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
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
Circulation Systems
• Land/Sea Breezes• Chinook Winds
– Foehn, Zonda, Santa Ana
• Desert Winds– Dust devil– Haboob – caused by strong thunderstorm
downburst
• Mountain/Valley Breezes
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
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 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
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
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