Download Weather Patterns and Severe Weather

Chapter 14 Lecture Outline
Weather Patterns
and Severe
Weather
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
Air Masses
A Cold
Canadian Air
Mass
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
Air Masses
 Four basic types of air masses
 Continental polar (cP)
 Continental tropical (cT)
 Maritime polar (mP)
 Maritime tropical (mT)
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
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
Air Masses
Air Masses
Satellite image of lake-effect snow storm
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
Air Masses
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
Fronts
Warm front
• Warm air replaces cooler air
• Shown on a map by a line with red
semicircles
• Shallow slope (1:200)
• Clouds become lower as the front nears
• Slow rate of advance
• Light-to-moderate precipitation
Fronts
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
Fronts
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
Fronts
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
Midlatitude Cyclones
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
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
Midlatitude Cyclones
Cloud Patterns of
Typical Mature MiddleLatitude Cyclone
Midlatitude Cyclones
Development
Cold Fronts and Warm Fronts
Midlatitude Cyclones
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
Midlatitude Cyclones
Cyclones and Anticyclones
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
Thunderstorms
Thunderstorms
Stages of development
• All thunderstorms require
– Warm air
– Moist air
– Instability (lifting)
• High surface temperatures
• Most common in afternoon and
early evening
Thunderstorms
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
Thunderstorms
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
Tornadoes
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
– May be associated with huge
thunderstorms called supercells
Mesocyclone Development
• Supercell thunderstorms
– Large, long-lasting thunderstorm with a single
violent rotating updraft
– Strong vertical wind shear
– Outflow never undercuts updraft, updrafts may
exceed 90kts and can cause large sized hail
– 3 types:
 Classic - CL
 high precipitation, HP
 low precipitation, LP
Some of the features associated with a classic tornado-breeding supercell thunderstorm as viewed from the southeast.
The storm is moving to the northeast.
Intense thunderstorms often can create flash flood
conditions especially if storms are “training”
A wall cloud photographed southwest of Norman, Oklahoma.
• Lightning and Thunder
– Causes of electrification of clouds
– graupel and hail fail into region of
supercooled water , water freezes,
releasing latent heat and keeping the
hailstone warmer than surrounding ice
crystal nuclei
• Net transfer ….+ ions from warmer to colder,
this leaves larger hail stones negatively
charged and smaller ice crystals positively
charged
When the tiny colder ice crystals come in contact with the much larger and warmer
hailstone (or graupel), the ice crystal becomes positively charged and the hailstone
negatively charged. Updrafts carry the tiny positively charged ice crystal into the upper
reaches of the cloud, while the heavier hailstone falls through the updraft toward the
lower region of the cloud
The generalized charge
distribution in a mature
thunderstorm.
• The Lightning Stroke
– A discharge of static electricity
– Positive charge on ground, cloud to ground
lightning
• Thunder
–
–
–
–
Lightning heats air to 54,000deg F – hotter than Sun’s surface
Explosive expansion of air - shock wave
Sound travels at 330m/s or 1100 ft/s, so delay… about 5 sec per mile
Sound is refracted upward in unstable atm and we do not hear
lightning at approximately 15km away … Heat Lightning
Thunder travels outward from the lightning stroke in the form of waves.
If the sound waves from the lower part of the stroke reach an observer before the waves
from the upper part of the stroke, the thunder appears to rumble.
If the sound waves bend upward away from an observer, the lightning stroke may be
seen, but the thunder will not be heard….”heat lightning”
The four marks on the road surface represent areas where lightning, after striking a car
traveling along south Florida’s Sunshine State Parkway, entered the roadway through
the tires. Lightning flattened three of the car’s tires and slightly damaged the radio
antenna. The driver and a six-year-old passenger were taken to a nearby hospital,
treated for shock, and released.
Average lightning flash density per square kilometer per year from 1997 to 2010. Notice
that in the United States, Florida is the most lightning-prone state. (Data from the North
American Lightning Detection Network. Courtesy of Vaisala.)
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 hr
Intensity measured by the Fujita
intensity scale, or EF- scale
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
Tornadoes
Doppler Radar image of tornado near Moore , OK…..May 3, 1999!
