GPH- 100P-02

Ahrens Extreme Weather and Climate
Ch. 7
Q’s 1, 2, 4, 10, 13, 16, 19, 20

1. What is the primary difference between a cloud droplet and a raindrop?
A typical cloud droplet is 100 times smaller than a raindrop. Clouds are composed of many small droplets.

2. Describe how the process of collision and coalescence produces precipitation.
To produce the many collisions necessary to form a raindrop, some cloud droplets must be larger than others. Larger drops may form on large condensation nuclei, such as salt particles, or through random collisions of droplets. The amount of time a droplet spends in a cloud, the larger it will be. In warm clouds (above freezing at all levels) strong updrafts slow the rain drops down, which allows it to grow larger because it spends more time in the cloud. As the droplet grows to 1 mm, the pull of gravity suspends the particle against the updraft. It will eventually collide with other small droplets begin to descend slowly, picking up size and speed. As it falls faster it reaches a size of 5mm at which point it is a rain drop.

4. How does the ice-crystal (Bergeron) process produce precipitation? What is the main premise behind this process?
The Bergeron Process proposes that ice crystals and liquid cloud droplets must co-exist in clouds at temperatures below freezing. This process of rain formation is extremely important in middle and high latitudes, where clouds are able to extend upwards into regions where air temperatures are below freezing. These are known as cold clouds. Above the freezing level, liquid water droplets are known as supercooled liquid droplets. The reason behind this is the lack of ice nuclei, which typically consist of clay minerals and bacteria that share similar geometry to an ice crystal. In the subfreezing air of a cloud, many supercooled liquid droplets will surround each ice crystal. Both the liquid droplets and the ice crystals are in equilibrium, meaning that the number of molecules leaving the surface of both the droplet and the ice crystal must equal the number of molecules returning. This difference in vapor pressure causes water vapor molecules to move (diffuse) from the droplet toward the ice crystal. The removal of vapor molecules reduces the vapor pressure above the droplet. Since the droplet is now out of equilibrium with its surroundings, it evaporates to replenish the diminished supply of water vapor above it. This process provides a continuous source of moisture for the ice crystal, which absorbs the water vapor and grows rapidly. The ice crystal then grows even larger by colliding with supercooled liquid droplets, at which point the liquid droplets stick to the ice crystal. This is called accretion. The icy matter that forms is called graupel, which falls, splinters and forms ice crystals that act as seeds. These crystals then form snowflakes, which are an aggregate of crystals, which may melt before it reaches the ground.

10. Why is it never too cold to snow? no matter how cold the air becomes, it always contains some water vapor that could produce snow. In fact, tiny ice crystals have been observed falling at temperatures as low as 53F. We air with “no snow” because the coldest winter weather occurs on clear, calm nights—conditions that normally prevail with strong high pressure areas that have few if any clouds.

13. What is the difference between freezing rain and sleet?
Freezing rain is a supercooled liquid drop that freezes upon impact. Sleet is a snowflake that partially melts then re-freezes and is frozen before it reaches the surface.

16. Describe how icing can be hazardous to aircraft. icing reduces aircraft efficiency by increasing weight. Icing has other adverse effects, depending on where it forms. On a wing or fuselage, ice can disrupt the air flow and decrease the plane’s flying capability. When ice forms in the air intake of the engine, it robs the engine of air, causing a reduction in power. Icing may also affect the operation of brakes, landing gear, and instruments.

19. How do the atmospheric conditions that produce sleet differ from those that produce hail?
Hail typically occurs in the summer, in large thunder clouds that cycle supercooled water droplets for 5-10 minutes. Sleet, typically occurs during winter time, and involves a falling snowflake, that melts and refreezes before hitting earths surface.

20. Where are the wettest and driest regions of the world located?
Mawsynram, on the crest of the southern slopes of the Khasi Hills in northeastern India, is considered the wettest place in the world as it receives an average of 467 inches of rainfall each year. Arica in northern Chile holds the world record for lowest annual rainfall—0.03 inches.