Severe Weather Thunderstorms, Tornadoes, Hurricanes, etc…

In this section, we will learn… Tornadoes, hurricanes, blizzards, and thunderstorms are severe weather phenomena that impact society and ecosystems. Hazards include downbursts (wind shear), strong winds, hail, lightning, heavy rain, and flooding. The movement of air in the atmosphere is due to differences in air density resulting from variations in temperature. Many weather conditions can be explained by fronts that occur when air masses meet. E4.3A Describe the various conditions of formation associated with severe weather (thunderstorms, tornadoes, hurricanes, floods, waves, and drought). E4.3B Describe the damage resulting from and the social impact of thunderstorms, tornadoes, hurricanes, and floods. E4.3C Describe severe weather and flood safety and mitigation. E4.3D Describe the seasonal variations in severe weather. E4.3E Describe conditions associated with frontal boundaries that result in severe weather (thunderstorms, tornadoes, and hurricanes).

Thunderstorms Thunderstorms are most often associated with a Cumulonimbus Cloud This is a cloud that forms when air is forced high into the atmosphere (against its will) Cold fronts often create this condition This produces an UNSTABLE environment The process of cooling, condensing, cloud formation, saturation and precipitation occurs quickly Cold fronts often create the proper conditions for this to occur

Thunderstorm Formation Thunderstorms have three stages… The Cumulous Stage: Warm air rising and condensing to form clouds The Mature Stage: Strong updraft of warm, moist air, AND a strong downdraft of wind and precipitation (hail and lightning can occur in this stage too) The Dissipation Stage: The surface has been cooled, cutting off the warm, moist air that “fuels” the storm. Once the precipitation has fallen from the sky, the storm is over

Lightning Lightning is believed to be created by friction between the rising updraft and the falling downdraft in a cumulonimbus cloud This friction creates a separation of charges (kinda like rubbing your feet across the carpet in your socks) Eventually there is a discharge between positive and negative charges, creating a “spark” (kinda like touching that doorknob and getting a shock) On a larger scale, that “spark” is a bolt of lightning

Thunder When lightning strikes, the “bolt” can be as hot as 30,000 degrees The air surrounding that bolt of electricity expands rapidly (remember… warm air expands), and then contract quickly as it cools. Because of the extremely high temperatures involved, the air expands and contracts rapidly enough to break the sound barrier (767 mph) The thunder is a “sonic boom”

Wind & Downdrafts What goes up, must come down! Thunderstorms are no different, when warm air rises high into the atmosphere, it eventually cools and comes back down toward the surface Cumulonimbus clouds can be as tall as 40,000 feet, so… It can fall very quickly toward the surface, producing strong winds (called downdrafts)

Hail

Thought to be the 1st photograph of a tornado (1884) Tornadoes Thought to be the 1st photograph of a tornado (1884)

Tornadoes Violent windstorms that take the form of a rotating column of air, or vortex, that extends downward from a cumulonimbus cloud. Because of the lower pressure in the center vortex, air near the ground rushes into the tornado from all directions. Air streams inward, it is spiraled upward around the core until it eventually merges with the airflow or the parent thunderstorm deep in a cumulonimbus tower.

Tornadoes Some tornadoes consist of a single vortex. However, within many stronger tornadoes are smaller intense whirls known as suction vortices, that orbit the center of the larger tornado → Multiple Vortex Tornadoes. Usually die out in less than a minute.

Tornadoes Suction vortices are responsible for most of the narrow, short waves of extreme damage that sometimes are through tornado tracks. Most reports of several tornadoes at once actually were multiple vortex tornadoes.

Supercell Structure

Tornado Development Less than 1% of thunderstorms produce tornadoes. Most intense tornadoes are associated with supercells (big, cumulonimbus cloud structures). Tornado formation begins with the development of a mesocyclone. Mesocyclone- Vertical cylinder of rotating air that develops in the updraft of a severe thunderstorm.

Tornado Development

Tornado Development *See supercell diagram for location. The mesocyclone within the cloud stretches vertically and narrows horizontally, causing winds speeds to accelerate. Air stretches downward until a portion of the cloud protrudes below the cloud base, producing a dark, slow rotating wall cloud. *See supercell diagram for location.

Tornado Classification Fujita Intensity Scale- Assesses the damage produced by a storm as it relates to wind speed. F0- Moderate; 40-72 mph F5- Severe; 261-318 mph Path of Destruction Duration Problem: Doesn’t take into account for structural integrity of objects. The ENHANCED Fujita Scale (EF – Scale) is now used in the US. It DOES take into account structural damage (but still uses a 0-5 numerical scale)

Watches & Warnings Tornado Watch Tornado Warning Conditions are ideal for a tornado to be created. Tornado Warning An actual tornado has been sighted in the area or is indicated by weather radar.

