© 2014 Pearson Education, Inc. Chapter 2 Lecture McKnight's Physical Geography 11e Lectures Chapter 7 Atmospheric Disturbances © 2014 Pearson Education, Inc. Andrew Mercer Mississippi State University

© 2014 Pearson Education, Inc. Learning Goals of This Chapter Define air mass, and explain how they are named. Identify the names and properties of the six different types of air masses. Define source region, and identify the source regions and associated air masses that affect North America. Define front, and identify the four types. Describe the formation of a cold front and the weather patterns as one passes a location. Describe the formation of a warm front and the weather patterns as one passes a location.

© 2014 Pearson Education, Inc. Learning Goals of This Chapter Describe the formation of a stationary front and the weather patterns on either side of the front. Describe the formation of an occluded cold front and the weather patterns as one passes a location. Explain the formation of a midlatitude cyclone. Explain the stages in the development of a midlatitude cyclone. Explain the typical weather pattern associated with the passage of a midlatitude cyclone with the center to the north of the location. Explain the typical weather pattern associated with the passage of a midlatitude cyclone with the center to the south of the location.

© 2014 Pearson Education, Inc. Learning Goals of This Chapter Define midlatitude anticyclone, and explain the associated weather patterns. Define easterly wave, and describe the associated weather characteristics. Explain the formation of a hurricane. Describe the development and structure of a hurricane. Identify areas where hurricanes originate and common their common tracks. Explain hazards associated with hurricanes and the types of damage they cause.

© 2014 Pearson Education, Inc. Learning Goals of This Chapter Explain the three stages in the development of a thunderstorm. Explain the formation of a mesocyclone. Explain the formation of tornadoes. Explain the ways storms can be monitored to enable informed decisions about personal safety.

© 2014 Pearson Education, Inc. Atmospheric Disturbances Air Masses Fronts Midlatitude Cyclones Midlatitude Anticyclones Easterly Waves Tropical Cyclones: Hurricanes Localized Severe Weather

© 2014 Pearson Education, Inc. Air Masses Properties of an air mass –Large (diameter > 1600 km) –Uniform horizontal properties –Recognizable entity; travel as one Origins of air masses –Remains over a uniform land or sea surface long enough to acquire its uniform characteristics

© 2014 Pearson Education, Inc. Air Masses Air mass classification –Two letter classification system –Lowercase letter indicates moisture content c – continental, dry m – maritime, humid –Uppercase letter indicates source region P – polar source region T – tropical source region A – arctic source region E – equatorial source region

© 2014 Pearson Education, Inc. Air Masses Source regions Properties

© 2014 Pearson Education, Inc. Fronts Definition of a front History of the name “front” Clash over midlatitudes between polar and tropical air masses Four primary frontal types: –Cold front – cold air advancing –Warm front – warm air advancing –Stationary front – no advance of air masses –Occluded front – cold air overtakes warm air

© 2014 Pearson Education, Inc. Fronts Cold Front –Protruding “nose” of cold air –Faster than warm fronts –Lift warm air ahead of cold fronts –Identified by blue line with triangles pointing in direction of frontal motion

© 2014 Pearson Education, Inc. Fronts Warm Front –Gentle slope of warm air rising above cool air –Slow cloud formation and precipitation –Indicated by red line with semicircles pointing in the direction of warm air motion

© 2014 Pearson Education, Inc. Midlatitude Cyclones Exist between 35° and 70° latitude Roughly 1600 km in size Central pressure near 990 to 1000 mb Converging counterclockwise circulation in Northern Hemisphere Circulation creates fronts Westward tilt with increasing elevation in Northern Hemisphere

© 2014 Pearson Education, Inc. Midlatitude Cyclones Weather changes behind front –Temperature –Winds –Pressure Cyclone movement –Steered by jet stream –System has a cyclonic wind circulation –Cold front advances faster than center of the storm

© 2014 Pearson Education, Inc. Midlatitude Cyclones Life cycle of a cyclone – cyclogenesis to occlusion

© 2014 Pearson Education, Inc. Midlatitude Cyclones Upper-level divergence and convergence related to cyclogenesis

© 2014 Pearson Education, Inc. Midlatitude Cyclones Occlusions – occluded front –cold front catches warm front, removing the energy of the storm (which is the warm air) –occlusions mark the end of the cyclone’s life –marked as a purple line with alternating triangles and half circles in direction of advancing cold air

