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 – 148 in 13 states, 315 deaths, $600 million in damage May – 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 ( ) – 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

Hurricane Mitch