Chapters 10, 11, and 12

Weather of Middle Latitudes

Severe Weather

Tropical Systems

Air Masses• Types

– 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– 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 – 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