Tornadoes October 3, 2007

Tornadoes  A tornado is a rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground  Known as waterspouts when they form over oceans. If the circulation remains aloft, it is known as a funnel cloud.

Tornadoes – Characteristics & Dimensions  A wide variety of shapes and sizes. Diameters  Diameters: typical 300 to 2000 ft Duration  Duration: usually a short lived phenomena, only a few minutes; some have lasted for hours Movement  Movement: is generally about 50km/hr (30 mph) over an areas about 3-4 km (2-2.5 mi) long SW to NE  Wind speeds: from 40 mph to 280 mph

 The most damaging aspect of a tornado is wind speed.  Tornadoes are spawned by frontal boundaries, squall lines, MCCs, and tropical cyclones.  The most severe tornadoes are spawned by severe thunderstorm cells known as supercells.  Tornadoes are preceded by a falling barometer, a S or SE wind, humid air, and a thunderstorm Tornadoes

Tornadoes winds actually are funneled and strengthen by an interstate overpass structure and in hallways  Opening windows during a tornado increases the pressure on the opposite wall and increases the chances the building will collapse  Flat roofs are more vulnerable than steep roofs, which impede air flow  Attic vents help equalize pressure

Tornadoes Figure 8.23Figure 8.24

 Supercell Tornado Development  Begins with the development of a mesocyclone: a large rotation region within the cloud interior which develops in the presence of vertical wind shear  From the surface aloft, winds shift direction while speed increases

 Supercell Tornado Development  Strong updrafts tilt the rotation region to a vertical position while the diameter decreases  With a spatial decrease comes an increase in speed as dictated by the conservation of angular momentum

Supercell Tornado Figure 8.23

When a large number of tornadoes (6 or more) forms over a particular region, this is a tornado outbreak One thunderstorm can spawn several tornadoes, referred to as families When tornadoes are likely to form during the next few hours, a tornado watch is issued Once a tornado is spotted, a tornado warning is issued

Nonsupercell Tornado Formation  Related to strong convection along a convergence zone

 Tornadoes predominate in North America, especially in south-central states of Texas, Oklahoma, Kansas, Nebraska, and Iowa, and throughout the Midwest.  This region is commonly called tornado alley.  Have occurred in EVERY state  A secondary concentration occurs in the southeastern U.S., especially Florida and Mississippi.

There are more tornadoes in the Midwest and Great Plains, but more people die in the southeast Lack of basements – no where to hide and less support for the upper floors of the house E or NE side of house (including basement) safer than W or SW side

 In the U.S. alone, we can expect to have on average about 100 tornadoes annually, so this is a common phenomenon.  Most occur between 10 AM and 6 PM, and tornado season runs from March to July, with May seeing the heaviest occurrence.

Tornadoes Figure 8.25

When a tornado is approaching from the SW, its strongest winds are on its southeast side

Tornado Damage Figure 8.23

 Tornadoes are classified using the Fujita scale according to their wind speed (which is inferred from damage) and ranges from:  C lass 0 (minimal damage and wind speeds of about 70 mph) to a Class 5 tornado (massive damage, cars picked up, wind speeds up to 300 mph).  Only one tornado on average per year is an F5.

Tornado Damage As wind speed doubles, the force of the wind exerted on an object increases by a factor of four. So, the 200mi/hr winds of an F4 tornado exerts 4 times as much force on a building as do the 100mi/hr winds of an F1 tornado

Folklore Raining cats and dogs, toads and frogs, bald chickens The updraft in a tornado is so strong that precipitation cannot fall through it – clear weather in the center Color of tornado depends on color of local soils