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The Nature of Storms. Chapter 13. 13.1 Thunderstorms. At any given moment, nearly 2000 thunderstorms are occurring around the world. . Abundant source of moisture in the lower levels of the atmosphere A mechanism to lift the air to allow moisture to condense and release latent heat

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13 1 thunderstorms
13.1 Thunderstorms

At any given moment, nearly 2000 thunderstorms are occurring around the world.

conditions of formation of thunderstorms

Abundant source of moisture in the lower levels of the atmosphere

  • A mechanism to lift the air to allow moisture to condense and release latent heat
  • The portion of the atmosphere where clouds grow must be unstable, to allow rising and cooling of air necessary to stop the cloud growth.
Conditions of Formation of Thunderstorms
13 2 severe weather
13.2 severe weather

Occasionally weather events come together in such a way that there is a continuous supply of surface moisture, causing more severe storms.

Lightning is electricity caused by the rapid rush of air in a cumulonimbus cloud.

A lightning bolt forms when friction between updrafts and downdrafts separates electrons from some of their atoms creating positive ions and negative ions.

This creates regions of air with opposite charges.

A channel of negatively charged air moves toward the ground and a channel of positively charged ions rushes upward from the ground to meet it, creating an illumination called lightning.


Lightning heats the air to around 30000°C, whichis five times HOTTER than the surface of the Sun!

  • As the super-heated air rapidly expands and contracts, it creates a sound called thunder.
  • Because sound travels more slowly than light, you typically see lightning before you hear thunder even though they are actually generated at the same time!
other dangers of thunderstorms

Hail: precipitation in the form of balls or lumps of ice, occurring most frequently in the central US. Most common during Spring

  • Flood: occur when a thunderstorm moves slowly over one location, dumping all its rain in one place
Other Dangers of Thunderstorms


A tornado is caused by wind shear, which is when wind speed and direction change suddenly with height– causing a horizontal rotation near the Earth’s surface

An updraft can shift this rotating air into the vertical position

As updrafts speed up the rotation, air pressure in the center decreases, creating a pressure gradient between the inside and outside of the tornado – creating violent winds

Although devastating, tornadoes typically last only a few minutes.

  • Before reaching the ground, it’s called a funnel cloud
  • Often associated with a super cell, which is the most severe thunderstorm
  • Air is made visible by dust and debris drawn into the swirling column or by the condensation of water vapor into a visible cloud
  • Over the area it covers, few storms on Earth can match a tornado’s violence
classification and distribution

The Fujita Tornado Intensity Scale ranks tornadoes according to the path of destruction, wind speed, and duration

  • Bottom range is F0 - winds up to 118km/hr
  • Top range is F5 – winds of more than 500km/hr
  • Most tornadoes do not exceed the F1 category

Tornadoes can occur at any time and at any place, although some times and locations are more conducive to their formation

  • Most occur in the spring, during late afternoon and evening, when temperature differences are greatest
  • Most common in the central United States due to colliding cP and mT air masses
  • More than 700 tornadoes touch down each year in the United States
Classification and Distribution
13 3 tropical storms
13.3 Tropical Storms

The most violent storm on Earth is within the calm, sunny tropics.


Tropical Cyclones

  • Large, rotating, low-pressure storm
  • The strongest tropical cyclones are known in the United States, Mexico, and the Caribbean as hurricanes
  • In the western Pacific Ocean, these storms are referred to as typhoons
  • People living near the Indian Ocean refer to these storms as cyclones


As water evaporates from the ocean, heat is stored in the form of latent heat

The heat is released as the air rises and water vapor condenses into clouds and rain

The rising air creates a low pressure system at the ocean surface and more air moves in to replace it

The Coriolis Effect causes the air to turn counterclockwise, resulting in the rotation of the cyclone

Air moving towards center rises and rotates faster, further decreasing pressure in the center

Tropical Cyclones / Hurricanes require two basic conditions to form:

  • Abundant supply of warm water
  • Some sort of disturbance to lift the warm air and keeprisingit

These conditions exist in all tropical oceans except the South Atlantic Ocean and the Pacific Ocean west of the South American coast because waters in these areas are cooler.

stages of a hurricane

Tropical Disturbance - beginning stages, causes the air to rise

Tropical Depression - when a disturbance acquires a cyclonic rotation around a center of low pressure

Tropical Storm - when winds of the depression reach 65km/hr

Hurricane - when winds of the storm reach 120km/hr

Eye - calm center of the storm

Eyewall - a band immediately surrounding the eye where the winds are the strongest

Stages of a Hurricane
classifying hurricanes

Saffir-Simpson Hurricane Scale - classifies hurricanes according to wind speed, air pressure in center, and potential for property damage

  • Ranges from Category 1 (74 mph winds) to Category 5(+155 mph winds)
  • At Category 3it is classified as a major hurricane
Classifying Hurricanes
hurricane hazards

Hurricanes cause a lot of damage, especially along coastal areas

  • Much of the damage results from violent winds
  • Strong winds move onshore and are responsible for another major threat, storm surges, which is when winds move a mound of water over land
  • Storm surges can be as high as 6m above normal sea level
  • Heat released through condensation of vast amounts of water vapor fuels hurricanes. This condensation produces great amounts of rain
Hurricane Hazards
13 4 recurring weather
13.4 Recurring Weather

Persistent or repetitive weather can negatively affect agriculture, transportation, and recreation.



  • Natural occurrence in which water spills over the sides of a stream’s banks onto land
  • Can be caused by long thunderstorms, hurricanes, and mild storms that stay in the same area for extended times
  • The main cause of thunderstorm-related deaths in the United States each year


  • A drought is an extended period of below average rainfall
  • Caused by large high-pressure systems in an area for an extended period of time
  • One of the most extreme droughts occurred during the 1930’s in the central United States (Dust Bowl)
Heat Wave

Cold Wave

A Cold Wave is an extended period of below normal temperatures, which can be caused by high pressure systems of continental polar (cP) or Arctic air

Wind-Chill: because wind transports heat away from the body, the temperature may feel cooler than it really is

The wind-chill factor tells how cold it actually feels to the human body

  • A Heat Wave is an extended period of above normal temperatures
  • Can result from long term high-pressure systems
  • Few clouds to block the blazing Sun
  • System barely moves because the air currents guiding the high-pressure system are weak
  • Increases rate of humidity
  • Can cause heatstroke, sunstroke, and/or death
heat index

Due to humidity, air may feel warmer than the real temperature

The heat index tells how warm it feels to the human body

Heat Index