15.3
4.2.6
SEVERE WEATHER
Explain the conditions and mechanisms for the development and behaviour of thunderclouds, hurricanes, and tornadoes.
All severe weather events begin with warm, light air rising rapidly to very high, cold levels causing an unstable updraft.
The earth uses storms to release energy into the atmosphere from the heat energy stored in warm water and air.
THUNDERSTORMS
Thunderstorm
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a storm event that includes lightning discharges, strong gusting winds, and torrential rain.
Life Cycle of a Thundercloud (cumulonimbus)
Stage 1: Cumulus Cloud Formation

warm, moist (mT) air mass of a Low pressure system rises rapidly over a colder (P) air mass (High pressure)

updrafts (Low pressure) pull in more warm, moist air, push this humid air very high to form thick cumulus clouds
Stage 2: Mature Thunderhead

updrafts continue to pull humid air so high up that it freezes (fusion)

fusion (liquid to solid) is exothermic and releases heat, causing the air to rise even higher, increasing the updraft

turbulence in the cloud causes water droplets and ice crystals to mix together, forming larger droplets, possibly hail

heavy rain begins when updrafts cannot keep the large water droplets / ice crystals suspended

the falling rain cools the air and pulls the air with it causing downdrafts (High pressure)
Lightning

turbulence in the cloud also causes electrons to be transferred from one droplet/crystal to another
o
o
o
o
positive (+) ice crystals on the top and negative (–) water droplets at the bottom
Excess electrons in the bottom of the cloud can either go up to the top of the cloud or down to the ground
The lightning bolt superheats the air causing it to expand quickly, producing a high pressure wave
The pressure wave moves faster than the speed of sound, producing a sonic boom = thunder
Stage 3: Final Stage

updraft stops and the rain and downdraft remove the remaining energy of the stormcloud
TORNADOES
Tornado
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a violently rotating column of air that is in contact with the ground
usually forms a rapidly swirling cumulonimbus cloud (funnel cloud)
 wind speeds between 65 and 175 km/h
 only about 75 m to 4 km wide
Wind Shear
difference in wind speed and wind direction in a short distance
occurs near fronts, mountains, and the jet stream
Tornado Alley
central US (Oklahoma, Kansas) up to southern Ontario
Formation
areas where many warm moist (mT) air (Low) meets cold dry (cP) air (High)
wind shear conditions are produced at these fronts
wind shear causes a horizontal spiral that becomes a vertical updraft
Water spout
tornado over water
HURRICANE
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Tropical Cyclone

a tropical low pressure system  an organized system of clouds and thunderstorms

has counterclockwise rotation in the Northern Hemisphere  anticyclonic rotates clockwise in NH

produces thunderstorms, strong winds, heavy rains, and storm surges.
Types  defined by maximum sustained winds and geographic location

Tropical Depression
has sustained winds over 37 km h–1

Tropical Storm
has sustained winds over 65 km h–1

Hurricane
has sustained winds over 120 km h–1
o Hurricane
Atlantic Ocean (Caribbean, Florida, Carolinas) and eastern Pacific Ocean (Hawaii, California)
o Typhoon
western Pacific Ocean (Philippines)
o Cyclone
everywhere else = Southern Hemisphere, Indian Ocean
Development

Equatorial surface water of the Atlantic Ocean near Africa are heated by the sun

Atlantic Ocean currents move these waters westward toward the Caribbean

Deep warm ocean water supply energy for hurricane – over 26C to a depth of 60m

The warm moist air rises rapidly up to tropopause  low pressure system with cumulonimbus clouds and intense updrafts

Coriolis Effect causes the rising air to spin counter-clockwise (cyclonic)
The storm continues to intensity as energy from warm deep ocean water is transferred to the atmosphere
The storm loses energy over land and over cooler water (no warm water), or if there is wind shear (cuts off cloud top)
Features
Eye
calm central area of the hurricane
Eye Wall
ring of cumulonimbus clouds, very low pressure, and intense winds
Storm Surge
high sea levels (up to 20m higher than normal) caused by reduced air pressure  flooding
4.2.7
Compare tornadoes and hurricanes in terms of size, duration, and damage caused.
Tornado – Hurricane Comparison
Tornado
Wind Speed
Size
Duration
Destruction
h–1
65 – 500 km
compact  less than 1 km wide
short life  less than 30 minutes
very intense in a small area
Hurricane
120 – 250 km h–1
huge  500 km wide
long life  several days
intense in a large area  more damage