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ATMO 201- Chapter 6.pdf

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Texas A&M University
Atmospheric Sciences
ATMO 201
Don Conlee

Chem 201 Monday, September 30, 2013 11:10 PM Fig 3 I. The Bergeron process (Fig 1) a. At a given subfreezing temperature, the saturation vapor pressure over water is greater than that over ice (more molecules, more vapor pressure) b. Along with Wegener and Findeisen, discovered that ice crystals grow to form precipitation because of the different saturation vapor pressures above supercooled water and ice Fig 1 II. Growth of ice crystals (fig 3) a. The ice crystal reaches its terminal velocity and starts to fall b. It can collide with supercooled water drops, which freeze to it – accretion or riming. It can collide with other ice crystals, and break apart, forming even more ice crystals c. It can collide with other ice crystals, which can stick to it – aggregation – this produces snowflakes Fig 2 d. As it gets near the ground, these might fall as snow, or melt and fall as rain Fig 4 III. Precipitation and cloud types (fig 2) a. Precipitation in cold convection clouds () starts quickly/ cc or Bergeron b. Starts out as ice/ less liquid water c. Warm Stratus just a draft: small drizzle IV. Growth Mechanicals for Ice Crystals (Fig 4, 5, 6 & 7) a. Cloud seeding: inject cloud with small particles that act as condensation nuclei, triggering the precipitation process b. NEED CLOUDS: seeding does not generate clouds c. Cold clouds with a low ratio of ice crystals to droplets best d. Man-made: Dry ice, silver iodide e. Tricky business – mixed results, Inadvertent seeding Fig 7:The streaks of falling Fig 5 Fig 6:Natural seeding by precipitation that evaporate cirrus clouds may form before reaching the ground V. Rain bands of precipitation a. Layered clouds (nimbus Straus) w/ less intense updrafts rain will fall at fairly steady rate; uniform raindrops are called virga (fall streaks a downwind of a mountain better name for cold clouds). b. Cumblounibus cloud, there may be no rain in the updraft portion. chain. Notice that heavy VI. Snow (Fig 8) snow is falling only in the a. Temp. and vapor content in the loud determines the types of snowflake that forms habit seeded areas. b. Habit: c. The temperature near the surface can also affect the type of snowflake we see at the ground: i. If it falls through warm air (near or just above freezing), it can start to melt, and the wet flakes can stick to each other to create “aggregates” – big fluffy flakes – good for snowballs ii. If it falls through cold (well below freezing), dry air, we get powder d. Snowfall is measured in two ways – depth of snow, and water equivalent (Fig 9) Fig 8 i. Wet snow will have 6 inches of snow per 1 inch of water Dentrites!!! ii. Dry snow will have 20 inches of snow per 1 inch of water iii. New, dry snow is a very good insulator: can actually protect plants from extreme cold a crystal or crystalline mass VII. Sleet, freezing rain, rime (fig 10 & 11) with a branching, treelike a. Sleet: snowflake melts as it falls, then freezes again before reaching the ground structure. i. Requires a deep subfreezing layer near the ground a natural treelike or mosslike b. Freezing rain: snowflake melts as it falls, remains melted, but freezes on contact when it hits the cold marking on a piece of rock or ground, trees, cars, power lines, etc.
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