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CU-Boulder ATOC 1050 - Lecture 17

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3/30Lecture 17Anti-Cyclonic motion is associated with circulation around a central region of High / LowPressure.Cyclonic motion is associated with circulation around a central region of High / Low Pressure System.According to the Three Cell Model, we can expect Rising / Sinking air and High / Low surface pressuresat the Equator.According to the Three Cell Model, we can expect Rising / Sinking air and High / Low surface pressuresat 30o North and South Latitude.“Cyclonic” Winds in the Northern Hemisphere rotate CCW / CW.“Anti-Cyclonic” Winds in the Southern Hemisphere rotate CCW / CW.CHAPTER 10 : AIR MASSESAir Masses- An immense body of air, usually 1600 km (994 miles) or more across and severalkilometers thick. The air is characterized by homogeneous physical properties(temperature and moisture content) at any given altitudeIn an air mass, what two properties are relatively homogeneous? Temperature and MoisturecontentAir Masses : Source Regions1. Must be an extensive and physically uniform area (a lot of land or a lot of water)2. Must be a region dominated by stationary or slow-moving anticyclones (High Pressure /Atmospheric Subsidence) with extensive areas of calm or light windsSource regions for air masses are associated with stationary or slow-moving anti-cyclonicsystems with extensive areas of calm or light winds. TrueAir Masses : Naming- Air masses are named for the latitude and the moisture content of the source region1. The latitude of the source region indicates temperature of the air massArctic (A) - crazy coldPolar (P) - coldTropical (T) - warm2. The nature of the surface of the source region indicates the moisture content of the airmass. The two categories are:Maritime (m) - over waterContinental (c) - over landA Trick to Remember…Air Mass Classification- Moisture Content -- “MC”- Latitudinal Source Regions “PAT”Air Masses : Naming1. Latitude (arctic-a, polar-p, tropical-t)2. Moisture Content (maritime-m, continental-c)3. The combinations are:The three “latitudinal” source region categories for classifying air masses are: Arctic, Polar, TropicalAn air mass that has formed and spent a significant amount of time offshore of Florida would beconsidered: Maritime TropicalAir Masses : ModificationAs air masses move from one place to another:1. The air mass will influence the location and2. The location will influence the air massThe air mass will be modified by the surface below:- Temperature- Moisture- StabilitySmall Scale Example :Cool Dry Air mass moves over Warm Moist Surface Air over the LakeTemp of Moist Air above the Lake drops to Dew Point TempSaturation / Condensation OccursSteam Fog -- also known as Sea SmokeAir Masses : cP & cA Characteristics- Characteristics : cold, dry, stable- Source region : northern Canada and Alaska- Weather : cold, dry air, and lake effect snowWhat type of Air Mass is associated with “Lake Effect” Snow? cPAir Masses : mP Characteristics- Characteristics : cold, moist, conditionally unstable- Source region : N. Pacific and N. Atlantic Oceans- Weather : rain or snowAir Masses : mT Characteristics- Characteristics : warm/hot, moist, unstable- Source region : sub-tropical Pacific and Atlantic Oceans and the Gulf of Mexico- Weather : precipitationAir Masses : cT Characteristics- Characteristics : warm/hot, dry, unstable- Source region : northern Mexico- Weather : clear skies, hot temps, no precipitationCHAPTER 10 : FRONTSFronts : are boundary surfaces that separate air massesTypes:1. Cold2. Warm3. Stationary4. OccludedCold FrontsCold, dry air replaces the warm, moist airHow is it located on a weather map?- Large changes in temperature over a short distance- Change in moisture content of air- Shift in wind direction- Low pressure regions- Cloud and precipitation patternsCold Fronts- Frontal Wedging : cold air advancing / lifting warm air- The slope of a cold front - the ratio of vertical rise to horizontal distance - isapproximately 1:50- Cold front move quickly with a speed of approximately 25 knots or more- Thunderstorms, heavy rain, and strong winds occur at the frontWeather : a narrow band of precipitation along the cold front with potential severe thunderstormsdirectly along the frontSquall Line : a line of active showers and thunderstorms that develop parallel to and often aheadof an advancing cold frontBackdoor Cold Fronts : cold fronts that move in from the east, or northeast (regular cold frontstend to move towards the south, southeast, or east). Backdoor cold front usually move out ofCanada and into the northeastern United States \Warm FrontsWarm, moist air replaces cold, dry air- Overrunning : the rising of warm air over cold- The slope of a warm front - the ratio of vertical rise to horizontal distance - isapproximately 1:3000- Average speed of a warm front is approximately 10 knots (½ of an average cold front)- Light precipitation might start falling with nimbostratus clouds overhead (transitionsfrom snow to rain)Why do Cold Fronts move faster than Warm Fronts?1. The direction of secondary circulation2. Evaporative cooling from precipitation into the cold sectors* temp drop adds to the density* cold front → constructive* warm front → destructiveStationary Fronts- A nearly stationary (not moving) boundary between two air masses- The airflow on both sides of a front is neither toward the cold air mass or warm air mass,but instead parallel to the line of the front- Weather : expect to have clear or cloudy skies with possible rainOccluded Fronts- Form when a cold front catches up to a warm frontCold-Occluded Front (cold occlusion)- The air behind the cold front is colder than the air ahead of the warm frontWarm-Occluded Front (warm occlusion)- The air behind the cold front is not as cold as the air ahead of the warm frontTake A Moment…Air Masses are named based upon their Latitudinal Source Region and MoistureContent.A cT air mass has hot and dry air.A mP air mass has cold and humid air.cP air masses create Lake Effect Snow.As cold dry air masses move over the comparatively warmer Great Lakes in the winter,stability decreases.Cold Fronts travel faster than Warm Fronts.The advancement of Warm fronts is known as Overrunning Nimbostratus clouds are theresult of Warm Front advancement.4/1Lecture 18During an occluded front, where is the warmest air generally located? AloftThe warm air south of the low's center and between the two fronts is known as the warm sector.CHAPTER 10 : AIR MASSES, FRONTS, &

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