Lecture Notes 21 Extreme Weather II .doc

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11 Apr 2012
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Lecture 21
Extreme Weather II
Dramatic Thunderstorms
Warm Air
Expands in volume
less dense
low pressure
Rises
Holds more humidity
Molecules move more
Unstable
Cold Air
Contracts in volume
denser
high pressure
sinks
holds less humidity
molecules move less
more stable
Thunderstorm development
Meteorological event associated with a localized storm cloud producing thunder
and lightning
Thunderstorms are composed of individual cells developing over 20-30 minutes
As one cell dies, another may develop nearby
What is the 3 stages of development?
1. Cumulus stage
Thunderstorms occur when warm and moist air is lifted upward
Mechanical lifting
Cold, dense air undercuts warm, less-dense air
Thermal lifting
Warm, less-dense air flows up and over cold, dense air along a gentle
slope
Updraft of warm and moist air--> condensation of water vapour--> release of la-
tent heat--> cloud becomes warmer
Warm and moist air continues to rise as long as it is less dense than the sur-
rounding air
No precipitation yet
2. Mature stage
Cloud reaches maximum vertical development
Ice crystals and water droplets become too heavy to be supported by the up-
drafts
Downdrafts
Heavy precipitation
Thunder and lightning
3. Dissipating stage
Precipitation-induced downdrafts throughout the cloud
Cloud sinks and shrinks
Light rain
Thunderstorms in Canada Western Canada
Frequent in Alberta, east of the Rockies
Mountain peaks are heated by the sun, producing updrafts of warm air
Warm air holds humidity from the Pacific coast
Surrounding air is cold
Thunderstorms in Canada Eastern Canada
Frequent in Southern Ontario
High humidex
Distribution of thunderstorms in Canada
number of days with at least one cloud-to-ground lighting bolt per 20 km x 20
km
What other weather phenomena is related to thunderstorms?
Gust of winds
Lighting and thunder
Hail
Tornadoes
Flash floods
Why does lighting occur?
Electrical unbalance
Charges separate during the development of cumulus
positive region at the top
negative region at the base
When electrical unbalance between positive and negative regions becomes too
strong
lighting occurs
What is the 4 steps for the development of a lighting bolt?
1. initiation : charge in balance
2. Stepped ladder: negative charges move downward in intermittent steps
3. Connection: a positive discharge leaps up from the ground
4. return stroke: connected path flashes bright during charge exchange between
cloud and ground
Where can lighting bolts can travel?
between points within a cloud
from a cloud to clean air
from a cloud to an adjacent cloud
What is thunder?
high temperature of lightning bolt causes surrounding air to expand explosively
Explosion creates sound waves
What is Hail?
Hail forms when updrafts carry water droplets high into extremely cold regions of
the troposphere
Hailstone fall when they become too heavy to be supported by the updraft
fall to the ground at speeds
v< 100 km/h
What is hailstones?
Accretionary onion-shell structure
hailstones add most of their mass during updrafts
Hail in Canada
Both Alberta and Southern Ontario have frequent thunderstorms
Hail is much more frequent in Alberta than in Southern Ontario
In Alberta: abundant cold air supply from the Rockies
Favourable to hailstone formation
In Southern Ontario: not enough cold air
What are Tornadoes?
Funnel cloud coming out of a cumulonimbus and extending toward the ground
with air spinning at high speed
Small in scale
Strike quickly
Violent
Wind speed can exceed 100 km/h
Tornado formation Regional scale
In North America, 3 air masses moving in different directions spin a developing
thundercloud
Low-altitude warm, moist air from the Gulf of Mexico
Mid-altitude cold air from Canada or the Rockies
High-altitude jet stream
Supercell thunderstorm scale
Vertical updraft movement
Single-cell thunderstorm
Updraft movement tilted by strong winds
Might lead to the development of a supercell thunderstorm
Most tornadoes are associated with supercell thunderstorms
Cumulonimbus cloud
Taller than cumulus cloud
Abundant ice crystals in top region
Often develops an anvil shape
Most tornadoes are produced within cumulonimbus clouds during supercell thun-
derstorms
Why do only some cumulonimbus clouds form tornadoes not fully understood
Single-cell thunderstorm - cumulus cloud
Supercell thunderstorm - cumulonimbus cloud
Tornado formation
Vortex scale
Alternative hypothesis of tornado formation at vortex scale
Colliding cold and warm air create a horizontal vortex
Updrafts lift vortex upwards
As vortex is uplifted and stretched, its spin velocity increases
Fujita scale
Fujita scale ranks tornadoes by:
1. Damage: “sfter the fact" ranking
2. Wind speed
What are the six levels of increasing severity?
– F0, F1, F2, F3, F4, F5