© 2014 Pearson Education, Inc. Midlatitude Anticyclones Anticyclones - high pressure systems –Subsiding, diverging winds at the surface –Flow clockwise around an anticyclone –Move slightly slower than cyclones Relationship to cyclones –Occur independently, but have a functional relationship –Anticyclone follows cyclone –Anticyclones typically behind cyclone’s cold front

© 2014 Pearson Education, Inc. Midlatitude Anticyclones Relationship to midlatitude cyclones

© 2014 Pearson Education, Inc. Midlatitude Anticyclones Occurrence and distribution –Typically 6–15 cyclones exist worldwide –More numerous and better developed in winter than in summer –Move more equatorward during summer

© 2014 Pearson Education, Inc. Easterly Waves Easterly wave characteristics –Oriented N–S –Little cyclonic circulation –Convergence behind wave, divergence ahead of wave –Can intensify to tropical cyclones

© 2014 Pearson Education, Inc. Tropical Cyclones: Hurricanes Tropical cyclone definition Tropical depression – winds < 38 mph Tropical storm – winds 38–74 mph Hurricane – winds > 74 mph –Typhoons –Baguios –Cyclones

© 2014 Pearson Education, Inc. Tropical Cyclones: Hurricanes Hurricane characteristics –Prominent low pressure center, winds spiral inward –Steep pressure gradient and strong winds –Warm moist air enters storm to form rain and release latent heat –Eye wall and eye –Anticyclonic winds aloft, divergence aloft

© 2014 Pearson Education, Inc. Tropical Cyclones: Hurricanes Hurricane origin –Over warm water –A few degrees N or S of equator –No significant wind shear –Hurricane season Hurricane movement –Irregular tracks within the flow of the trade winds –Typically begin moving east–west; some curve poleward

© 2014 Pearson Education, Inc. Tropical Cyclones: Hurricanes Damage and destruction –High winds, torrential rain, and isolated tornadoes –Primary destruction – storm surge flooding Saffir-Simpson scale

© 2014 Pearson Education, Inc. Localized Severe Weather Thunderstorms –Violent convective storms –Accompanied by thunder and lightning –Formation stages Cumulus stage Mature stage Dissipating stage –Atmospheric conditions prone to thunderstorm formation

© 2014 Pearson Education, Inc. Localized Severe Weather Lightning –Electric discharge in thunderstorms –Separation of charges due to ice particles in a cloud –Positive charges on Earth’s surface –Lightning types Cloud to ground Cloud to cloud Within cloud –Thunder

© 2014 Pearson Education, Inc. Localized Severe Weather Tornadoes –Deep low pressure vortex, typically less than 400 meters in diameter –Fast winds, sometimes in excess of 300 mph –Originate above ground, water vapor condenses into funnel cloud –Contains vapor and debris

© 2014 Pearson Education, Inc. Localized Severe Weather Tornado formation –Vertical wind shear creates rotation with horizontal axis –Horizontal rotation tilted into vertical by thunderstorm updraft –Mesocyclone and tornado development

© 2014 Pearson Education, Inc. Localized Severe Weather Distributions of tornadoes (US)

© 2014 Pearson Education, Inc. Localized Severe Weather Distributions of tornadoes (global)

© 2014 Pearson Education, Inc. Localized Severe Weather Tornado classification

© 2014 Pearson Education, Inc. Summary Storms can impact the landscape through damaging winds and flooding rains. Air masses form in regions of stagnant air and are important for the weather in the midlatitudes. Fronts are the boundaries between different air masses. There are four primary types of fronts. Midlatitude cyclones are low pressure systems that are responsible for a majority of the weather in the midlatitudes.

© 2014 Pearson Education, Inc. Summary Midlatitude anticyclones are related to midlatitude cyclones. Easterly waves are minor tropical disturbances responsible for thunderstorms in the tropics. Hurricanes are strong tropical cyclones that cause catastrophic wind and storm surge flooding damage. Thunderstorms are localized strong weather phenomenon that include thunder, lightning, and heavy rainfall.

© 2014 Pearson Education, Inc. Summary Lightning results from charge separation within a cloud. Thunder is caused by superheating of the atmosphere by lightning and the resulting sound waves. Tornadoes are violent vortices associated with strong, rotating thunderstorms.