Earth image Storms - Thunderstorms, Tornadoes, and Hurricanes

Dr. Pamela Gore
Georgia Perimeter College


  1. Explain the causes of thunderstorms.
  2. Explain the causes of lightning and thunder.
  3. Be able to determine the distance to a thunderstorm.
  4. Discuss the dangers of thunderstorms.
  5. Discuss safety during a thunderstorm.
  6. Discuss tornadoes.
  7. Discuss when and where tornadoes are most likely to happen.
  8. Discuss safety during a tornado.
  9. Discuss the characteristics of a hurricane.
  10. Discuss the causes and types of damage from a hurricane.
  11. Discuss when and where hurricanes occur.
  12. Discuss how hurricanes are named.
  13. Discuss hurricane safety.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S1E1a. Identify different types of weather and the characteristics of each type.
  • S6E4b. Relate unequal heating of land and water surfaces to form large global wind systems and weather events such as tornados and thunderstorms.
  • S6E6a. Explain the role of the sun as the major source of energy and its relationship to wind and water energy.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Because the earth turns daily on an axis that is tilted relative to the plane of the earth's yearly orbit around the sun, sunlight falls more intensely on different parts of the earth during the year. The difference in heating of the earth's surface produces the planet's seasons and weather patterns.
  • The cycling of water in and out of the atmosphere plays an important role in determining climatic patterns. Water evaporates from the surface of the earth, rises and cools, condenses into rain or snow, and falls again to the surface. The water falling on land collects in rivers and lakes, soil, and porous layers of rock, and much of it flows back into the ocean.

This section addresses, in whole or in part, the following National Science Education Standards:
  • The sun is the major source of energy for phenomena on the earth's surface, such as growth of plants, winds, ocean currents, and the water cycle. Seasons result from variations in the amount of the sun's energy hitting the surface, due to the tilt of the earth's rotation on its axis and the length of the day.
  • Global patterns of atmospheric movement influence local weather. Oceans have a major effect on climate, because water in the oceans holds a large amount of heat.
  • Clouds, formed by the condensation of water vapor, affect weather and climate.

See photos of miscellaneous storm damage in Georgia, 1997-2007.

Thunderstorms and lightning

Image courtesy NOAA.

Thunderstorms occur in every state, and every thunderstorm is accompanied by lightning.

What causes thunderstorms?

The sun warms the Earth, and the Earth warms the air above it through the process of conduction. The warm air rises by convection. Warm air rising through cooler air continues to rise and is called unstable air. The warm air rises, and at some point, condensation occurs high in the cloud. Precipitation occurs (rain and sometimes hail), and a cold downdraft forms as the rain falls.

Thunderstorm formation requires three things:

Three types of thunderstorms:

Dangers associated with thunderstorms

All thunderstorms are dangerous. Every thunderstorm produces lightning, which kills more people every year than tornadoes. An average of 80 people are killed and 300 injured each year by lightning. In addition, heavy rain from thunderstorms can lead to flash floods, which are the number one cause of deaths associated with thunderstorms (more than 140 each year). Strong winds, hail, and tornadoes may be associated with some thunderstorms. Lightining also causes many fires each year.

Hail causes more than $1 billion in damage to property and crops each year. You do not want to own a new car dealership when there is thunderstorm with large hail. Several hundred new and used cars were damaged by hail in Gwinnett Co., GA in February 2005 - not to mention privately owned vehicles in the area.

See photos of large hail in Georgia February 21, 2005.

The flash of lightning and the boom of thunder occur at the same time. Lightning is seen before the thunder is heard, however, because light travels faster than sound.

By counting the number of seconds between the lightning flash and the sound of the thunder, you can tell roughly how far away the storm is. Every five seconds represents one mile. This results from the fact that light travels at about 186,000 miles per second, and sound travels at only about 1,100 feet per second (or about 0.21 miles per second). If the storm is a mile away, the lightning flash reaches you virtually instantaneously, but it takes about 5 seconds for the sound of the thunder to reach you.

Thunderstorms affect small areas when compared with large storms like hurricanes and winter storms. The typical thunderstorm is about 15 miles in diameter and lasts for about 30 minutes. Nearly 1,800 thunderstorms are occurring at any given time around the world, and more than 20 million lightning strikes a year.

What causes lightning and thunder?

The convection in the air within a thunderstorm separates positive and negative charges. Lightning occurs when the negative charges (or electrons) near the bottom of the cloud are attracted to the positive charges (or protons) on the ground. (The accumulation of electric charges has to be greater than the insulating properties of the air.)

When this happens, an invisible stream of negatively charged air streams down towards an object on the ground (such as a tree, boat, tower, chimney, person outdoors, etc.) where positive charges have clustered due to the pull of the negative charges in the thunderhead. An electrical connection is made and the protons rush up to meet the electrons in the clouds, producing the visible flash of lightning.

Lightning causes thunder because it heats the air. A bolt of lightning is incredibly hot and can heat the air to between 50,000 degrees Fahrenheit in a fraction of a second. That is hotter than the surface of the sun!

