Lecture 5: Severe Weather

What is the Energy Balance on Earth?

There is an equilibrium between incoming radiation and outgoing radiation

• Earth intercepts only a small portion of the Sun’s total radiation

• It is this energy from the Sun that drives the hydrologic cycle and all the weather phenomena on Earth

The atmosphere is composed of what? (%s)

78% Nitrogen

21% oxygen

1% water vapour, carbon dioxide, ‘trace gases’

What in the atmosphere leads to cloud development and precipitation?

Water vapour

What causes water vapour?

evaporation from large bodies of water

Structure of the atmosphere

Clouds are in what layer of the atmosphere?

Troposphere

Where is the ozone layer found?

Stratosphere

What does the prefix of a cloud describe?

Height

What does the suffix of a cloud describe?

Appearance

Prefixes & Suffixes 

Prefixes:  Suffixes:

High cloud:   cirro-  Puffy:   -cumulus

Mid-level cloud:   alto-  Flat:   -stratus

Low cloud:   strato-

Clouds that produce precipitation contain what is their name?

“nimb”

What is a cumulonimbus cloud?

A cloud that produces lightening, thunder, and heavy rain

What is a nimbostratus cloud?

A cloud that produces prolonged light to moderate precipitation

Clouds diagram

What is a front?

A boundary between two air masses

• The name of the front describes the air behind it

• Fronts generally move from west to east

• At a cold front, dense cold air undercuts warm air

• At a warm front, the less dense warm air overrides cold air

Is this image a cold front or a warm front?

Clod front

• the air behind it is cold 

• arrows point to direction of movement (west-east)

Colf front vs Warm front (diagram)

Thunderstorm development requires what 3 things?

1. water vapour

2. a large difference in temp between air at the ground and air aloft

3. rising air (or a lifting mechanism, i.e. a front)

What are the 3 stages of thunderstorm development?

1. Cumulus

2. Mature

3. Dissipative 

Most pass through all 3 stages in 1 hr

What is the only cloud that can produce hail?

Cumulonimbus

How is hail produced?

Updrafts in the cloud repeatedly force a water droplet upward

• The droplet develops a ring of ice around it each time it enters the cold part of the cloud

• the ball of ice eventually becomes heavy enough to fall to the ground

What is lightening?

A spark of electricity occurring in a cloud

What causes thunder? 

Lightning heats the air causing the air to expand thus creating a shockwave (thunder)

• Sometimes the atmosphere refracts thunder making it inaudible

What is the main requirement for lightening?

A cumulonimbus cloud containing a region of opposite charges

• The interaction of ice crystals, hailstones, and water droplets result in a separate distribution of charges in the cloud

What is a tornado?

A rotating column of low-pressure air touching the ground that forms within a supercell thunderstorm

• different from a ‘funnel cloud” (rotating column that doesn’t touch ground)

What are the 4 characteristics of tornadoes?

• Most range between 100 and 200 metres wide

• They travel from the southwest toward the northeast at an average speed of 50 km/h

• They tend to exist for <20 mins with a defined life cycle

• The most common season for tornadoes is summer in Canada and Spring in the U.S.

What are the 3 stages tornadoes exist in?

1. Organizational stage

2. Mature stage

3. Rope stage

What occurs in the organizational stage of a tornadoes life cycle?

Wind shear causes rotation to develop

• A funnel cloud protrudes from above

• Dust and debris rotate beneath

What occurs in the mature stage of a tornadoes life cycle?

Most severe damage occurs

What occurs in the rope stage of a tornadoes life cycle?

The tornado stretches out and weakens

Tornado formation diagram

How do we classify tornadoes?

Enhanced Fujita Scale (EF)

• Tornadoes are classified on a scale of EF0 to EF5, based on the damage produced

What classifies as an EF5 tornado?

• complete devastation

• Wind speed over 322 km/h

• Less than 1% of tornadoes

What 2 places experience the most tornadoes on Earth? (tornado alleys)

1. U.S.

2. Canada

Where is the U.S. tornado alley?

Kansas & Oklahoma

Where is Canada’s tornado alley?

Southwestern Ontario

Why do tornado alleys exist?

• They are areas where air asses commonly collide

• They are areas of relatively flat land (this allows for undisturbed rotation)

Why does Canada’s tornodo alley exist?

• Tornadoes in Ontario occur when a southwesterly wind brings warm, moist air from the Gulf of Mexico

• The warm air may interact with cooler lake breezes

Notable tornadoes: Super outbreak

• April 3, 1974

• On this day 148 tornadoes touched down between Ontario and Alabama

• one of the strongest tornadoes in the outbreak killed 23 people

Largest tornado outbreak in history:

2011, April 25th - 28th

• In the southeast U.S., 358 tornadoes touched down

• 324 people were killed (mainly in Alabama)

• Overall, there were more deaths from tornadoes in 2011 than any other year since 1925

Notable tornadoes: Joplin tornado

May 22, 2011 - An EF5 tornado caused 161 deaths in Joplin, Missouri

• This tornado was the costliest in U.S. history ($2.8 B) and the deadliest in the U.S. since 1947

Notable tornadoes - Goderich tornado

Killed 1 person and destroyed much of the town core on Aug. 21, 2011

• It was the first EF3 tornado to touch down in Ontario in 15 years

• The tornado was spotted over Lake Huron on RADAR and a warning was issued 12 minutes before it reached the town

Notable tornadoes - Moore tornado

On May 20, 2013, an EF4 tornado in Moore, Oklahoma caused 24 deaths

• A tornado warning was issued for the area 16 minutes in advance

  • Advances in weather technology have greatly improved warning time

What are the 2 types of cyclones?

