© 2014 Pearson Education, Inc. Weather Patterns and Severe Weather Chapter 14 Lecture Outline Natalie Bursztyn Utah State University Foundations of Earth Science Seventh Edition

© 2014 Pearson Education, Inc. Focus Question 14.1 Which air mass is associated with lake-effect snow? What causes lake-effect snow?

© 2014 Pearson Education, Inc. Air Masses Characteristics Large body of air – 1600 km (1000 mi) or more across – Several km thick Similar temperature at any given altitude Similar moisture at any given altitude Move and affect a large portion of a continent

© 2014 Pearson Education, Inc. Air Masses

© 2014 Pearson Education, Inc. Air Masses Source region Where an air mass acquires its properties Classification of an air mass Two criteria used to classify air masses: 1.By the latitude of the source region Polar (P) – High latitudes: cold Tropical (T) – Low latitudes: warm 2.By the nature of the surface in the source region Continental (c) – Form over land: dry Maritime (m) – Form over water: humid

© 2014 Pearson Education, Inc. Air Masses Four basic types of air masses 1. Continental polar (cP) 2. Continental tropical (cT) 3. Maritime polar (mP) 4. Maritime tropical (mT)

© 2014 Pearson Education, Inc. Air Masses Air masses and weather cP and mT air masses important in N. America North America (east of the Rocky Mountains) – Continental polar (cP) From northern Canada and interior of Alaska – Winter: Brings cold, dry air – Summer: Brings cool relief

© 2014 Pearson Education, Inc. Air Masses Air masses and weather North America (east of the Rocky Mountains) – Continental polar (cP) Responsible for lake-effect snows – cP air mass crosses the Great Lakes – Air picks up moisture from the lakes – Snow occurs on the leeward shores

© 2014 Pearson Education, Inc. Air Masses

© 2014 Pearson Education, Inc. Air Masses Satellite image of lake-effect snow storm

© 2014 Pearson Education, Inc. Air Masses Air masses and weather North America (east of the Rocky Mountains) – Maritime tropical (mT) From the Gulf of Mexico and the Atlantic Ocean Warm, moist, unstable air Brings precipitation – Continental tropical (cT) Southwest and Mexico Hot, dry – Maritime polar (mP) Brings precipitation to the western mountains Occasional influence: causes the Northeaster

© 2014 Pearson Education, Inc. Air Masses

© 2014 Pearson Education, Inc. Focus Question 14.1 Which air mass is associated with lake-effect snow? What causes lake-effect snow? Continental polar (cP) air masses are associated with lake-effect snow. As the cold dry air passes over the great lakes it picks up heat and moisture, then, as the air mass crosses land again it loses the moisture as snowstorm precipitation due to air mass instability.

© 2014 Pearson Education, Inc. Fronts Boundaries that separates air masses of different densities Air masses retain their identities – Warmer, less dense air forced aloft – Cooler, denser air acts as wedge

© 2014 Pearson Education, Inc. Fronts Warm front Warm air replaces cooler air Shown on a map by a line with red semicircles Small slope (1:200) Clouds become lower as the front nears Slow rate of advance Light-to-moderate precipitation

© 2014 Pearson Education, Inc. Fronts

© 2014 Pearson Education, Inc. Fronts Cold front Cold air replaces warm air Shown on a map by a line with blue triangles Twice as steep (1:100) as warm fronts Advances faster than a warm front Associated weather is often violent – Intensity of precipitation is high – Duration of precipitation is short Weather behind the front is dominated by – Cold air mass – Subsiding air – Clearing conditions

© 2014 Pearson Education, Inc. Fronts

© 2014 Pearson Education, Inc. Fronts Stationary front Flow of air on both sides of the front is almost parallel to the line of the front Surface position of the front does not move Occluded front Active cold front overtakes a warm front Cold air wedges the warm air upward Weather is often complex Precipitation is associated with warm air being forced aloft

© 2014 Pearson Education, Inc. Fronts

© 2014 Pearson Education, Inc. Focus Question 14.3 Briefly explain how flow aloft aids the formation of cyclones at the surface.

