Foundations of Earth Science
Weather Patterns and Severe Weather
Chapter 14 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition
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Define air mass.
Describe the classification and weather associated with air masses.
Focus Questions 14.1
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Air mass characteristics
Large body of air
1600 km (1000 mi) or more across
Several kilometers thick
Similar temperature at any given altitude
Similar moisture at any given altitude
Move and affect a large portion of a continent
Air mass weather
Air Masses
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Air Masses
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Source region
Where an air mass acquires its properties
Classification of an air mass
Two criteria used to classify air masses:
By the latitude of the source region
Polar (P) and Arctic (A)
High latitudes: cold
Tropical (T)
Low latitudes: warm
By the nature of the surface in the source region
Continental (c)
Form over land: dry
Maritime (m)
Form over water: humid
Air Masses
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Five basic types of air masses
Continental polar (cP)
Continental arctic (cA)
Continental tropical (cT)
Maritime polar (mP)
Maritime tropical (mT)
Air Masses
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Air Masses
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Air masses and weather
cP and mT air masses important in North 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
Air Masses
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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
Air Masses
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Air Masses
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Air Masses
Satellite image of lake-effect snow storm
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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
Air Masses
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Air Masses
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Compare and contrast typical weather associated with a warm front and a cold front.
Describe an occluded front and a stationary front.
Focus Questions 14.2
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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
Overrunning
Fronts
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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
Fronts
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Fronts
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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
Fronts
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Fronts
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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
Fronts
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Fronts
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Summarize the weather associated with the passage of a mature mid latitude cyclone.
Describe how aloft is related to cyclones and anticyclones at the surface.
Focus Questions 14.3
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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
Midlatitude Cyclones
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Midlatitude Cyclones
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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
Midlatitude Cyclones
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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
Midlatitude Cyclones
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Midlatitude Cyclones
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Midlatitude Cyclones
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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
Midlatitude Cyclones
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Midlatitude Cyclones
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List the basic requirements for thunderstorm formation.
Locate places on a map that exhibit frequent thunderstorm activity.
Describe the stages in the development of a thunderstorm.
Focus Questions 14.4
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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
Thunderstorms
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Thunderstorms
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Stages of development
All thunderstorms require
Warm air
Moist air
Instability (lifting)
High surface temperatures
Most common in afternoon and early evening
Thunderstorms
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Thunderstorms
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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
Thunderstorms
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Thunderstorms
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Summarize the atmospheric conditions and locations that are favorable to the formation of tornadoes.
Discuss tornado destruction and tornado forecasting.
Focus Questions 14.5
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Tornadoes
Tornado – 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
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Tornadoes
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Tornadoes
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Tornadoes
Occurrence and development
Associated with severe thunderstorms
Product of interaction between thunderstorm updrafts and tropospheric winds
Average of 1297 in the United States between 2000 and 2014
Most frequent from April through June
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Tornadoes
Occurrence and development
General atmospheric conditions
Occur most often along a cold front
Associated with huge thunderstorms called supercells
Cold, dry cP air meets warm, humid mT air
Greater contrast = more intense storm
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Tornadoes
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Tornadoes
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Tornadoes
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Tornadoes
Characteristics
Diameter 150–600 m (500–2000 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
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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 km2 (25,000 mi2)
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
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Identify areas of hurricane formation on a world map.
Discuss the conditions that promote hurricane formation.
List the three broad categories of hurricane destruction.
Focus Questions 14.6
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Hurricanes
Most violent storms on Earth
Hurricane
Intense centers of low pressure
Form over tropical oceans
Characterized by intense convective activity and strong cyclonic circulation
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Hurricanes
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
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Hurricanes
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Hurricanes
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Hurricanes
Parts of a hurricane
Eyewall
Near the center
Rising air
Intense convective activity
Wall of cumulonimbus clouds
Greatest wind speeds
Heaviest rainfall
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Hurricanes
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Hurricanes
Eye
At the very center
About 20 km (12.5 mi) diameter
Precipitation ceases
Winds subsides
Air gradually descends and heats by compression
Warmest part of the storm
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Hurricanes
Hurricane formation and decay
Energy from condensing water vapor
Develop most often in late summer
Form in all tropical waters except the South Atlantic and Eastern South Pacific
Other tropical storms
Tropical depression
Winds do not exceed 61 km (38 mi) per hour
Tropical storm
Winds 61–119 km (38–74 mi) per hour
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Hurricanes
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Hurricanes
Diminish in intensity as:
They move over cooler ocean water
They move onto land
The large-scale flow aloft is unfavorable
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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
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Hurricanes
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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
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Hurricanes
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Hurricanes
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