• May 20, 2013 Moore OK…..again
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
Hurricanes
Hurricane Wind Patterns
Hurricanes
Parts of a hurricane
•
Eyewall
– Near the center
– Rising air
– Intense convective activity
• Wall of cumulonimbus clouds
• Greatest wind speeds
• Heaviest rainfall
Hurricanes
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
Hurricanes
• Hurricane formation and decay
– Form in all tropical waters except the
• South Atlantic(rare)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)
Hurricanes
Hurricanes
Diminish in intensity as:
• They move over cooler ocean water
• They move onto land
• The large-scale flow aloft is unfavorable
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
Hurricanes
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
• Resonsible for large number of deaths
– Wind damage
– Inland flooding from torrential rains
Hurricanes
2012 Season
•
In 2012, there were 19 tropical cyclones, 19 tropical storms, 10 hurricanes,
and 2 major hurricanes. Damage in this season was around $77.57 billion
and deaths were around 354. The majority of these damages and deaths
were caused by Hurricane Sandy and Hurricane Isaac.
Hurricane Sandy
October 22, 2012 – October 31, 2012
2013 Season
With 14 tropical storms, two hurricanes, and no major hurricanes,activity fell far
below the predictions. The season's impact was minimal; although 15 tropical
cyclones developed, several were weak or remained at sea. Tropical Storm
Andrea killed four people after making landfall in Florida and moving up the East
Coast of the United States
2014 Season
The season's first tropical cyclone, Arthur, developed on July 1, ahead of the long-term
climatological average of July 9. Early on July 3, the system intensified into a hurricane,
preceding the climatological average of August 10.[16] After continuing to steadily
intensify, it moved ashore between Cape Lookout and Cape Hatteras as a Category 2
hurricane, becoming the first U.S. land falling cyclone of that intensity since Hurricane
Ike in 2008.Upon moving inland, Arthur became the earliest known hurricane to strike
the North Carolina coastline on record.
2015 Season
The season officially began on June 1, 2015, and ends on November 30, 2015. These
dates historically describe the period each year when most tropical cyclones form in the
Atlantic basin and are adopted by convention. However, the formation of tropical
cyclones is possible at any time of the year.
The first storm, Ana, developed a month before the official start of the season, becoming
the first pre-season tropical or subtropical cyclone since 2012's Beryl, the earliest-forming
cyclone since 2003's Ana, and the earliest cyclone on record to strike the United States.
Despite an ongoing El Niño event, the season started unusually early, and August and
September so far have featured 8 tropical cyclones. With the classification of Tropical
Storm Ida, this season has featured more named storms than the previous season;
however this season is still slightly below normal according to its Accumulated cyclone
energy Index. In early October, Joaquin became the strongest hurricane in the Atlantic
since Igor in 2010. But Wait………there’s MORE !
The 2016 Atlantic hurricane season
The most active and costliest Atlantic hurricane season since 2012 as well as the deadliest since
2005. The season officially started on June 1 and will end on November 30.[1] The season began
nearly five months before the official start, with Hurricane Alex forming in the Northeastern
Atlantic in mid-January, the first Atlantic January hurricane since Hurricane Alice in 1955.
The strongest, costliest and deadliest storm of the season so far is Hurricane Matthew, the
southernmost Category 5 Atlantic hurricane on record, and the first Category 5 hurricane to form
in the Atlantic since Felix in 2007. With up to 1,655 deaths attributed to it, Hurricane Matthew is
the deadliest Atlantic hurricane since Stan of 2005. Following the development of Hurricane
Nicole, which reached major hurricane status, it was the first time that two Category 4 or stronger
hurricanes had formed in the month of October, with the other being Hurricane Matthew. Due to
Nicole becoming a major hurricane, this season was the first to have more than two major
hurricanes since 2011.
Most forecasting groups predicted above average activity due to a developing La Niña and
warmer than normal sea surface temperatures. Overall, the forecasts have been fairly accurate.
So far, twelve of the fifteen developed tropical cyclones (except Fiona, Ian and Lisa) have
impacted land, and seven of those storms caused loss of life, directly or indirectly. At least 1,739
people have died as of October 28, making this season the deadliest since 2005.