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UNC-Chapel Hill GEOG 111 - Hydrometeors and Precipitation Rates

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GEOG 111 1st Edition Lecture 23 Outline of Last Lecture I. Cloudsa. Cloud typesII. Precipitationa. Two processesi. BergeronOutline of Current Lecture I. Two processes for growing hydrometeorsa. Bergeron processb. Collision-CoalescenceII. Controls on precipitation rateCurrent LectureI. Two processes for growing hydrometeors (review)a. Bergeron – grows ice crystals through multiple processes involving all 3 states of water// often produces ice crystals that act as seeds for growing hydrometeors below in the warmer portions of cloudsi. Requires a cold cloud (temperature < -10 C) where ice crystals, water vapor and super-cooled water coexist1. High level clouds (cirrus and cirrostratus)2. In the coldest time of the year (winter) the Bergeron process may occur in lower altitude clouds as well and can even create snowii. Processes:1. Deposition of vapor  ice crystal growth (vapor to ice)a. Freezing also occurs but mainly deposition2. Removal of water around ice crystal so relative humidity drops3. Evaporation of cloud water  vapor 4. So ice crystals grow at the expense of cloud water5. Rising air motions keep growing snowflake aloft until it becomes too heavyiii. The Bergeron process provides a distinct glazed and/or streaked appearance in cloudsb. Collision-coalescence – hydrometer grows as it collides with smaller droplets through the skyi. It is the only precipitation formation process that operates in warm cloudsii. Can supplement the Bergeron process in cold clouds (e.g. super-cooled droplets colliding)These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.iii. Large cloud droplet falls through smaller cloud droplets accumulating them along the wayiv. Often contributes to the seeder-feeder process: process in which you have precipitation forming from the Bergeron (seeds) and Collision-coalescence process (warmer droplets that are feeding the seeds)1. Two scenarios:a. Rain droplets from melted snowflakes (seeds) are fed by the warm cloud water dropletsb. Cloud water droplets freeze on falling snowflakes, in a process called rimingi. The snowflakes grow in size as it falls through the warmer part of the cloudII. Controls on precipitation rate (amount of precipitation/time)a. Amount of water vapor condenses/deposited out of atmospherei. Air temperature (higher temp ~ higher SVP) and proximity to warm body of water (how much water is available)ii. Rate of lifting1. Convective lift can be 10-50 times faster than frontal liftb. Distinguish between two precipitation typesi. Stratiform (from nimbostratus) – cause sudden changes in temperature, humidity and pressure of air1. Where warm/tropical air meets cool air and warm air is forced to rise2. Less intense with over a longer period of time than convective precipitation ii. Convective (from cumulonimbus) – precipitation falls as showers with rapidly changing intensity over a certain area for a relatively short time1. Mainly in a conditionally unstable or moist


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