Physics 137 - Introduction to the Atmosphere and Weather                                                                             Homework Key - Assignment #6
Revised Winter 2005

6-QR15. Which clouds are associated with each of the following characteristics: (a) lightning; (b) heavy rain showers; (c) mackerel sky; (d) mares’ tails; (e) halos; (f) light continuous rain or snow; (g) hailstones; (h) anvil top.

    The cloud characteristic-cloud type associations are as follows:
        (a) lightning - cumulonimbus,
        (b) heavy  rain showers - cumulonimbus,
        (c) mackerel sky - cirrocumulus,
        (d) mares’ tails - cirrus,
        (e) halos - cirrostratus,
        (f) light continuous rain or snow - nimbostratus or stratus,
        (g) hailstones - cumulonimbus,
        (h) anvil top - cumulonimbus.

6-QT2. Explain why icebergs are frequently surrounded by fog.

    The moist air over the ocean is nearly saturated. As it flows into the vicinity of an iceberg, the iceberg chills the air to its dew point, thus producing a fog.

6-QT4. While driving from cold air (well below freezing) into much warmer air (well above freezing), frost forms on the windshield of the car. Does the frost form on the inside or outside of the windshield? How can the frost form when the air is so warm?

    The frost forms on the outside of the windshield because the outside surface is at a temperature below freezing. This fact is what makes the formation of frost possible. Saturated warm air is chilled to below freezing upon contact with the cold glass. Deposition or condensation with freezing occurs.

6-QT10. Near the shore of an extremely large lake, explain why steam fog is more likely to form during the autumn and advection fog in early spring.

   In the autumn the cooling air is likely to be colder than the still warm waters of the lake. The air immediately adjacent to the water is nearly saturated and is warmed by the lake waters. As that nearly saturated warm air rises it mixes with and is cooled by the air above the lake. The mixture is saturated and steam fog is created.
    In the early spring advection carries warmer air from the surrounding land over the still cold waters of the lake. The warm air is chilled by the lake to a temperature below its dew point thus producing advection fog.

6-QT15. The sky is overcast and it is raining. Explain how you could tell if the cloud above you is a nimbostratus or a cumulonimbus.

    There are several clues that could be sought: (1) Large raindrops would indicate cumulonimbus; smaller drops, nimbostratus. (2) Intense, but variable rainfall would imply cumulonimbus; light or moderate, but steady rainfall, nimbostratus. (3) Strong, shifting surface winds would imply cumulonimbus; light steady surface winds or calmness would indicate nimbostratus. (4) A short period of rainfall would favor cumulonimbus; a persistent rainfall would favor nimbostratus. (5) Any accompanying lightning, thunder or hail would definitely imply cumulonimbus.

6-PE1. The data in the chart below represent the dew-point temperature and expected minimum temperature near the ground for various clear winter mornings in a southeastern city. Assume that the dew point remains constant throughout the night. Answer the following questions about the data. (a) On which morning would there be the greatest likelihood of observing visible frost? Explain why. (b) On which morning would frozen dew most likely form? Explain why. (c) On which morning would there be black frost with no sign of visible frost, dew, or frozen dew? Explain. (d) On which morning would you probably only observe dew on the ground. Explain why.

Morning 1 Morning 2 Morning 3 Morning 4 Morning 5
Dew-point Temperature 2C (35F) -7C (20F) 1C (34F) -4C (25F) 3C (38F)
Expected Minimum Temperature 4C (40F) -3C (27F) 0C (32F) 4.5C (24F) 2C (35F)

    (a) For visible frost to form the minimum temperature must be below the dew point and both must be below freezing. Only Morning 4
  satisfies all these requirements.
    (b) For frozen dew to form the dew point must be above freezing and the minimum must be at or below freezing. Only Morning 3
meets these requirements.
    (c) Black frost requires a minimum which is below freezing but above the dew point. Only Morning 2 meets these requirements.
    (d) Liquid dew requires a minimum which is above freezing but below the dew point. Only Morning 5 meets these requirements.