Name____________________________________T.A./Section_________________ 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 Hemisphere. 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) PGF X CF 1 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. 2 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 = 2 (sin )*v, where 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 = 2 (sin )*v, where v is velocity. 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) L H Water H L Land 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. 3 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 (< 20 m) 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) 4 -21 C at 4 km 4 Height (km) 3 -17 C at 2 km Layer A 0-2 km Layer B 2-4 km 2 1 0 -20 -10 -1 C at 0 km 0 10 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 answer: or cumulus cirrus stratus .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. 5 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 ...