Download Chapter 19 Weather Patterns and Severe Storms

Chapter 19
Weather Patterns
and Severe Storms
Standard: Evaluate the patterns and
relationships in the composition
and movement of the
atmosphere around the earth.
Forecasting Scales
• From Largest Scale to smallest:
• Global Scale
– What generally occurs around the globe
– Think idealized global circulation and what the type of
general weather occurs at those latitudes
• Synoptic Scale
– Concentrates on different pressure regions
• Cyclones and Anticyclones and the frontal zones
created by them
• Mesoscale and Microscale
– Smaller than synoptic
– Focuses on small, individual storm cells and the
phenomenon associated with them
• Such as: thunderstorms, tornadoes, and hurricanes
Air masses
Characteristics
• Large body of air
• 1600 km (1000 mi.) or more across
• Perhaps several kilometers thick
• Similar temperature at any given altitude
• Similar moisture at any given altitude
• Move and affect a large portion of a
continent
A cold
Canadian
air mass
Air masses
Source region – the area where an air
mass acquires its properties
Classification of an air mass
• Two criteria are used to classify air masses
• 1. By the latitude of the source region
• Polar (P)
• High latitudes
• Cold
• Tropical (T)
• Low latitudes
• Warm
Air masses
Classification of an air mass
• 2. By the nature of the surface in the source
region
• Continental (c)
• Form over land
• Likely to be dry
• Maritime (m)
• Form over water
• Humid air
Air masses
Classification of an air mass
• Four basic types of air masses
•
•
•
•
Continental polar (cP)
Continental tropical (cT)
Maritime polar (mP)
Maritime tropical (mT)
Air masses are classified on the
basis of their source region
Air masses
Air masses and weather
• cP and mT air masses are the most
important air masses in North America,
especially east of the Rockies
Air masses
Air masses and weather
• 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
• 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 of
the lakes
Air masses
Air masses and weather
• Maritime tropical (mT)
• From the Gulf of Mexico and the Atlantic
Ocean
• Warm, moist, unstable air
• Brings precipitation to the eastern United
States
• Continental tropical (cT)
• Southwest and Mexico
• Hot, dry
• Seldom important outside the source region
Air masses
Air masses and weather
• Maritime polar (mP)
• Brings precipitation to the western mountains
• Occasional influence in the northeastern United
States causes the "Northeaster" in New
England with its cold temperatures and snow
Fronts
Boundary that separates air masses of
different densities
• Air masses retain their identities
• Warmer, less dense air forced aloft
• Cooler, denser air acts as wedge
Fronts
Types of fronts
• Warm front
•
•
•
•
Warm air replaces cooler air
Shown on a map by a red line with semicircles
Small slope
Clouds become lower as the front nears
• Cirrus then strato-form clouds
• Slow rate of advance
• Light-to-moderate precipitation
Warm front
Fronts
Types of fronts
• Cold front
•
•
•
•
•
Cold air replaces warm air
Shown on a map by a blue line with triangles
Slope is twice as steep as warm fronts
Advances faster than a warm front
Associated weather is more violent than a
warm front
• Generates Cumulonimbus clouds
• Intensity of precipitation is greater
• Duration of precipitation is shorter
Fronts
Types of fronts
• Cold front
• Weather behind the front is dominated by
• Cold air mass
• Subsiding air
• Clearing conditions
Cold front
Fronts
Types of 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
Formation
of an
occluded
front
Idealized global circulation
Middle-latitude cyclone
Primary weather producer in the middlelatitudes
Life cycle
• Form along a front where air masses are
moving parallel to the front in opposite
directions
• Continental polar (cP) air is often north of the
front
• Maritime tropical (mT) air is often south of the
front
Middle-latitude cyclone
Life cycle (cont.)
• Frontal surface takes on a wave shape
with low pressure centered at the apex of
the wave
• Flow of air is counterclockwise cyclonic
circulation
• Warm front and cold front form
• Cold front catches up to warm front and
produces an occlusion
• Warm sector is displaced aloft
• Pressure gradient weakens and fronts
discontinue
Stages in
the life
cycle of a
middlelatitude
cyclone
Middle-latitude cyclone
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
Middle-latitude cyclone
Idealized weather (cont.)