When the air is heated that quickly, it expands rapidly, and then collapses back together making a shockwave that results in the sound we call thunder. Lightning always produces thunder. If you see lightning but don't hear any thunder (so-called "heat lightning"), that means the lightning is too far away from you for the sound waves to reach you.

The average flash of lightning could turn on a 100-watt light bulb for more than 3 months.

Lightning can occur from cloud-to-ground, from cloud-to-cloud, within a cloud, or cloud to air.

Your chances of being struck by lightning are estimated to be 1 in 600,000. Those chances can be reduced if you follow safety rules. Most lightning deaths and injuries occur when people are caught outdoors, and most happen in the summer.

In the southeastern and western states, most thunderstorms occur during the afternoon. In the Plains states, thunderstorms are common in the late afternoon and at night.

Ball lightning appears as a glowing sphere which drifts horizontally through the air. It is typically the size of a grapefruit, but can be much larger or smaller. It may hover a few feet or tens of feet above the ground, and usually lasts only a few seconds, but sometimes persists much longer. Sometimes it disappears silently, and other times it explodes with extreme violence. It has been observed inside airplanes, submarines, and homes (and can pass through closed glass windows), and appears to be harmless. There is no accepted theory that explains ball lightning yet (because it has not yet been created in the laboratory), but there is much speculation and controversy.

St. Elmo's Fire is similar, but it remains attached to an object, rather than moving through the air. St. Elmo's Fire is a type of continuous electric spark called a "glow discharge," seen during thunderstorms when the ground below the storm is electrically charged, and there is high voltage in the air between the cloud and the ground. The voltage rips the air molecules apart, pulling the electrons and protons away from one another, and the gas begins to glow. St. Elmo's Fire is plasma or a glowing mixture of separate proton clusters and electrons. It may be related to ball lightning and is sometimes mistaken for ball lightning.

Important Terms

A severe thunderstorm watch is issued by the National Weather Service when the weather conditions are such that a severe thunderstorm is likely to develop. A severe thunderstorm has winds at least 58 miles per hour or hail at least 3/4 inch in diameter.) When you hear a thunderstorm watch, go to a safe place right away and listen to the radio or television for more information.

A severe thunderstorm warning is issued when a severe thunderstorm has been sighted or ideintified by weather radar. A warning is more severe than a watch. If you are not already in a safe place, get there at once! Listen to a battery-operated radio or television and wait for the "all clear."

Safety during a thunderstorm

Check this website for additional safety precautions.


Union City, Oklahoma tornado. Image courtesy NOAA.

What are tornadoes?

Tornadoes are the most violent type of storm, with wind speeds reaching up to 300 miles per hour. Tornadoes develop from powerful thunderstorms and appear as rotating, funnel-shaped clouds.

Tornadoes cause damage when they touch down on the ground. They snap the tops off of trees, blow off roofs, and may demolish buildings entirely. The path of a tornado may be as much as one mile wide and 50 miles long.

When and where do tornadoes occur?

Tornadoes can form any time of the year, but the typical season runs from March to August.

Tornadoes can occur in any state. But there is an area in the central U.S. which experiences more than the usual number of tornadoes. This ares is called "Tornado Alley", and includes Arkansas, Iowa, Kansas, Louisiana, Minnesota, Nebraska, North Dakota, Ohio, Oklahoma, South Dakota, Texas.

Important Terms

Tornado Watch -- Conditions are such that tornadoes are likely to form. Stay tuned to the radio or television news.

Tornado Warning -- A tornado has been sighted. Take shelter immediately!

What should I do if a tornado is coming my way?

Image courtesy NOAA.


Tornado in downtown Atlanta, GA, March 14, 2008.


Damage associated with the April 1998 tornado that struck DeKalb County, Georgia, and the Georgia Perimeter College Dunwoody Campus.
Upper left: Trees that have been broken off above the ground at the Georgia Perimeter College Dunwoody Campus.
Upper right: House with damaged roof, trees broken off about roof level, and piles of pine tree branches.
Lower left: House with damaged roof and piles of pine tree logs.
Lower right: Car that took a direct hit from a pine tree.
Images copyright Pamela Gore, April 2008.


Hurricane Floyd satellite image. 1999. Courtesy of NOAA.

What are hurricanes?

Hurricanes are severe tropical storms with strong sustained winds (at least 74 miles per hour) and heavy rain. When hurricanes make landfall, the heavy rain and strong winds will damage or destroy buildings, trees, bridges, cars, etc. The heavy rains cause severe flooding many miles inland.

At the coast, a large dome of water, 50-100 miles across, hits the coastline where a hurricane makes landfall. This is called a storm surge. The surge of high water, topped by heavy wind-driven waves is devastating and causes tremendous property damage. The pounding waves can also reshape the coastline and cut inlets through islands. If a storm surge arrives at high tide, the height of the water will be even greater. This is a major reason why you MUST stay away from the ocean during a hurricane warning or hurricane.

Hurricane clouds rotate counter-clockwise around an "eye" or central area with sinking air, light winds, and few or no clouds. The most violent winds are those immediately surrounding the eye of the storm.

Tropical Depression - An organized system of clouds and thunderstorms and low level circulation with maximum sustained winds of 38 miles per hour or less.

