Natural Disasters

Prof. Stephen A. Nelson

Exceptional Weather - Thunderstorms, Tornadoes, Nor'easters, & Drought

Thunderstorms

Thunderstorms occur anywhere that warm moist air has absorbed enough heat to make the air less dense than the surrounding air. This commonly occurs along cold fronts, but can occur in other places as well, particularly where daytime heating forms hot air near the Earth's surface.

As the warm moist air rises it begins to cool and water begins to condense into tiny droplets that form clouds. Condensation of the water droplets in the clouds releases the latent heat of evaporation, adding heat to the rising air, thus decreasing its density and allowing it to rise to higher levels in the atmosphere. This rising air, called an updraft,  starts to build clouds to heights of up to 6 km. 

Further rising and cooling within the clouds causes more condensation, as well as the formation of ice crystals which release further latent heat and build cloud heights up to 12 km.  Eventually the water droplets and ice crystals in the clouds become so large that they can no longer be supported by the uprising air mass, and they begin to fall forming rapid downdrafts on the leading edge of the cloud.  In the mature stage of thunderstorm development updrafts and downdrafts operate side by side within the cloud.  This is the most dangerous stage of a thunderstorm because of the high winds accompanying the downdrafts, the heavy rain, as well as thunder, lightning, and possible hail and tornado development.  Eventually the cloud reaches the dissipating stage as the downdrafts drag in so much cool dry air that it prevents further updrafts of warm moist air. With lack of updrafts of warm moist air, the cloud begins to dissipate and eventually it stops raining. Thunderstorms can form as single cells, with only one cloud mass, or as multiple cells, with several clouds moving along a similar path.

Although thunderstorms can occur nearly everywhere, they show an unequal distribution through the United States.  Areas that receive the highest number of thunderstorms are areas where warm moist air moves northward from the Gulf of Mexico.  As seen in the figure to the right, areas in southwestern Florida have over 100 days per year with a thunderstorm.

Hail - Hail is a rain of semi-spherical, concentrically layered ice balls that are dropped from some thunderstorms.  Hail rarely kills people, but it does heavy damage to agriculture, roofs, and automobiles.  The conditions necessary to form hail during a thunderstorm are:

• Large thunderstorms with high cloud tops formed from hot moist rising air.
  
  
• Upper level cold air with a large temperature contrast between the upper level air and the rising moist air.
  
  
• Strong updrafts within the thunderstorm to keep hailstones suspended in the cloud while layers of ice are added to the stones.  When the stones become too large to be suspended by the cloud they fall to the surface as hail.

Although thunderstorms are most common along the Gulf coast, thunderstorms that produce hail are more common in the mid-continent region where temperature contrasts between upper air masses and the rising hot air are greater.  Hail can range from pea-size stones to grapefruit size stones.
             

Lightning - Lightning is the electrical discharge from clouds that causes thunder. Thus, lightning occurs from all thunderstorms.  Lightning is the major cause of forest fires and results in many deaths. 

Between 1954 and 2013, an average of 51 deaths per year in the U.S.

131 died in 1969 (deadliest year)

90% of lightning strike victims survive.

Deaths from lightning have a similar distribution to the occurrence of thunderstorms, with Florida having both the most thunderstorms and the most lightning deaths.

Deaths from lightning usually occur outdoors as seen in the table here.

Lightning is caused by an imbalance of electrical charge between and within clouds and the ground.  Most of you have simulated lightning by walking across a carpet on a dry day and then touching a metal object like a door knob. Static electrical charge builds on your body and is discharged to the door knob as a bolt of electricity.  

During the buildup of a thundercloud, charged particles of water droplets and ice become separated in the cloud. Positively charged particles are moved to the top of the cloud and negatively charged particles are moved to the bottom of the cloud.

The thundercloud then begins to interact with the ground.  The negatively charged part of the cloud induces a build-up of positive charges on the ground.  Similarly beneath the upper positively charged part of the cloud negative charges are induced in the ground below. When the difference in voltage between the oppositely charged parts of the cloud and the ground become great enough, the electricity is discharged as a bolt of lightning.

Note that lightning can travel from the cloud to the ground, from the ground to the cloud, and within the cloud itself.  lightning travels at speeds of about 100,000 miles per hour, and usually includes several strokes that all occur within about 1/2 of a second.  The discharge of electricity during a bolt of lightning heats up the air surrounding the bolt causing rapid expansion of the air.  It is this rapid expansion of the air that causes the sound we call thunder.

Tornadoes

Tornadoes are funnel shaped clouds that are associated with thunderstorms.  Tornadoes have wind velocities higher than hurricanes (up to 500 km/hr [318 miles per hour]), but affect a much smaller area than hurricanes.  Over 70% of the tornadoes that occur on Earth occur in the great plains of the United States.  Between 1920 and 2004 tornadoes caused 11,560  deaths in the United States, an average of 136 deaths per year. Tornado awareness has reduced the death toll over the last 40 years.   

Tornado Intensity  - The intensity of a tornado is classified by the Fujita tornado intensity scale shown in the table below.   This was recently (2007) modified and is now called then Enhanced Fujita Scale.    Since actually wind velocities during a tornado can only rarely be measured, both scales rely on post-tornado inspections to determine the damage done to various types of structures.  This damage is then compared with damage done to similar structures in experiments.  The wind velocity and the Fujita Scale rating of the tornado are then based on the comparison.   The Enhanced Fujita scale was developed to more accurately reflect the correlation between wind speed and damage based on data collected over the years since the development of the original Fujita Scale. Note that wind velocities are generally lower for the Enhanced Fujita Scale.