• Changes in weather associated with the
passage of a middle-latitude cyclone
• Changes depend on the path of the storm
• Weather associated with fronts
• Warm front
• Clouds become lower and thicker
• Light precipitation
• After the passage of a warm front winds
become more southerly and
temperatures warm
Middle-latitude cyclone
Idealized weather
• Changes in weather associated with the
passage of a middle-latitude cyclone
• Weather associated with fronts
• 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, and
temperatures drop
Cloud
patterns
typically
associated
with a
mature
middlelatitude
cyclone
Satellite view of a cyclone over the
eastern United States
Middle-latitude cyclone
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
• Cyclone
• Low pressure system
• Surface convergence
• Outflow (divergence) aloft sustains the low
pressure
Middle-latitude cyclone
Role of air aloft
• Anticyclone
•
•
•
•
High pressure system
Associated with cyclones
Surface divergence
Convergence aloft
Severe weather types
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
Average number of days per year
with thunderstorms
Severe weather types
Thunderstorms (cont.)
• Stages of development
• All thunderstorms require
• Warm air
• Moist air
• Instability (lifting)
• High surface temperatures
• Most common in the afternoon and early
evening
Severe weather types
Thunderstorms (cont.)
• Stages of development
• Require continuous supply of warm air and
moisture
• Each surge causes air to rise higher
• Updrafts and downdrafts form
• Eventually precipitation forms
• Most active stage
• Gusty winds, lightning, hail
• Heavy precipitation
• Cooling effect of precipitation marks the end of
thunderstorm activity
Stages in the development
of a thunderstorm
Thunderstorms
• Necessary ingredients:
– 1. Low-level southerly flow (mT) pushed up by
a continental polar (cP) air mass
• The mT provides latent energy
– 2. Southwesterly mid-level flow (cT)
• Provides an inversion (cap) not allowing the
release of latent energy
– 3. Jet stream
• Eliminates the cap (cT) allowing for dramatic
release of latent energy
• Evacuates mass of air
RADAR
• Radar is also used to measure the rate of
rainfall
• Uses radio wave impulses to measure
precipitation
• Waves are reflected off of the precipitation
• The reflected signal is called an echo
• Measures: type of precipitation, intensity, and
storm direction
• Echo is brighter when precipitation is more intense
• Also uses the Doppler effect to measure
precipitation: Doppler Radar
http://weather.unisys.com/radar/rad_cp.html
Thunderstorm Types
• Single cell: one ordinary cell that goes through
all three stages and dissipates out
• Multi-cell: several single cells clustered together
– Lasts several hours with large hail, damaging winds,
flash flooding, and isolated tornadoes
• Squall lines: line of thunderstorms that can
extend for hundreds of miles.
– Lasts several hours with hail and damaging winds
– The “gust front” continually lifts warm moist air to
sustain the life of the storm
– Shown on radar by a bow echo
– May produce downbursts and microburst and even
some sustained straight-lined winds called derechos
- similar to those in July 2006 in St. Louis
Radar Image of a Bow echo
Thunderstorm types (cont.)
• Supercell: single cell t-storms that last for
many hours
– Significant tornadoes and large hailstones
(golfball size)
– Extreme winds and flash flooding
– Characterized by: cyclonic rotating updraft
due to vertical wind shear
– May form a wall cloud indicating possible
tornadic activity.
Multi-cell
Squall-line
Super cell
Wall cloud
Double Vortex
• Thunderstorms can develop 2 vortices
– One cyclonic and one anticyclonic
• 2 vortices create an internal flow that sustains the life of
the T-storm
– Must have a warm moist inflow of air
• Cyclonic vortex can develop strong updraft leading to
large hail and possible tornadoes
• Anticylconic vortex is associated with downdrafts and
downbursts as well as precipitation
– May develop a gust front with winds greater than 100 mph
– Straight line winds known as Derechos
– St. Louis storms of summer 2006: largest power outage in the
history of St. Louis http://www.crh.noaa.gov/lsx/?n=july_2006
Severe weather types
Tornadoes
• Local storm of short duration
• Features
• Violent windstorm
• Rotating column of air that extends down from
a cumulonimbus cloud
• Low pressures inside causes the air to rush into
• Winds approach 480 km (300 miles) per hour
• Smaller suction vortices can form inside
stronger tornadoes
• Smaller whirls of wind (tiny tornadoes within)
Smaller suction vortices in
tornadoes (fig 18.16)
Severe weather types
Tornadoes
• Occurrence and development
•
•
•
•
