3 30 Lecture 17 Anti 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 pressures at the Equator According to the Three Cell Model we can expect Rising Sinking air and High Low surface pressures at 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 MASSES Air Masses An immense body of air usually 1600 km 994 miles or more across and several kilometers thick The air is characterized by homogeneous physical properties temperature and moisture content at any given altitude In an air mass what two properties are relatively homogeneous Temperature and Moisture content Air Masses Source Regions 1 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 winds Source regions for air masses are associated with stationary or slow moving anti cyclonic systems with extensive areas of calm or light winds True Air Masses Naming Air masses are named for the latitude and the moisture content of the source region 1 The latitude of the source region indicates temperature of the air mass Arctic A crazy cold Polar P cold Tropical T warm 2 The nature of the surface of the source region indicates the moisture content of the air mass The two categories are Maritime m over water Continental c over land A Trick to Remember Air Mass Classification Moisture Content MC Latitudinal Source Regions PAT Air Masses Naming 1 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 Tropical An air mass that has formed and spent a significant amount of time offshore of Florida would be considered Maritime Tropical Air Masses Modification As air masses move from one place to another 1 The air mass will influence the location and 2 The location will influence the air mass The air mass will be modified by the surface below Temperature Moisture Stability Small Scale Example Cool Dry Air mass moves over Warm Moist Surface Air over the Lake Temp of Moist Air above the Lake drops to Dew Point Temp Saturation Condensation Occurs Steam Fog also known as Sea Smoke Air Masses cP cA Characteristics Characteristics cold dry stable Source region northern Canada and Alaska Weather cold dry air and lake effect snow What type of Air Mass is associated with Lake Effect Snow cP Air Masses mP Characteristics Characteristics cold moist conditionally unstable Source region N Pacific and N Atlantic Oceans Weather rain or snow Air Masses mT Characteristics Characteristics warm hot moist unstable Source region sub tropical Pacific and Atlantic Oceans and the Gulf of Mexico Weather precipitation Air Masses cT Characteristics Characteristics warm hot dry unstable Source region northern Mexico Weather clear skies hot temps no precipitation CHAPTER 10 FRONTS Fronts are boundary surfaces that separate air masses Types 1 Cold 2 Warm 3 Stationary 4 Occluded Cold Fronts Cold dry air replaces the warm moist air How 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 patterns Cold Fronts Frontal Wedging cold air advancing lifting warm air The slope of a cold front the ratio of vertical rise to horizontal distance is approximately 1 50 Cold front move quickly with a speed of approximately 25 knots or more Thunderstorms heavy rain and strong winds occur at the front Weather a narrow band of precipitation along the cold front with potential severe thunderstorms directly along the front Squall Line a line of active showers and thunderstorms that develop parallel to and often ahead of an advancing cold front Backdoor Cold Fronts cold fronts that move in from the east or northeast regular cold fronts tend to move towards the south southeast or east Backdoor cold front usually move out of Canada and into the northeastern United States Warm Fronts Warm 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 is approximately 1 3000 Average speed of a warm front is approximately 10 knots of an average cold front Light precipitation might start falling with nimbostratus clouds overhead transitions from snow to rain Why do Cold Fronts move faster than Warm Fronts 1 The direction of secondary circulation 2 Evaporative cooling from precipitation into the cold sectors temp drop adds to the density cold front constructive warm front destructive Stationary 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 rain Occluded Fronts Form when a cold front catches up to a warm front Cold Occluded Front cold occlusion The air behind the cold front is colder than the air ahead of the warm front Warm Occluded Front warm occlusion The air behind the cold front is not as cold as the air ahead of the warm front Take A Moment Air Masses are named based upon their Latitudinal Source Region and Moisture Content 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 the result of Warm Front advancement 4 1 Lecture 18 During an occluded front where is the warmest air generally located Aloft The warm air south of the low s center and between the two fronts is known as the warm sector CHAPTER 10 AIR MASSES FRONTS MID LATITUDE CYCLONES Polar Front Theory Norwegian Cyclone Model Majority of weather in the mid latitudes Florida to Alaska is generated by mid latitude cyclones The Norwegians were the first to develop an atmospheric model that