11 Apr 2012
School
Department
Course
Professor
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