Download Chapter 19 Weather Patterns and Severe Storms * Air masses

Chapter 19
Weather Patterns and Severe Storms
*
Air masses
* Large bodies of air with similar temperature and humidity characteristics over a large
area
*
*
*
Formation of Air Masses
* Source regions
• Regions where air masses form
• Surface conditions are transferred to overlying air
• Air must sit over source for some time to attain the surface characteristics
*
• Maritime, continental
* Temperature
•
*
• Canada and Asia origin
•
• Stable conditions
• Extremely cold in winter, but pleasant in summer
• Can push as far south as Florida
* Maritime Polar (mP) Air Masses
• Originates in upper latitude oceans
•
•
• Can lead to large New England snow storm called a _____________________
* Continental Tropical (cT) Air Masses
• Originates over the desert southwest of U.S. and northern Mexico
• ry
• Inherently unstable
• Little if any precipitation
•
* Maritime Tropical (mT) Air Masses
• Originates over low latitude oceans (__________________)
•
• Inherently unstable
•
• Responsible for summer afternoon thunderstorms
*
• Cold air advances towards warmer air
•
•
• Steep angle produces intense rising motions
• Can produce heavy precipitation over a short period of time
1
•
•
Can move rapidly (fastest moving front)
*
•
•
•
•
•
•
•
•
Warm air advances towards colder air
Warmer air lifts over cold air (overrunning)
Brings warmer and more humid conditions
Winds becomes southerly
Shallow sloping front
•
•
•
•
•
Neither air mass moves
Warmer air is displaced aloft
Overrunning precipitation occurs on the colder north side of front
Persistent weather can last for days
Doesn’t move as fast a cold front
*
*
*
*
•
• Warm is forced aloft and the storm weakens
• Point of occlusion is where storm reaches it’s maximum strength
The Life-Cycle of a Mid-latitude Cyclone
* Developed by Vilhelm Bjerknes and called the polar front theory
(______________________________)
* _______________________ – formation of cyclones
• Begins along the stationary polar front
• Warm air attempts to push north and cold air attempts to push south
• Low pressure forms developing a ripple in the stationary front
• Warm and cold fronts develop
* Mature cyclones
• Deep low pressure center forms from surface convergence
•
• Strong storms exist along the cold front
•
lighter precipitation last longer along the warm front
• If cold enough, heaviest snow falls on the northwest side of the surface low
pressure
*
• Faster moving cold front catches the slower moving warm front and cold air
forces the warmer air aloft
•
• After occlusion storm weakens (_____________________)
* Idealized Weather in a mid-latitude cyclone
Interaction of surface and upper-level patterns
* The upper atmosphere and the surface are inherently connected
* Troughs develop behind cold fronts
* Ridges develop behind warm fronts
2
*
* Upper level divergence relates to surface cyclones
* Upper level convergence relates to surface anticyclones
Thunderstorms
* About 40,000 thunderstorms daily
* Each produces lightning
*
*
* Cumulonimbus clouds
* Most occur in the state of Florida
* Types of lightning
• ________________ – standard lightning appearance
• Sheet – in cloud lightning
• _______ – lightning the size of a basketball and can last for several minutes (web link)
• ______________ – tall sharp objects glow with a continuous barrage of strikes
(web link)
• ________ – short-lived electrical bursts from the tops of thunderstorms (movie)
• _________ – upward moving electrical jets from the tops of thunderstorms (movie)
• From space
*
• Rapid expansion of air due to excessive heating creates a shock wave
• Lag between the stroke and resulting thunder due to speed differences between
light and sound
– Time in seconds / 5 yields distance in miles
*
*
*
Life cycle
•
•
– Contains only updrafts, no precipitation
Mature stage
–
•
*
*
– only downdrafts, light precipitation
• The whole cycle usually lasts less than 1 hour
Severe Thunderstorms
* Defined
•
•
•
* Mesoscale convective complex’s (MCCs)
* Mesoscale convective systems (MCSs)
*
*
Tornadoes
*
* Tornado characteristics and dimensions
• Size ranges from tens of yards to over 2 miles
3
*
*
• Typically last for a few minutes, some last for hours
• Movement = 30 mph over 2-2.5 miles
• Winds range from 40 mph - 325 mph
* Tornado formation
• Squall lines, MCCs, supercells, tropical cyclones
* Supercell tornado development
• Mesocyclone –
– Wind shear is necessary
– Updrafts lift the horizontal column of air
– 20% develop into tornadoes
•
– Lowered cloud base hangs down from the base of the cloud (back side
of storm)
• Funnel forms
–
• Ground contact indicates tornado
* The location and timing of tornadoes
• The U.S. has the most tornadoes
– Tornado Alley
– Texas = highest state frequency
–
• Late spring = greatest overall frequency
* Tornado damage
• Winds = greatest damage
– Flying debris = greatest injuries
•
– Rates the damage caused by tornadoes
* Fatalities
• Relatively few deaths
• Average of 760 tornadoes = people average 91 deaths
– Most associated with a few large tornadoes
– Mobile homes, autos sites of most deaths
– Safest area = building basement
* Watches and warnings
• Severe Weather Watch
– Favorable atmospheric conditions for severe weather development
• Severe Weather Warning
–
Hurricanes around the globe
* Atlantic
• No hurricanes exist in the Southern Atlantic
* Western Pacific –
• Has the highest frequency of storms and stronger systems
* Indian Ocean and Australia –
Hurricane characteristics
* Most powerful of all storms
4
• Sustained winds of (74 mph – 210 mph)
• Average diameter = (350 mi)
• Central pressure = ~ 990 mb to 870 mb
Exist on large scales
*
*
*
*
*
*
*
Hurricane structure
*
•
• Region of descending air
• Often cloud free
• Average diameter = 15 miles
• Easily detected by radar and satellite
* Eye wall –
• Most intense winds and rain
* Cloud/rain bands –
• Heavy bands of thunderstorms around the eye
• Often contains weak tornadoes
• Easily detected by radar
Hurricane intensity scale
*
Hurricane Formation
* Tropical disturbance (wave) – disorganized storms with little or no circulation
• most come from Africa
* ____________________ – disturbances intensify and establish one closed isobar
• Winds < 37 mph
* ________________________ – increased organization
• Winds < 74 mph
* Hurricane (typhoon, cyclone) – strong circulation
•
Necessary Conditions for Hurricane formation
*
* Unstable atmosphere without vertical shear
* Upper level outflow enhances storm development
Other hurricane notes
* Most frequent in late summer and early autumn
*
*
Hurricane Destruction and Fatalities
* Wind – gusts can exceed 200 mph
* Heavy rain – rainfall can exceed 30 inches
• Flooding creates the most fatalities
* Tornadoes - Usually form in right front quadrant
*
• Rise in water level associated with pressure drop
• Strong winds blowing from the same direction cause water to rise up
End of Chapter 19
5