© 2014 Pearson Education, Inc. Midlatitude Cyclones Primary weather producer in the middle latitudes Idealized weather – Middle-latitude cyclones move eastward across the United States First signs of their approach are in the western sky Require two to four days to pass over a region Largest weather contrasts occur in the spring Changes in weather associated with the passage of a middle-latitude cyclone Changes depend on the path of the storm

© 2014 Pearson Education, Inc. Midlatitude Cyclones

© 2014 Pearson Education, Inc. Midlatitude Cyclones Weather associated with fronts Warm front – Clouds become lower and thicker – Light precipitation After the passage of a warm front: – Winds become more southerly – Temperatures warm

© 2014 Pearson Education, Inc. Midlatitude Cyclones Cold front Wall of dark clouds Heavy precipitation – Hail and occasional tornadoes After the passage of a cold front: – Winds become more northerly – Skies clear – Temperatures drop

© 2014 Pearson Education, Inc. Midlatitude Cyclones

© 2014 Pearson Education, Inc. Midlatitude Cyclones

© 2014 Pearson Education, Inc. Midlatitude Cyclones Role of air aloft Cyclones and anticyclones – Generated by upper-level air flow – Maintained by upper-level air flow – Typically are found adjacent to one another

© 2014 Pearson Education, Inc. Midlatitude Cyclones

© 2014 Pearson Education, Inc. Focus Question 14.3 Briefly explain how flow aloft aids the formation of cyclones at the surface. It maintains cyclonic and anticyclonic circulation. So long as divergence aloft is equal to or greater than the surface inflow, the low pressure and its accompanying convergence can be sustained.

© 2014 Pearson Education, Inc. Focus Question 14.4 What are the basic requirements for the formation of a thunderstorm?

© 2014 Pearson Education, Inc. Thunderstorms Features Cumulonimbus clouds Heavy rainfall Lightning Occasional hail Occurrence 2000 in progress at any one time! 100,000 per year in the United States Most frequent in Florida and eastern Gulf Coast region

© 2014 Pearson Education, Inc. Thunderstorms

© 2014 Pearson Education, Inc. Thunderstorms Stages of development All thunderstorms require – Warm air – Moist air – Instability (lifting) High surface temperatures Most common in afternoon and early evening

© 2014 Pearson Education, Inc. Thunderstorms

© 2014 Pearson Education, Inc. Thunderstorms Stages of development Continuous supply of warm air and moisture – Each surge causes air to rise higher – Updrafts and downdrafts form Eventually precipitation forms – Gusty winds, lightning, hail – Heavy precipitation Cooling effect of precipitation marks the end of thunderstorm activity

© 2014 Pearson Education, Inc. Thunderstorms

© 2014 Pearson Education, Inc. Focus Question 14.4 What are the basic requirements for the formation of a thunderstorm? Thunderstorms form when warm, humid air rises in an unstable environment. Various mechanisms can trigger the upward air movement needed to create thunderstorm-producing cumulonimbus clouds.

© 2014 Pearson Education, Inc. Focus Question 14.5 Why do tornadoes have such high wind speeds?

© 2014 Pearson Education, Inc. Tornadoes Local storm of short duration Features: – Rotating column of air that extends down from a cumulonimbus cloud – Low pressure inside – Winds approach 480 km (300 mi) per hour – Smaller suction vortices can form inside stronger tornadoes

© 2014 Pearson Education, Inc. Tornadoes

© 2014 Pearson Education, Inc. Tornadoes Occurrence and development Average of 770 each year in the U.S. Most frequent from April through June Associated with thunderstorms Exact cause is not known Formation of tornadoes – Occur most often along a cold front – Associated with huge thunderstorms called supercells

