Questions for Thought

1. Explain the reasoning behind the wintertime expression, “Clear moon, frost soon”? On clear, calm nights, objects and the earth surface cools off by emitting infrared radiation. This cooling effect is more rapid on clear night than cloudy nights since there is no clouds to emit back to the surface. Hence the phrase “clear moon, frost soon”. The cool surface is then mixed with air above it, which will eventually cool to the dew point and water vapor will appear on the cooled surfaces. If the temperature continues to fall in the freezing range then the dew will freeze and frost will form.

2. Explain why icebergs are frequently surrounded by fog? Fog surround icebergs due to advection fog. This type of fog is caused by the iceberg surface temperature being colder than the moist ocean air moving over the iceberg. As the warmer air moves over the iceberg heat transfer by conduction occurs and the air is cooled to its saturation point eventually forming fog. 3. During a summer visit to New Orleans, you stay in an air-conditioned motel.  One afternoon, you put on your sunglasses, step outside, and within no time your glasses are “fogged up”. Explain what has apparently caused this? New Orleans is a humid location and averages a dew point temperature of 72F degrees and contains a lot of water vapor. The air-conditioned room cools your glasses down near or below the outside dew point temperature. As you step outside into the humid, warm and moist air, water vapors condenses onto the glasses forming fog.

4. While driving from cold area (well below 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? When driving through the cold area the outside of the window will cool to below freezing. When the windshield is exposed to warm air, it is chilled to saturation point and produces frost through deposition pg116

5. Why are really clean atmosphere and really dirty atmosphere undersirable? Dirty atmosphere have an abundance of nuclei often leads to thicker fogs than clean atmosphere there by reducing visibility. If fog is too thick then the sun cannot penetrate to heat the surface. Additionally, dirty atmosphere can led to acid fog with impurities such as oxides of sulfur and nitrogen. This is harmful to people health. A really clean atmosphere can also be a nuisance because the condensation process needs airborne particles for water vapors to condense on or condensation nuclei. Without it condensation would not exist and we would not have cloud droplets.

6. Why do relative humidities seldom reach 100 percent in polluted air? When the air is polluted, water vapors condenses upon hygroscopic particles when the relative humidity is lower than 100%. Hence condensation can take place in the atmosphere even when the relative humidity is about 75 percent. So the haze or fog is formed before the relative humidity has chance to ever reach 100%. Pg118

7. Why are advection fogs rare over tropical water? The surface of tropical water is not sufficiently cooler than the air above the water hence advection fog is rare pg120

8. A January snowfall covers central Arkansas with 5 inches of snow. The following day, a south wind brings heavy fog to this region. Explain what has apparently happened. The 5 inches of snow cooled the surface in central Arkansas. As the warmer moist south wind moves over the progressively colder surface. The air is cool to its saturation point and fog is formed by advection fog pg121

9. If all fog droplets gradually settle earthward, explain how fog can last (without disappearing) for many days at a time. If the fog is too thick then the sunlight is blocked and the fog may not dissipate. This is common during the day in the central valley of California, where a strong inversion also slows the dissipation by keeping the warm air above the fog. At night, radiation cooling take effect to cool the air temperature and fog forms again. This process can last for days and even weeks. Pg120

10. 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? During autumn in the northern hemisphere it is safe to say the air is likely to be cold while large body of water such as lake are slow to emit their heat away. When the cold air moves over the warmer water steam fog is produced. In early spring, if there is significantly cooler difference between the cold surface and the air above the cold surface then the warm air is chilled and advection fog is created. Pg120-122

11. The air temperature during the night cools to the dew point in a deep layer, producing fog. Before the fog formed, the air temperature cooled each hour about 2 C. After the fog, the air temperature cooled by only 0.5 C each hour. Give two reasons why the air cooled more slowly after the fog formed? During the night the air is cooled more slowly because the fog traps the infrared energy that is emitted from the earth and secondly, latent heat is released as fog is formed.pg 124

12. On a winter night, the air temperature cooled to the dew point and fog formed. Before the formation of fog, the dew point remained almost constant. After the fog formed, the dew point began to decrease. Explain why? Fog is made of tiny liquid droplet that forms near the ground and Dewpoint is the temperature which the air must be cooled for saturation to occur. Once the fog stops forming the amount of water vapor will begin to decrease which will also decrease dewpoint.

13. Why can you see your breath on a cold morning? Does the air temperature have to be below freezing for this to occur? Moist air from your mouth meets the cold air outside and mixes with it. The mixture becomes saturated and a tiny cloud is formed with exhaled breath. . The air temperature does not have to be below freezing for this to occur, just close to saturation

14. Explain why altocumulus clouds might be observed at 6400 (21,000 ft) above the surface in Mexico City, Mexico, but never at that altitude above Fairbanks, Alaska? Large temperature changes affect at different locations affect which clouds are observed. In tropical region, the air temperature is low enough to freeze all liquid above 6000M however, further up north the same temperature may exist at a lower altitude pg 127

15. The sky is overcast and it is raining. Explain how you could tell if the cloud above you is a nimbostratus or a cumulonimbus? Pg 129/131 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

16. Suppose it is raining lightly from a deck of nimbostratus clouds. Beneath the clouds are small, ragged, puffy clouds that are moving rapidly with the wind. What would you call these clouds? How did they probably form? Stratus clouds are ragged appearing clouds that are beneath the deck of nimbostratus. They are usually form when a thick fog lifts. Pg 130

17. You are sitting inside your house on a sunny afternoon. The shades are drawn and you look out the window and notice the sun disappears for about 10 seconds. The alternate light and dark period lasts for nearly 30 minutes. Are the clouds passing in front of the sun cirrocumulus, altocumulus, stratocumulus or cumulus? Give reasonable explanation for your answer? Since the sun is disappearing rapidly at 10 seconds intervals it is safe to assume the cloud is from the low group family of clouds. Cirrocumulus and altocumulus belong to the high and middle group respectively. Stratocumulus pg 130-131 The higher the cloud, the slower it appears to move. If the sun disappears for about 10 seconds to reappear, then the cloud is certainly low. That would exclude the cirrocumulus and altocumulus that are both high clouds, the former made of ice crystal. If it was a stratocumulus, it would be a constant veil since the stratus is a layer resulting from an inversion.

That leaves the cumulus that is the nice puffy cauliflower-like cloud that, if low enough, could appear to move fast, interrupting the sun on and off, as seen throguh the shades of a window.

Exercises;
1. The data in table 5.6 below represents the dew point temperature and expected minimum temperature near the ground for various clear winter morning in a southeastern city. Online 2. If a ceiling balloon rises at 120m (about 400ft) each minute, what is the ceiling of an overcast deck of stratus clouds 1500m (about 5000ft) thick if the balloon disappears into the clouds in 5 minutes? 2000 ft

he ceiling is defined as the base of a cloud deck. If the balloon rises 400 feet every minute and it takes 5 minutes to reach the cloud deck, then 400ft/min x 5 min = 2000 feet. If I am understanding the question properly it is immaterial that the cloud deck is 5000 feet thick