Tornado Frequency in the US

Tornado Frequency Around the World

Cool Tornado Pictures

Hurricanes

Hurricane Formation The hurricanes that strike the Caribbean, Gulf of Mexico and the US originate in Africa They begin as low pressure storm systems over land The Trade Winds (one of the systems of global winds) blow them out over the warm water of the Atlantic Ocean

Hurricane Growth Remember that low pressure pulls air in at the surface… and pushes it to higher levels in the atmosphere Air temperatures at high levels in the atmosphere are colder, and the warm, moist air over the ocean begins to form clouds (BIG Cumulonimbus clouds) Lots of Latent Heat released during condensation Creates a very unstable environment

Continued Growth Remember that the water over the Atlantic Ocean near Africa is warm (80°F) and has lots of moisture in it (mT air masses) As precipitation begins, the rain and cool downdrafts of the Cumulonimbus system are not enough to cut off the updraft (it’s too warm) The warm, moist air continues to fuel the system of Cumulonimbus clouds, making them bigger, and bigger, and bigger

Hurricane Movement Once the growing low pressure system is out over the warm water of the Atlantic Ocean, the Trade Winds blow it to the west Because the oceans are relatively smooth, there isn’t much to stand in its way and slow it down With 2,000 to 3,000 miles of warm ocean water to move across… the system has the time and the fuel to become severe

Tropical Depressions, Tropical Storms and Huricanes Low pressure begins as a Tropical Disturbance The low pressure systems that blow out over the waters of the Atlantic Ocean begin as Tropical Depressions (a low pressure system is in fact a “depression” of pressure) If winds in the system reach 39mph, the system becomes classified as a Tropical Storm (and it is given a name) If the system continues to gain strength and winds speeds reach 74 mph, it is officially classified as a hurricane

Hurricanes, Typhoons and Cyclones We give different names to Tropical Storms that develop, depending on their location In the Atlantic Ocean we call them Hurricanes In the Pacific Ocean we call the Typhoons In the Indian Ocean we call them Cyclones They are all basically the same type of storm

Hurricane Dangers The most dangerous component of a hurricane is the “Storm Surge” As the storm makes landfall, a mound of ocean water (driven by strong winds AND the “sucking” power of the strong low pressure system) is pushed on shore More people die as a result of drowning, than do of strong winds

Hurricane Classification The Safir-Simpson Scale Type KNOTS MPH millibars inches of mercury Feet Meters  Damage Depression less than 34 less than 39 ------ Tropical Storm 35 - 63 39 - 73 Category 1 64 - 82 74 - 95 greater than 980 greater than 28.94 3 - 5 1.0 - 1.7 minimal Category 2 83 - 95 96 - 110 979 - 965 28.50 - 28.91 6 - 8 1.8 - 2.6 moderate Category 3 96 - 113 111 - 130 964 - 945 27.91 - 28.47 9 - 12 2.7 - 3.8 extensive Category 4 114 - 135 131 - 155 944 - 920 27.17 - 27.88 13 - 18 3.9 - 5.6 extreme Category 5 136+ 156+ less than 920 less than 27.17 19+ 5.7+ catastrophic

A Hurricane’s “Death” As a hurricane makes landfall, the supply of warm, moist air that was fueling it is cut off As the downdrafts and rain cool the land, the strength of the updraft slows Without an updraft, the system will dissipate, and eventually die out (just like a thunderstorm) This may take days and 100’s of miles Hurricane Ike comes to Michigan in 2008 (my front yard in Commerce Township) >

Hurricane Katrina In August of 2005, Tropical Storm Katrina developed over the Bahamas As it approached Florida, it developed into a Category I hurricane It lost some strength over the land (turned back into a Tropical Storm), but picked right back up when it moved back over the warm water of the Gulf of Mexico

Hurricane Katrina As the storm moved across the Gulf of Mexico, it RAPIDLY developed (from a Category II to a Category V in only 9 hours) Wind speeds reached 175 mph A mound of water was pushed toward Louisiana and Mississippi… even though wind speeds slowed (to a Category III) by the time it made landfall

Flooding While the storm surge associated with Katrina was estimated to be 28 feet above normal sea level In addition, much of the city is BELOW sea level A systems of levees and dams were supposed to protect the city, but this was too high 85% of the city flooded

The Aftermath The most destructive and costliest natural disaster in the history of the US More than $81 billion dollars in damage More that 1800 people died (700 still listed as missing) More than 90,000 square miles declared a Federal Disaster Area More than 3 million people were left without power

In Addition… There were unforeseen economic effects Gas and Oil production shut-downs Unemployment Insurance Company Bankruptcies Relocation/Flight from the region Environmental Effects Oil Spills Erosion Sewage Loss of Marine and Animal life Looting and Violence