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UNC-Chapel Hill GEOG 111 - Snow

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GEOG 111 1st Edition Lecture 26 Outline of Last Lecture I. The Circumpolar Vortex (review)II. Wave Cyclonesa. ExampleOutline of Current Lecture I. The Circumpolar Vortex (review)II. Snowa. Significanceb. Formation c. Atmospheric patternsCurrent LectureI. Snow – complex aggregations of ice crystals that develop in cold clouds in connection with the Bergeron processa. Significancei. Substantial part of the global land surfaceii. Snow changes the climateiii. Natural hazards1. Avalanches2. Killer flooding 3. Southeastern not used to snowfall  chaosb. Formationi. Cold air + moistureii. Snow particle type varies by temperature and moistureiii. Deformation of crystals1. In the cloud environment2. On the groundiv. Total snow accumulation depends on:1. Snowfall rate/durationa. The heaviest snowfall events display long durations and/or high snowfall rates2. Wetness of the snowa. Typical  1:10 ratio of snow to melted precipitation (E.g. 1” snow to 1” rain)b. Dry snow  20:1c. Wet  5:1These 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.3. Antecedent temperature of surface – this relates to conditions during prior day (e.g. solar insolation) and surface characteristics (e.g. specific heat and albedo)v. Ingredients for a perfect snowstorm1. Air that’s cold enough near the surfacea. Strong anticyclone to the north helps as it provides a replenishing supply of cold, dry airb. Not that melting snowflakes, evaporating rain droplets, or sublimating snowflakes can cool down the air significantlyc. Dry air and high precipitation rates are helpful2. Slow moving, wave cyclone that moves south of the area3. A relative absence of warm advection aloft (i.e. no warm nose)4. Frontal surface higher above surface (surface front farther to the south)vi. Example: March 1993 Super storm1. Over 30 inches in NC2. Cyclone progresses to the Northc. Atmospheric patternsi. Low pressure center, at surface, rotating counterclockwise, further away from warm front colder the air is1. Heavy snowstorm is when cyclone is in the Southii. Vertical temperature profile:1. Column below freezing (ideal)2. Isothermal profile, close to freezing (more realistic)a. When it snows up high it’s frozen but melts as it fallsb. Snow melts into rain, which causes enough cooling to bring the near surface air to freezingiii. Northwest flow snowfall1. Occurs as > 50% of mountain snowfall events2. Low level moisture scavenged from lakes and forced upward along mountains by orographic lifting3. Occurs after a


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