lecture 18: hurricanes and tornadoes Flashcards
tropical cyclones
most destructive of weather disasters, affecting huge areas (thousands of km2) with 3 types of hazard:
- high winds (up to 240km)
- storm surges (raising sea level up to 6m)
- heavy rains and subsequent flooding (landsliding)
typhoon mangkut
2018- southern coast of china
120km/h winds
hurricane michael
oct 2018, florida
hurricane formation
tropical cyclones are storms with 1 minute sustained winds >120km/h, form between 5-20 degrees N/S
subdivision of tropical cyclones geographically
hurricanes: south atlantic and mexico
typhoons: southeast asia
cyclones: southwest indian and australia
why arent tropical cyclones formed
S atlantic: high winds
SE Pacific: insufficient water temperature
red sea/persian gulf: insufficient surface area
equator: insufficient coriolis effect
hurricane formation
start by summer heating of tropical oceans with a) warm shallow water >27-50 degrees to enhance evaporation b) unstable air for thunderclouds to form c) weak vertical wind shear to stop storm from breaking up
4 stages of hurricane formation
- tropical disturbance low pressure draws in surrounding thunderstorms with weak winds
- storms coalesce into tropical depression and winds strenghten (37-63km)
- tropical storm has stronger winds that start to form cyclonic pattern, can now be named
- tropical cyclone has winds >120km/h that cannot reach the core due to rotation, forms eye of storm
path of hurricane
follows a curve due to coriolis effect (earths rotation)
- deflected to the right in northern hemisphere and left in southern hemisphere
hurricane decline
winds decline when warm water energy source is cut off as they go over land or reach colder waters
- can interact with other weather systems in mid-latitude and gain strength again
typhoon freda
Oct 1962, killed 46 people in US and 7 in canada
- came up west coast mass of water vapour blew up
- took 2 weeks to travel 8000km across the pacific
3 steps of hurricane
- air moves towards low pressure eye over warm sea, evaporating water
- humid air spirals up the eyewall. Rising water vapor cools and condenses, forming clouds which releases latent heat of condensation intensifying the updrawft
- cold air is sucked down the eye
rotation of hurricanes in hemispheres
anticlockwise in northern
clockwise in southern
eye and eyewall
eye: circular region of calm clear weather at hurricane centre
eyewall: surrounding cyclinder of upward spiralling winds that are the strongest in the entire hurricane
hurricane energy
storm motion speed along its track is added or subtracted from storm wind speed to give the actual wind speed experienced on the ground
storm motion is 30km/h: storm winds 160km/h safe semicircle is 130, unsafe is 190km/h, might be 200km/h in eye
what happens when eyewall passes overhead
intense winds as eyewall passes overhead followed by winds dying away as the eye itslef passes overhead
after eye passes, sudden intense eyewall winds in opposite direction
how is storms energy calculated
terajoules (1TJ = 1,000,000,000,000J)
based on wind speed and size of wind field
hurricane scale
strength measured on Saffir- simpson scale
C1: minimal, winds damage trees & mobile homes
C2: moderate, trees blown down and major damage to roofs and mobile homes
C3: extensive, large trees down, mobile homes destroyed and structural damage to small buildings
C4: extreme, all signs blown down, damage to windows, doors, roofs, coastal builidngs and flooding
C5: catastrophic: damage to windows, doors and roof, small buildings turned overhead and blown away
north atlantic hurricane season
1 jun - 30 nov (peak in mid aug to late oct)
lifespan: 1-30 days
diameter: 200-1300km
eye: 16-70km
north atlantic hurricanes
start at 10-20N, move westward by easterly trade winds than northwards by coriolis effect and eastwards at midlatitudes by westerly winds
- depending on size it will take different paths
subtropical ridge or bermuda/azores high
high pressure zones that can furthur influence path of north atlantic hurricanes
hurricane mitigation
land use planning: avoid population centres in low lying coastal lands
securing roof, eliminate eaves from roof, strap roof to walls in attic, no thin ashfalt roofs, building codes for mobile homes
tornadoes
most intense of all atmospheric phenomenon with winds up to 500km/h
can damage buildings, blow off roofs, blow buildings off foundations and demolish buildings , can batter people and buildings up with projectiles
tornado formation: single cell thunderstorm
warm moist air rises vertically in single updraft
moisture condenses from coolign air
falling rain causes cool downdraft surpassing updraft
limits thunder-storm intensity and reduces chances of tornado
tornado formation: supercell thunderstorm
wind shear shifts updraft, tilting the thundercloud into anvil
seperates warm up and cool down
allows for more intense thunderstorms and higher chance of tornado forming
- responsible for tornado outbreak in may 2013 oklahoma
where are the most tornadoes int he world
US with 1200 per year
- mostly in Texas and midwest (tornado alley)
canada is at the end of the alley and experiences 100 per year
why do many tornadoes form at mid west
- high altitude, high speed jet stream winds from west
- mid altitude cool dry air from northwest
- low altitude warm moist air from gulf of mexico to south
- converging air masses cause atmospheric instabilities and intense rotation/thunderstorms
tornado fujita scale
6 level ratin based on: wind speed,path length, path width, storm damage
F0: light, >half all tornados, short length/width
F1: moderate, 1/4 of tornados, damage to roofs sheds and garages, push cars of roads
F2: considerable, 1/6, roof and chimneys torn off, vehicles lifted off ground and large trees snapped
F3: severe, <5%, uproot forests, roofs blown off, metal building collapse, heavy vehicles lifted
F4: devastating, <1%, steel/concerete buildings lose walls, cars tossed 100m, bark stripped from trees
F5: incredible, rare, few seconds but incredible strength, carries vehicles and homes great distances