Average of 770 each year in the United States
Most frequent from April through June
Associated with severe thunderstorms
Exact cause of tornadoes formation is not
known
• Conditions for the formation of tornadoes
• Occur most often along a cold front
• During the spring months
• Associated with huge thunderstorms called
supercells
Formation of Tornadoes
Severe weather types
Tornadoes
• Characteristics
• Diameter between 150 and 600 meters (500
and 2000 feet)
• Speed across landscape is about 45 kilometers
(30 miles) per hour
• Cut about a 10 km (6 miles) long path
• Most move toward the northeast
• Maximum winds range beyond 500 kilometers
(310 miles) per hour
• Intensity measured by the Fujita intensity scale
Enhanced Fujita Scale
DERIVED EF
SCALE
FUJITA SCALE
F
Numb
er
Fastest
3 Second
1/4Gust
mile
(mph)
(mph)
EF
3 Second
Numb
Gust
er
(mph)
OPERATIONAL EF
SCALE
EF
Numb
er
3
Seco
nd
Gust
(mph)
0
40-72
45-78
0
65-85
0
65-85
1
73-112
79-117
1
86-109
1
86-110
2
113-157
118-161
2
110-137
2
111-135
3
158-207
162-209
3
138-167
3
136-165
4
208-260
210-261
4
168-199
4
166-200
5
261-318
262-317
5
200-234
5
Over 200
Average annual tornado incidence
per 10,000 square miles for a 27
year period
Paths of
Illinois
tornadoes
(1916 – 1969)
Severe weather types
Tornadoes
• Tornado forecasting
• Difficult to forecast because of their small size
• Tornado watch
• To alert the public to the possibility of
tornadoes
• Issued when the conditions are favorable
• Covers 65,000 square km (25,000 square
miles)
• Tornado warning is issued when a tornado is
sighted or conditions are indicated by weather
radar
Severe weather types
Tropical Cyclones
(aka) Hurricanes in the U.S. (Atlantic and
Gulf Coast)
• Known as
• Typhoons in the western Pacific
• Cyclones in the Indian Ocean
• Most violent storms on Earth
• To be called a hurricane
• Wind speed in excess of 119 kilometers (74
miles) per hour
• Can reach up to 300 km/hr (186mph)
• Rotary cyclonic circulation
• Low pressure <980mb (<28.94 “ Hg)
Severe weather types
Hurricanes
• Profile
• Form between the latitudes of 5 degrees and
20 degrees
• Energy source is warm ocean water (83°F or
warmer)
• Occur in Summer months for N. Hemisphere (June –
November)
• North Pacific has the greatest number per year
• Can generate 50 foot ocean waves
Severe weather types
Hurricanes
• Profile
• Parts of a hurricane
• Eyewall
• Near the center
• Rising air
• Intense convective activity
• Wall of cumulonimbus clouds
• Greatest wind speeds
• Heaviest rainfall
Severe weather types
Hurricanes
• Profile
• Parts of a hurricane
• Eye
• At the very center
• About 20 km (12.5 miles) diameter
• Precipitation ceases
• Winds subsides
• Air gradually descends and heats by
compression
• Warmest part of the storm
Cross section of a hurricane
Hurricane Katrina
Severe weather types
Hurricanes
• Hurricane formation and decay
• Form in all tropical waters except the
• South Atlantic and Eastern South Pacific
• Cool water
• Energy comes from condensing water vapor
• As water condenses, it releases heat
energy to the atmosphere
• Develop most often in late summer when
warm water temperatures provide energy
and moisture
• June – November for U.S.
Hurricane stages
• Stages
• Tropical disturbance – winds less than 61
km (38 miles) per hour
• Tropical depression – winds do not exceed
61 kilometers (38 miles) per hour
• Tropical storm – winds between 61 to 119
km (38 and 74 miles) per hour
• Hurricane – winds greater than 119 km (74
miles) per hour.
Severe weather types
Hurricanes
• Hurricane formation and decay
• Diminish in intensity whenever
• They move over cooler ocean water
• They move onto land
• The large-scale flow aloft is unfavorable
Severe weather types
Hurricanes
• Destruction from a hurricane
• 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
• Enhances height of ocean waves
• Saffir-Simpson scale ranks the relative
intensities of hurricanes
• Measures hurricane intensities
Severe weather types
Hurricanes
• Destruction from a hurricane
• Categories of hurricane damage
• Storm surge - large dome of water 65 to 80
kilometers (40 to 50 miles) wide sweeps
across the coast where eye makes landfall
• Wind damage
• Strongest winds are on the northeast
portion of hurricanes in the U.S.
• Inland flooding from torrential rains
Hurricane Katrina
Damage from Katrina
Weather Station Symbols
Wind Speed
Sky Coverage
Wind Direction
Temperature in °F 60
Dew Point in °F 52
257 Air Pressure in mb
Weather conditions
At this weather station, the following conditions were measured:
1. Wind speed = 15 knots
4. Temperature = 60 °F
2. Wind Direction = NW
5. Dew Point = 52 °F
3. Sky Coverage = 25%
6. Barometric Pressure = 1025.7mb
7. Weather Conditions = Rain