© 2014 Pearson Education, Inc. Tornadoes

© 2014 Pearson Education, Inc. Tornadoes Characteristics Diameter 150–600 m ( ft) Speed 45 km (30 mi) per hour Can cut a 10 km (6 mi) long path Max winds over 500 km (310 mi) per hour Intensity measured by the Fujita intensity scale, or EF- scale

© 2014 Pearson Education, Inc. Tornadoes Tornado forecasting Difficult to forecast Tornado watch – To alert public to the possibility of tornadoes – Issued when the conditions are favorable – Covers 65,000 km 2 (25,000 mi 2 ) Tornado warning – Issued when a tornado is sighted or indicated by weather radar – Use of Doppler radar helps increase the accuracy by detecting the air motion

© 2014 Pearson Education, Inc. Tornadoes

© 2014 Pearson Education, Inc. Focus Question 14.5 Why do tornadoes have such high wind speeds? The high wind speed of tornadoes is directly related to extremely low pressures in the center of the vortex. Drawn by the much lower pressure in the center of the vortex, air near the ground rushes into the tornado from all directions. Because of the tremendous pressure gradient associated with a strong tornado, maximum winds can sometimes approach 480 kph.

© 2014 Pearson Education, Inc. Focus Question 14.6 Why do hurricanes not form near the equator? Explain the lack of hurricanes in the South Atlantic and eastern South Pacific.

© 2014 Pearson Education, Inc. Hurricanes Most violent storms on Earth To be called a hurricane: – Wind speed > 119 km (74 mi) per hour – Rotary cyclonic circulation Form between 5º and 20º latitudes Wind speeds reach 300 kph Generate 50-foot waves at sea – Typhoons in the western Pacific – Cyclones in the Indian Ocean – North Pacific has the greatest number per year

© 2014 Pearson Education, Inc. Hurricanes

© 2014 Pearson Education, Inc. Hurricanes Parts of a hurricane Eyewall – Near the center – Rising air – Intense convective activity Wall of cumulonimbus clouds Greatest wind speeds Heaviest rainfall

© 2014 Pearson Education, Inc. Hurricanes

© 2014 Pearson Education, Inc. Hurricanes Eye At the very center About 20 kilometers (12.5 miles) diameter Precipitation ceases Winds subsides Air gradually descends and heats by compression Warmest part of the storm

© 2014 Pearson Education, Inc. Hurricanes Hurricane formation and decay – Form in all tropical waters except the South Atlantic and Eastern South Pacific Energy from condensing water vapor Develop most often in late summer – Tropical depression Winds do not exceed 61 km (38 mi) per hour – Tropical storm Winds 61–119 km (38–74 mi) per hour

© 2014 Pearson Education, Inc. Hurricanes

© 2014 Pearson Education, Inc. Hurricanes Diminish in intensity as: They move over cooler ocean water They move onto land The large-scale flow aloft is unfavorable

© 2014 Pearson Education, Inc. Hurricanes Hurricane destruction Factors that affect amount of hurricane damage – Strength of storm (the most important factor) – Size and population density of the area affected – Shape of the ocean bottom near the shore Saffir-Simpson scale ranks the relative intensities of hurricanes

© 2014 Pearson Education, Inc. Hurricanes

© 2014 Pearson Education, Inc. Hurricanes Hurricane destruction Categories of hurricane damage – Storm surge Large dome of water 65 to 80 km (40 to 50 mi) wide sweeps across the coast where eye makes landfall – Wind damage – Inland flooding from torrential rains

© 2014 Pearson Education, Inc. Hurricanes

© 2014 Pearson Education, Inc. Focus Question 14.6 Why do hurricanes not form near the equator? Explain the lack of hurricanes in the South Atlantic and eastern South Pacific. The Coriolis effect is too weak at the equator to initiate sufficient storm rotation. The South Atlantic and eastern South Pacific have too cool of ocean waters to provide the necessary heat and moisture for hurricane formation.