in Motion: Air Pressure, Forces, and Winds

1) What is the underlying cause of the Coriolis force?

Rotation of the Earth.

2) For a geostrophic wind blowing in the upper atmosphere in the Northern Hemisphere, there is a force balance between the Coriolis force pulling it to the right, and the pressure gradient force pulling it to the left.

Chapter 10: Wind: Small-scale and Local Systems

1) If the air temperature above the ocean surface remains constant throughout the day, when would you expect a sea breeze to be strongest?

A sea breeze is driven by the difference in temperature between the sea surface and the land surface. The bigger the difference, the stronger the wind. Since you would expect the land surface to heat up during the day, until sometime around mid-afternoon, the sea-breeze would probably be strongest then (mid-afternoon).

2) In Los Angeles, a mountain range is located just inland from the coast as shown below. During the day, would you expect the thermal circulation winds caused by the land-sea interface to contribute to, or offset, the winds caused by the thermal circulation created between the flat coastal area and the mountains?

During the day, the sea breeze will blow from the ocean towards land. At the same time, a valley breeze will blow from the flat regions towards, and up, the heated slopes of the mountain range. Since this valley breeze is in the same direction as the sea breeze, they will contribute to one another.

Chapter 11: Wind: Global Systems

1) Name the two semipermanent highs that have the biggest influence on weather in the U.S., and describe which way the winds move around both of them.

The Pacific high remains over the Pacific ocean in the Northern hemisphere, and the Bermuda high remains over the Atlantic ocean in the Northern hemisphere. As with all highs, winds blow clockwise around both of these.

2) Give an example of how winds influence the ocean, and an example of how the ocean influences winds.

Winds are the driving force behind many of the oceanic circulations we looked at in class. Underneath most of the semipermanent highs and lows, we observe surface currents that move in a circular pattern similar to the winds above. The obvious example of how the ocean can influence winds is El Niño (which is actually caused by winds in the first place). The sea surface temperature in the equatorial Pacific influences winds and weather around the globe.

Chapter 12: Air Masses and Fronts

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

1. cP (continental polar): Cold and dry

2. mP (maritime polar): Cold and moist

3. cT (continental tropical): Warm and dry

4. mT (maritime tropical): Warm and moist

2) Fill in the blanks in the following sentence with either “moves over” or “slides under”. As a cold front passes, warm air moves over cold air, and as a warm front passes, warm air moves over cold air.

Chapter 13: Middle Latitude Cyclones

1) Upper-air support of a middle latitude cyclone simply describes divergence over the surface low and convergence over the surface high.

2) Explain how a baroclinic atmosphere (isotherms cross isobars) can lead to the sinking air / rising air circulation known as baroclinic instability.

When the atmosphere is baroclinic, temperature advection occurs as the winds blow from cold regions to warm ones, or vice-versa. When the winds bring cold air into warm regions, the cold air is denser than the surrounding warm air, so it sinks. Similarly, when winds bring warm air into cold regions, the warm air is lighter than the surrounding air, so it rises. A circulation pattern is created when air is sinking over one region and rising over another, and this condition is referred to as baroclinic instability.

Chapter 15: Thunderstorms and Tornadoes

1) What usually leads to the dissipation of an ordinary thunderstorm?

During the mature stage of a thunderstorm, increased mixing near the edges (entrainment) of the cumulonimbus cools down some of the air inside the cloud, causing it to sink. At the same time, falling precipitation essentially drags air with it, pushing the air down even faster. These two effects combine to create significant downdrafts, which eventually cut off the supply of warm moist air near the cloud base.

2) Place the following cloud types in order of when they would occur (form) in the sequence of steps leading up to formation of a thunderstorm and ultimately a tornado:

Wall cloud (4)

Cumulonimbus (3)

Tornado (6)

Fair weather cumulus (1)

Funnel Cloud (5)

Towering cumulus (2)

Chapter 16: Hurricanes

1) In which part of a hurricane is the wind speed and precipitation usually most intense? In the eye wall.

2) Why don’t many hurricanes reach the West coast of the United States?

First of all, prevailing winds leading from the regions where North Pacific hurricanes originate are Easterly, which pushes the developing storms towards the west, away from the West coast of the U.S. If a developing storm is pushed more towards the north on a trajectory that would reach the West coast, it usually encounters cold surface waters that cut off its moisture and energy supply before it can intensify.