Earth Science

Atmospheric Processes

Severe Weather

Study Tip
Hurricanes, typhoons, tornadoes, and cyclones… These terms can be confusing to differentiate from one another. A cyclone is the umbrella term that includes tropical storms and typhoons. Tornadoes and cyclones are considered different types of storms.


Thunderstorms form when ground temperatures are high, typically during late afternoons and early evenings in the summer.

The formation of a thunderstorm can be broken down into three stages:

Study Tip
Hurricanes, typhoons, tornadoes, and cyclones… These terms can be confusing to differentiate from one another. A cyclone is the umbrella term that includes tropical storms and typhoons. Tornadoes and cyclones are considered different types of storms.
  • Sun heats earth’s surface
  • Hot air rises
  • Air is moist, cumulus clouds form vertically
  • Cumulus cloud becomes large
  • Water within cloud becomes heavy
  • Cool, dry air enters cloud and begins to descend down the cloud
  • Downdraft pulls heavy water downward
  • Downdrafts in the cloud begin to dominate over updrafts
  • Since warm air can no longer rise, clouds stop forming
  • Convection ceases and water vapor does not condense
  • Storm begins to dissipate and clouds disappear from bottom to top

Thunderstorms: Thunder and Lightning

  • Thunder storms can form individually or as part of a squall line along a cold front.
  • Large amounts of energy collected within cumulonimbus clouds is released as electricity called lightning.
  • The rapid heating of the air surrounding a lightning strike produces a loud clap of thunder, which is a result of the rapidly expanding air.
Formation of a  thunderstorm
The three stages in the formation of a thunderstorm

Earth Science



  • Tornadoes form at the front of severe thunderstorms
  • Tornadoes generally last only a few minutes
  • When tornadoes form over water, they are referred to as waterspouts


  • Tornadoes are typically products of severe thunderstorms.
  • As air in a thunderstorm rises, the surrounding air races in to fill the gap,
  • This forms a funnel-shaped whirling column of air that extends down to the earth from the thundercloud.


  • Damages a small area but can destroy everything it passes
  • An average of 90 people are killed by tornadoes each year


  • Tornadoes form during the spring where maritime tropical and continental polar air masses meet.

Tornadoes: Measurement

The Fujita scale measures the intensity of tornadoes based on wind speed and damage.

F Scale
Light - tree branches fall and chimneys may collapse
Moderate - mobile homes, autos pushed aside
Considerable - roofs torn off houses, large trees uprooted
Severe - houses torn apart, trees uprooted, cars lifted
Devastating - houses leveled, cars thrown
Incredible - structures fly, cars become missiles
Maximum tornado wind speed

Earth Science

Differences between Cyclones and Tornadoes

Hundreds of meters
A few seconds to a few hours
Maximum tornado wind speeds can reach 318 mph
Tropical cyclone (hurricane)
Hundreds of miles
5-10 days
Category 5 (strongest) wind speeds up to 156 mph

Mid-latitude Cyclones

A cyclone is any rotating low-pressure system. Cyclones rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Mid-latitude cyclones form at a polar front where temperature differences are high between two air masses.

Mid-latitude Cyclones: Formation

  • Warm air at the cold front rises and creates a low pressure cell
  • Winds rush into the low pressure cell, creating a column of air
  • Air twists counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere
  • Air is moist so rain or snow falls

Mid-latitude Cyclones: Nor’easters

Nor’easters are mid-latitude cyclones in the Mid-Atlantic and New England states.

Cross-sectional view of a hurricane
A cross-sectional view of a hurricane


Hurricanes are special types of cyclones that form in the tropics. They are also referred to as tropical cyclones.

Earth Science

Hurricanes: Formation

Hurricanes arise in the tropical latitudes in summer and autumn when sea surface temperatures are over 28ºC.

  • Warm seas create a large humid air mass.
  • Warm air rises and forms a low pressure cell called a tropical depression.
  • Air begins to rotate around the low pressure.
  • As air rises, water vapor condenses, releasing energy from latent heat.
  • If wind shear is low, the storm will build into a hurricane within 2-3 days

Hurricanes: Storm Activity

  • Hurricanes can cover 800km (500 miles) in one day.
  • Hurricanes that form in the Northern Hemisphere originate in the trade winds and move to the west. Hurricanes switch direction and travel toward the north once they reach latitude of the westerlies.

Hurricanes: Saffir-Simpson Scale

Hurricanes are categorized based on wind speed on the Saffir-Simpson Scale

Estimated Damage
1 (weak)
Above normal; no real damage to structures
2 (moderate)
Some roofing, door, and window damage, considerable damage to vegetation, mobile homes, and piers
3 (strong)
Some buildings damaged; mobile homes destroyed
4 (very strong)
Complete roof failure on small residences; major erosion of beach areas; major damage to lower floors of structures near shore
5 (devastating)
Complete roof failure on many residences and industrial buildings; some complete building failures


Blizzards are large snowstorms with high winds.


  • Temperatures below –7º C (20º F); –12º C (10º F) for a severe blizzard.
  • Winds greater than 56 kmh (35 mph); 72 kmh (45 mph) for a severe blizzard.
  • Snow so heavy that visibility is 2/5 km (1/4 mile) or less for at least three hours; near zero visibility for a severe blizzard.

Blizzards: Formation

  • Occur across the middle latitudes and towards the poles
  • Usually part of a mid-latitude cyclone
  • Commonly occurs when the jet stream has traveled south and a cold, northern air mass comes into contact with a warmer, semitropical air mass.
  • Pressure gradient between the low-pressure and high-pressure parts of the storm create strong winds.

Earth Science

Blizzards: Lake-Effect Snow

Lake effect snow occurs when an air mass reaches the leeward side of a lake. The air mass is very unstable and drops a tremendous amount of snow.

Heat Waves and Droughts

Heat Wave:

  • According to the World Meteorological Organization a region is in a heat wave if it has more than five consecutive days of temperatures that are more than 9º F (5º C) above average.
  • A high-pressure area sitting over a region with no movement is the likely cause of a heat wave.


  • When a region gets significantly less precipitation than normal for an extended period of time, it is in drought.
  • Consequences to droughts include dust storms, blown over soil, and wildlife disturbance.
Concept Check
  • Compare and contrast tornadoes and cyclones.
  • Describe how a blizzard is formed.
  • Describe the causes of both of the following natural phenomena: heat waves and droughts.