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Lecture 5

EESA09H3 Lecture 5: lecture 5 note

Environmental Science
Course Code
Tanzina Mohsin

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Wind Lecture 5 Thunderstorms and Tornadoes
What is a thunderstorms?
It is a convective storm (associated with surface heating)
Common in southern Ontario in summer
Only storm have thunder and lightning
large thunderstorms can form tornadoes
Four Categories of thunderstorm
1. Ordinary
o Often develop within large air masses (midlatitude cyclone)
o Not necessarily near a frontal system
o Can form with little vertical wind shear(gradient/change of
horizontal wind speed with height)
Three stages
a. Cumulus
o association with clouds (thunder clouds)
o with heated surface, air rises
o Rising air cools, condense and form clouds
o Near 0 degrees, they form cumulonimbus
b. mature stage
o Rising air, more clouds extending upper part of tropospheric region
o Downward flow of wind forming downdraft with precipitation
o Gust front forces more air up into the updraft.
o Updraft and downdraft form a convective cell.
c. Dissipating stage
o When the Gust front moves past the updraft, the updraft weakens
o Rain starts to fall into updraft , cutting off the rising humid air
o Downdraft cuts off updraft and storm loses energy source and
o < 1 hour
o Diameter : 1km or less
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2. Multi-cell thunderstorm
Similar to ordinary except moderate wind shear (meaning storm tilts)
Storm tilts (downdraft tilted)
Downdraft forms downwind of updraft
downdraft not going to cut the updraft that easily so last longer
gust front of one storm initiates or induces another storm
3. Supercell thunderstorm
Form with strong vertical wind shear (air masses involved)
- Surface wind (mT) from south/southwest
- Upper level winds (CP air) from north/northwest
Along cold front of a midlatitude cyclone
100-600m in diameter
Tornadoes can spawn
Downdraft does not cutoff updraft
Storms can last for several hours
Hail can form (solid precipitation)
Microbursts can also form
Gust front forming along the cold front of
midlatitude cycle
Associated with (cT & mP) air causing
more rain because it has all the energy
form the midlatitude cyclone system
The anvil extend of the cloud can course
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Little downburst of wind within a supercell thunderstorm (eg, shower
Localized downdrafts
- Radial burst of surface wind
Aviation hazard
- Airplane crash August 1985 at Dallas-Fort Worth Airport, 100 lives
4. MCC (Mesoscale convective complexes)
Multiple thunderstorms (eg, ordinary + multi-cell)
Circular fashion
Covers over 100,00 square km
12 hours or more , self-sustaining
Heavy precipitation
important for farmers (irrigation purposes)
characteristic of thunderstorms
Squall Line (characteristic of super thunderstorms)
String of thunderstorms along the cold front (chance for tornado
November 15, 1989
- The Huntsville (eg, tornado)
Hail and tornadoes formed
Special characteristic of thunderstorm
Unique, no other storm has lightning
Charge separation occurs in the cloud (charge is carried by the cloud
droplets or ice crystals)
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