Chapters 10, 11, and 12
Weather of Middle Latitudes
Severe Weather
Tropical Systems
Air Masses
• Types
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cT – continental tropical
mT – maritime tropical
mP – maritime polar
cP – continental polar
A – arctic
• Air masses are modified three ways
– Exchange of heat or moisture
– Radiational heating or cooling
– Adiabatic heating or cooling with vertical motion
Fronts
• Stationary
– No lateral movement
– Wind blows approximately parallel to isobars
– If precipitation occurs it is light and it occurs on the
cold side
• Warm
– Cold air retreats and warm air advances
– Widespread steady precipitation ahead of front
– Light drizzle and fog along the front
Fronts
• Cold
– Cold more dense air displaces warm less dense
air
– Slope of the front is much steeper, so in warm
unstable air there is significant lift and storms
– Squall Line
• A band of intense thunderstorms that develop along
or ahead of a cold front
– If warm air is stable precipitation is brief and
showery in a narrow band close to the front
Back Door Cold Front
Fronts
• Occluded
– Cold Occlusion
• Air behind the advancing cold front (cP) is colder
than the cool air ahead of the warm front (mP)
– Warm Occlusion
• Air behind the advancing cold front (mP) is
relatively mild compared to cold air ahead of the
warm front (cP)
– Neutral Occlusion
• No temperature change, but showers present and a
shift in winds
Extratropical Cyclone
• A Low pressure system that
is a major weather maker for
mid-latitudes
• Cyclogenesis – the birth of a
cyclone
• An extratropical cyclone has
4 stages
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Incipient
Wave
Occlusion
Bent back occlusion
Extratropical Cyclone
• Triple Point
– Point where occluded, cold, and warm front meet
– Sometimes a secondary cyclone can form here
• Bomb
– A rapidly developing extratropical cyclone
– Central pressure drops at least 24mb in 24 hours
• Cyclolysis (Filling)
– When the central pressure in the low begins to rise
– Death of a cyclone
Idealized Mature Cyclone
Principal Cyclone Tracks
Circulation Systems
• Land/Sea Breezes
• Chinook Winds
– Foehn, Zonda, Santa Ana
• Desert Winds
– Dust devil
– Haboob – caused by strong thunderstorm
downburst
• Mountain/Valley Breezes
• Sea Breeze
• Land Breeze
Santa Ana Winds
Dust Devil
Haboob
• Valley Breeze
• Mountain Breeze
Thunderstorm Life Cycle
Thunderstorm Classification
• Single Cell Thunderstorm
– “pop up” storms in warm humid air masses that are shortlived
• Multicell Thunderstorms
– Lightning, thunder, and rain that persist for many hours
– Each cell may be at a different stage
– Two types
• Squall line
• Mesoscale Convective Complex (MCC)
– An area of many interacting thunderstorm cells (very large area)
• Supercell Thunderstorm
– Strong updraft with rotation that may spawn a tornado
Conditions for Thunderstorms
• Humid air in the middle to lower
troposphere
• Atmospheric instability
• A source of uplift
Severe Thunderstorms
• Must have at least one of the following
– Hailstone greater than ¾” in diameter
– Tornadoes or a funnel cloud
– Surface winds greater than 58 miles per hour
• For Development
– Vertical wind shear
– Mature synoptic scale cyclones
Thunderstorm Hazards
• Lightning (thunder)
• Downbursts
– Macro (>2.5mi, winds ~ 130mph, 30 min)
– Micro (<2.5mi, winds ~ 170mph, 10 min)
• Flooding
• Hail
• Tornado
Tornadoes
• Violently rotating column of air that is in contact
with the ground made visible by condensation,
dust, and/or debris
• Most violent, short lived, lots of damage
• 10% of severe tstorms produce tornadoes
• April 3-4 1974 – 148 in 13 states, 315 deaths,
$600 million in damage
• May 3 1999 – 70 in 3 states, 55 deaths, $1.1
billion in damage, (max wind = 318mph)
• March 18, 1925 – 1 in 3 states, 695 deaths, 3.5
hours, path 219 miles
Average Annual Number of Tornadoes Per 10,000 square miles
Why Tornadoes Are Dangerous
• Extremely
high winds
• Strong
updraft
• Subsidiary
vortices
• Abrupt drop
in air pressure
Hurricane
• A violent tropical cyclone that originates over
tropical ocean waters with maximum sustained
wind speed greater than 74mph
• Different from extratropical cyclone
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–
–
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Smaller, but more intense (lower central pressure)
No fronts
Upper air flow is anticyclonic
Presence of an eye and and eye wall
Hurricane Season in the western Atlantic is June 1 to
November 30, with the peak being September 10
Hurricane Hazards
• Heavy rains and floods
– Some rain can be extremely beneficial,
especially if suffering from a drought
• Strong winds
• Tornadoes
• Storm Surge
• 60% of all hurricane deaths are due to
flooding (1970-1999) – before 1970 storm
surge was major cause
Storm Surge
Saffir-Simpson Hurricane
Intensity Scale
Life Cycle of a Hurricane
• Tropical Disturbance – organized cluster of
cumulonimbus clouds over tropical seas
with a detectable low pressure center
• Easterly Wave – a ripple in the trade winds
featuring a weak trough of low pressure
• Tropical Depression – winds > 23mph
• Tropical Storm (name) – winds > 39mph
• Hurricane – winds > 74mph
Hurricane Tracks
Hurricane Gilbert - 1988
Hurricane Mitch - 1998