Tropical Storm - An organized system of strong thunderstorms with well-defined circulation with sustained winds of 39 to 73 miles per hour.

Hurricane - An intense tropical weather system with well-defined circulation with sustained winds of74 miles per hour or higher.

When and where do hurricanes occur?

In the U.S., the official hurricane seasonruns from June 1 to November 30, but hurricanes can happen any time of the year. Most occur from mid-August to late October.

Hurricanes form in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern Pacific Ocean. The Atlantic, Carribbean, and Gulf storms generally move to the northwest, where they may strike a number of islands, and ultimately may strike the Atlantic or Gulf coast of the US (or countries to the south), and move inland.

Hurricane intensity

Hurricanes are classified into five categories, on the basis of their wind speeds. Wind speed is directly related to potential to cause damage. The hurricane classification system is called the Safir-Simpson Hurricane Scale.

Category One -- Winds 74-95 miles per hour
Category Two -- Winds 96-110 miles per hour
Category Three -- Winds 111-130 miles per hour
Category Four -- Winds 131-155 miles per hour
Category Five -- Winds greater than 155 miles per hour

Hurricanes of category 3 or above are considered major hurricanes.

See Hurricane Ivan Impact Studies. Hurricane Ivan was a category 3 storm that came ashore near Gulf Shores, Alabama on September 16, 2004. In particular, see the before and after photo pairs.

Damage from Hurricane Ivan, September 16, 2004. The three buildings surrounded by sand are in Gulf Shores, Alabama.
The house in the pine forest is at Perdido Key, Florida. Compare with the tornado damage photos. Note that the pine trees are not snapped off at roof level like they are in the tornado damaged areas, but the house is completely demolished.
Images copyright Pamela Gore, March 2004.

Collapsed front of multistory building, Orange Beach, AL: This five-story building was perched on top of a dune that was eroded during Hurricane Ivan. The Gulf-front portion of the building collapsed. Compare the pilings in the pre and post-photos of the house adjacent to the multi-story structure to determine the scale of vertical erosion of the dune. In the post-storm photo, the lower, unpainted portions of the pilings were below sand level prior to Hurricane Ivan. Image courtesy U.S. Geological Survey.

Destroyed houses on top of a dune, Orange Beach, AL: These houses were on built top of a dune that was severely eroded during Ivan. Note the walkways in the pre-storm photo that once served as pathways down to the beach. Image courtesy U.S. Geological Survey.

Breach through barrier island, Pine Beach, AL: The island was severed by a breach that may have developed as the back bay drained excess water following the peak of the storm surge, although the breach could have been initiated by waves and surge from the Gulf side. More analyses are required to determine the forcing processes. Image courtesy U.S. Geological Survey.

Hurricane Katrina, August 2005, Gulf coast and New Orleans

Storm damage along the Mississippi coast from Hurricane Katrina, August 2005. Photo credit: U.S. Geological Survey.

Storm damage from Hurricane Katrina, August 2005. Dauphin Island, south of Mobile, AL, more than 110 km from where the eye of the storm came onshore. In the lower left corner of the second photo , you can see an oil rig that broke loose and washed ashore.

USGS Hurricane Katrina Impact Studies

What causes hurricanes?

Hurricanes gather heat and energy through contact with warm ocean waters. Water is evaporated from the sea, and the moisture powers the storm. A wind pattern forms near the ocean surface that spirals air inward. Bands of thunderstorms form, which allows the air to warm further and rise higher into the atmosphere.

How are hurricanes named?

Hurricanes are given names to help us identify storms and track them as they move across the ocean. There can be more than one hurricane at a time . We name them so we do not get confused about the various storms.

If you have the latitude and longitude coordinates from the U.S. National Weather Service, which is the federal agency that tracks hurricanes and issues warnings and watches, you can track a hurricane on a map.

In 1953, the U.S. National Weather Service began using female names for storms.

In 1979, the practice changed to use both women and men's names.

Atlantic Ocean hurricanes may have French, Spanish or English names since these are the major languages bordering the Atlantic Ocean where the storm occur.

Hurricanes are named alphabetically (except for Q, U and Z) by the National Weather Service. There are six lists of names that are rotated each year by the World Meteorological Organization. If a particular hurricane is very deadly or costly, then the name is retired and a new name added to the list.

Some past deadly hurricanes and tropical storms have been named: Ivan (2004), Opal (1995), Floyd (1999), Tropical Storm Alberto (1994), Andrew (1992), Hugo (1989), Agnes (1972).

Important terms

Hurricane Watch -- A hurricane is possible within 36 hours. Stay tuned to the radio and television for more information. The Hurricane Center is tracking the storm and trying to predict where it may come ashore.

Hurricane Warning -- A hurricane is expected within 24 hours. You may be told to evacuate. You should begin making preparations to evacuate. If your area is having an evacuation, remember to take your Disaster Supply Kit. Make plans for your pets if you must evacuate.

Hurricane safety

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created April 3-5, 2005
Image link fixed Sept 21, 2005
Links and images updated April 19, 2007
Links updated to facstaff August 20, 2008
Atlanta tornado pictures added August 23, 2008