Tornado Frequency - Tornadoes have occurred in every state of the U.S. except Alaska.  Because they are associated with strong thunderstorms, the frequency of tornadoes is closely related to the areas that have lots of thunderstorms, but also occur predominantly in the plains states where cold air descending from above the Rocky Mountains encounters warm air moving northward from the Gulf of Mexico. 

 
It is these contrasting air masses and their common paths of circulation which give rise to both the thunderstorms and the tornadoes.   

Since 1985, over 41% of deaths due to tornadoes in the U.S. have involved mobile homes. People in vehicles are much less likely to die.

2011 was a deadly year for tornadoes in the U.S. As of August 24, 1718 tornadoes had been reported in the U.S.,with 1239 confirmed. Worldwide, 566 people died, with 550 in the U.S. The deadliest and most destructive were the April 27 outbreak in the southeastern U.S. and the Joplin, Missouri EF 5 tornado on May 22.

• Between April 25 and 28, 2011 over 300 tornadoes swept through the southeastern U.S. and killed 305 people (41 in Tuscaloosa, Alabama).
  
  

• On May 22, 2011, An EF5 tornado destroyed much of the town of Joplin Missouri,and killed 159 people.See video at www.youtube.com/watch?v=XT7CtF5ljxY

• If at home:
  
  • Go at once to the basement, storm cellar, or the lowest level of the building.
    
  • If there is no basement, go to an inner hallway or a smaller inner room without windows, such as a bathroom or closet.
    
  • Get away from the windows.
    
  • Go to the center of the room. Stay away from corners because they tend to attract debris.
    
  • Get under a piece of sturdy furniture such as a workbench or heavy table or desk and hold on to it. Use arms to protect head and neck.
    
  • If in a mobile home, get out and find shelter elsewhere.
    
    
• If at work or school:
  
  • Go to the basement or to an inside hallway at the lowest level.
    
  • Avoid places with wide-span roofs such as auditoriums, cafeterias, large hallways, or shopping malls.
    
  • Get under a piece of sturdy furniture such as a workbench or heavy table or desk and hold on to it. Use arms to protect head and neck.
    
    
• If outdoors:
  
  • If possible, get inside a building.
    
  • If shelter is not available or there is no time to get indoors, lie in a ditch or low-lying area or crouch near a strong building. Be aware of the potential for flooding.   Use arms to protect head and neck.
    
    
• If in a car:
  
  • Never try to out drive a tornado in a car or truck. Tornadoes can change direction quickly and can lift up a car or truck and toss it through the air.
    
  • Get out of the car immediately and take shelter in a nearby building. If there is no time to get indoors, get out of the car and lie in a ditch or low-lying area away from the vehicle. Be aware of the potential for flooding.
    
    
• After the tornado:
  
  • Help injured or trapped persons  Give first aid when appropriate. Don't try to move the seriously injured unless they are in immediate danger of further injury. Call for help.
    
  • Turn on radio or television to get the latest emergency information.
    
  • Stay out of damaged buildings. Return home only when authorities say it is safe.
    
  • Use the telephone only for emergency calls.
    
  • Clean up spilled medicines, bleaches, or gasoline or other flammable liquids immediately.
    
  • Leave the buildings if you smell gas or chemical fumes.
    
  • Take pictures of the damage--both to the house and its contents--for insurance purposes.
    
  • Remember to help your neighbors who may require special assistance--infants, the elderly, and people with disabilities.
    
    
• Inspecting utilities in a damaged home:
  
  • Check for gas leaks--If you smell gas or hear a blowing or hissing noise, open a window and quickly leave the building. Turn off the gas at the outside main valve if you can and call the gas company from a neighbor's home.
    
  • If you turn off the gas for any reason, it must be turned back on by a professional.
    
    

Because tornadoes can strike anywhere and anytime there are thunderstorms, the best mitigation is for an educated populace to be aware of the conditions under which tornadoes develop and heed any tornado watches or warnings that are issued by a responsible agency, and practice the tornado safety tips listed above.  The only other mitigation that can reduce the damage produced by tornadoes is building codes that require structures to be constructed with extra reinforcing of wood frames and masonry.

Other Severe Weather Phenomena

Nor'easters
Nor'easters are extratropical cyclonic storms that originate outside of the tropics.  They are, like hurricanes, centers of low pressure with winds that circulate in a counterclockwise direction around the low pressure. 

Examples of questions on this material that could be asked on an exam

1. What conditions are necessary to form a thunderstorm?
   
   
2. What causes thunder?
   
   
3. What conditions are necessary for hail to form?
   
   
4. In the U.S., where are tornadoes most likely to occur and where are they least likely to occur?
   
   
5. What are the Fujita Scale and the Enhanced Fujita Scale and how are these values determined?
   
   
6. Why are mobile homes so potentially dangerous for tornado hazards?
   
   
7. If you are at home, where is the safest place to be during a tornado?
   
   
8. What methods can be employed to mitigate against tornado hazards?
   
   
9. In general, what causes a drought?

References

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