1. Tropical cyclones

2. Extratropical cyclones

Tropical cyclones:

• These only form over warm water, usually at latitudes 5-30°

• They include hurricanes and typhoons

• They contain high winds, heavy rain, and storm surges

Extratropical cyclones:

• These form over land or water in temperate regions at latitudes 30-70°

• They are associated with fronts and are also called mid-latitude cyclones

• They contain rain, snow, freezing rain, etc.

What are the 4 stages of Tropical cyclone development?

1. tropical disturbance

2. tropical depression

3. tropical storm

4. hurricane 

Tropical disturbance:

A large low-pressure area with unsettled weather

Tropical depression:

An unorganized area of thunderstorms

Tropical storm:

An organized area of storms with wind of 65-119 km/h

Hurricane:

A circle-shaped low-pressure area with winds of at least 120 km/h

Tropical cyclones require what?

A water temperature of at least 26 ^oC

What are the 3 components of a hurricane? 

1. eye

2. eyewall

3. spiral rain bands 

What is the eye?

A region in the center with light winds and clear to partly cloudy skies

What is the eyewall?

A ring of intense thunderstorms that whirl directly around the aye

• Most destructive component

What are spiral rain bands? 

Rings of tall clouds and heavy rain that exist throughout the hurricane

Anatomy of a hurricane diagram

How do we name Atlantic Ocean Hurricanes?

Alternating male and female names are used in alphabetical order (5 letters are skipped)

• When the list of names is exhausted, the remaining storms are named after the letters of the Greek alphabet (in order)

• some names are retired if they produce notable damage (Ex. Andrew, Katrina)

How fast do hurricanes typically travel?

Very slowly → <20 km/h

What direction does wind rotate in a hurricane?

counter-clockwise

If a hurricane is moving northwest, where will its highest winds be located?

The northeast

What is the most devastating effect of hurricanes?

Storm surges

What causes storm surges?

powerful winds that create a rapid rise in sea level

How do we classify hurricanes?

Hurricanes are classified by the Saffir-Simpson Scale

• The classification is based on wind speed

Where in North America are the regions most at risk?

• Atlantic coast

• Gulf of Mexico

What is the hurricane season?

The official hurricane season ranged from June 1^st to November 30^th

• Many hurricanes occur in August and early September because this is when the water is warmest

Hurricanes in Canada: Hurricane Fiona

September 24, 2022

• It caused 2 deaths and was a Category 2 hurricane when it made landfall in Nova Scotia

Hurricanes in Canada: Hurricane Hazel

October 15, 1954

• It killed 81 people when intense flash floods in Toronto swept away homes

• No other natural disaster has caused that many deaths in Canada to this day

What is fog?

a cloud with its base at the Earth’s surface

How does fog occur?

It occurs at night when the air cools to the dew point (at which point water vapor condenses into droplets)

• Fog can also form when warm air moves over a cold body of water

In the Great Lakes region, what hazard has caused the most deaths?

Snowstorms

What causes most deaths from snowstorms?

Heart attacks from shovelling snow

What are the very specific conditions for blizzards?

• Wind of at least 40 km/h

• Snow falling or blowing snow occurring

• Visibility less than 400 m

• All of these must occur for at least 4 hours

“the rule of 4’s”

What is lake-effect snow?

Snow caused by cold air moving over relatively warm water.

• Heavy snow falls downwind of lakes

• Snow belts are found downwind of the lakes (in Winter, the wind is often from the northwest)

Where in Southern Ontario typically receive lake effect snow?

London & Kitchener - Lake huron

Windsor - Lake Michigan 

• why these places have high annual snowfall

When do lake effect clouds diminish?

When ice appears on lakes

What is a haboob?

A sandstorm that occurs in arid and semi-arid regions

What causes a haboob to form?

Downdrafts on the leading edge of a thunderstorm

What is a dust devil?

a small spinning vortex of air formed over hot, dry land

• Goes from ground up (opposite of tornado)

What causes a dust devil to form?

• As hot air rises, the wind direction may change due to an obstacle

• This may result in a spinning column of air

What causes ice storms?

Mainly caused by freezing rain

• The weight of the ice can pull down trees and power lines

• Freezing rain is rain that freezes as soon as it lands on a surface

What are droughts? 

an extended period of unusually low precipitation

What hazard effects more people in North America than any other? How?

Droughts

• They cause water shortages that can lead to crop failure

• In developing countries, this may lead to malnutrition and famine.

What is wind chill?

a correction factor to an air temperature caused by the presence of wind making the air feel cooler than the temperature suggests

What is humidex? 

a correction factor to a temperature reading caused by high humidity making the air feel warmer than the temperature suggests

Alerts are broken into 3 categpries:

1. Watch

2. Warning

3. Advisory

What is a watch?

• An alert covering a wide area

• Conditions favour the development of hazardous weather, but has not been reported

Ex: tornado watch, winter storm watch

What is a warning?

• An alert that usually covers smaller areas

• It indicates that hazardous weather is currently occurring in the area

• More severe → actually happening

Ex: severe thunderstorm warning

What is an advisory?

It is issued to alert the public of less hazardous weather conditions

Ex: dense fog advisory