Geography Test 3 Review

Geography Test 3 Review - Test 3 Review Precipitation a...

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Test 3 Review Precipitation: a) Precipitation formation processes Collision-coalescence process (warm clouds): Collision: A large droplet (called the collector droplet) falls at a greater rate than the other droplets because of the laws of gravity. As the collector drop falls, it compresses the air in its path. The compressed air creates a small gust of wind that pushes the smaller droplets out of the way. The small gust of wind cannot push aside larger droplets, however, and the collector is able to collide with them. As a result, the collision efficiency is greater for the droplets that are not much smaller than the collector drops. MIDDLE SIZED IS GOOD. Too big are too fast, to small get pushed away. Coalescence: When a collector drop and a smaller drop collide, they can either combine to form a single, larger droplet or bounce apart. Most often the colliding droplets stick together (AKA COALESCENCE). The percentage of colliding droplets that join together is the coalescence efficiency. Most collisions result in coalescence. *Collision and coalescence together form the primary mechanism for precipitation in the Tropics, where warm clouds predominate. Ice Crystal/ Bergeron process (cool clouds): Unlike the clouds in the tropics, at least a portion of mid-latitude clouds have temperatures below the feezing point of ice. Cold Clouds have temperatures ABOVE 0ºC in the lower reaches and subfreezing conditions above. A well developed cumulus cloud might be composed entirely of water droplets in its lower portion, a combination of supercooled droplets and ice crystals in its middle section, and exclusively ice crystals in its upper reaches. Supercooled water droplets: Water below the freezing point but still in liquid form. *Just as condensation nuclei are needed for water to condense, ice nuclei are needed to freeze. Well, ice nuclei just aren’t that common, so there are a lot of liquid droplets that cannot freeze. THE COEXISTENCE OF ICE AND SUPERCOOLED WATER DROPLETS IS ESSENTIAL TO THE DEVELOPMENT OF MOST PRECIPITATION OUTSIDE THE TROPICS. The Bergeron process: Ice crystals hold on to the molecules and they doesn’t melt or evaporate easily. If it gets more than it gives (condensation vs. evaporation) then it grows. The crystal grows and takes all the molecules and gets too big and begins to fall. Ice crystals continue to grow through accretion and aggregation Accretion: occurs as the ice crystals fall through supercooled droplets and collide with them Aggregation: occurs when ice crystals collide and join together. b) Precipitation types
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1. The type of precipitation is determined by the temperature between the cloud base and the surface. EX: if the temp between a cloud and the surface is 12ºF, snow flakes will form.
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