SEVERE WEATHER: HAIL, TORNADOES AND WIND EVENTS

AOS 101 Weather and Climate

Lisha M. RoubertUniversity of Wisconsin-Madison

Department of Atmospheric & Oceanic Sciences

Severe Weather• Severe weather refers to any

dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmospheric conditions.

• High winds, hail, excessive precipitation, and wildfires are forms of severe weather.

Wind events

• Downburst• Squall lines• Tornadoes• Tropical cyclones• Waterspouts• Strong extratropical cyclones• Dust storm• Wildfires

Tornadoes• Tornadoes are defined as

violently rotating columns of high wind that rotate around regions of low pressure beneath a thunderstorm.

• These rotating columns of air are called “funnels”. These funnels are visible due to moisture that condenses in the rising and cooling air within a tornado and also due to the dust and debris that have been sucked into the vortex.

• Tornadoes usually rotate cyclonically and move towards the northeast along with the thunderstorm.

Wedge Tornado

Rope Tornado

Funnel clouds• There are tornadoes in which the

circulation does not extend to the ground. These are called funnel clouds instead of tornadoes.

• Funnel clouds are rotating columns of air not in contact with the ground. However, the violently rotating column of air may reach the ground very quickly - becoming a tornado! If there is debris being picked up or blown around by the "funnel cloud" - the rotating column of air has already reached the ground and it's a tornado!

What causes Tornados?

• Tornadoes in the winter and early spring are often associated with strong frontal systems that form in the Central US and move east

• During the spring in the Central Plains, thunderstorms develop along a dryline – separates very warm, moist area to the east from hot, dry air to the west.

• Tornadoes may form as thunderstorms fire along the dryline during the afternoon hours

Tornado Development• Tornadoes typically form in

association with supercell thunderstorms.

• A supercell thunderstorm is a special type of thunderstorm that can persist for several hours due to its organized internal structure.

• Supercell thunderstorms are characterized by a single, rotating updraft. They form in regions of strong vertical wind shear.

Wind Shear• Vertical wind shear is the change

in wind speed and/or direction with height. – Directional wind shear- the change

of wind direction with height– speed shear-the change of wind

speed with height. • Vertical wind shear induces a

"rolling" effect in the atmosphere, similar to the diagram below. This rolling effect becomes important when a thunderstorm forms because it is tilted upward and causes a thunderstorm to rotate.

Directional Wind Shear

Speed Shear

Supercell Tornadogenesis

• A rotating updraft is a key to the development of a supercell, and eventually a tornado. There are many ideas about how this rotation begins. One way a column of air can begin to rotate is from wind shear – when winds at two different levels above the ground blow at different speeds or in different directions.

• An example of wind shear that can eventually create a tornado is when winds at ground level, often slowed down by friction with the earth's surface, come from the southwest at 5 mph. But higher up, at 5000 feet above the same location, the winds are blowing from the southeast at 25 mph! An invisible "tube" of air begins to rotate horizontally. Rising air within the thunderstorm tilts the rotating air from horizontal to vertical – now the area of rotation extends through much of the storm.

• Once the updraft is rotating and being fed by warm, moist air flowing in at ground level, a tornado can form. There are many ideas about this too.

Mesocyclones• When the combination of wind speeds and

changing directions in a super cell is right the air will rotate, causing a mesocyclone to form. So the air near a mesocyclone is rotating and rising. Where air rises there aren't water droplets, so there won't fall any rain in that area. Since tornadoes are made of rising air, they never form in the neighbourhood of any rain. When the mesocyclone is powerful enough the actual tornado will form.

• Often, a tornado is located on the edge of the updraft, next to air that's coming down from the thunderstorm with falling rain or hail. This is why a burst of heavy rain or hail sometimes announces a tornado's arrival. Although the air is rising in a tornado, the funnel itself grows from the cloud toward the ground as the tornado is forming. When the funnel cloud touches the ground, it becomes a tornado.

How big are tornadoes?

• Most tornadoes are less than 1/4 of a mile wide on the ground - but they can also can exceed 1 mile in width! The McColl tornado in March 1984 (moving from the McColl S.C. area into NorthCarolina) was over 1 1/2 miles on the ground!

• Most tornadoes are on the ground 10 minutes or less - but in 1925 a tornado traveled 219 miles across Missouri, Illinois, and Indiana in 4 hours!

• In 1924, a tornado that started in Aiken County South Carolina traveled 135 miles into Florence County!

How strong are tornadoes?• While most tornadoes (69%) have winds

of less than 100 miles an hour, they can be much much stronger. Violent tornadoes (winds greater than 205 miles an hour) account for only 2% of all tornadoes, they cause 70% of all tornado deaths. – In 1931, a tornado in Minnesota lifted an

83-ton railroad train with 117 passengers and carried it more than 80 feet!

– Once a tornado in Oklahoma carried a motel sign 30 miles and dropped it in Arkansas!

– In 1975 a Mississippi tornado carried a home freezer more than one mile!

An F2 tornado near Seymour, Texas, in April 1979. F2 and F3 tornadoes are considered strong, packing winds of 113-206 mph that can cause major to severe damage.

Tornado Wind and Damage Scale

Tornado Scale Wind Speeds Damage Frequency of Occurrence

F0 40 to 72 MPH Some damage to chimneys, TV antennas, roof shingles, trees, and windows. 29%

F1 73 to 112 MPH Automobiles overturned, carports destroyed, trees uprooted 40%

F2 113 to 157 MPH Roofs blown off homes, sheds and outbuildings demolished, mobile homes overturned. 24%

F3 158 to 206 MPHExterior walls and roofs blown off homes. Metal buildings collapsed or are severely damaged. Forests and farmland

flattened.6%

F4 207 to 260 MPH Few walls, if any, standing in well-built homes. Large steel and concrete missiles thrown far distances. 2%

F5 261 to 318 MPHHomes leveled with all debris removed. Schools, motels, and other larger structures have considerable damage

with exterior walls and roofs gone. Top stories demolishedLess than 1%

How fast can tornadoes move?

• The average speed of a tornado is around 35 miles an hour - but they can remain almost stationary or move as fast as 70 miles an hour!

• The average tornado moves from southwest to northeast - but they can move in any direction and even change direction.

Tornado Video

http://www.weather.com/outlook/videos/vortex2-watching-tornado-develop-live-14517

Where do tornadoes occur?• Because a tornado is part of a severe convective storm,

and these storms occur all over the Earth, tornadoes are not limited to any specific geographic location. In fact, tornadoes have been documented in every one of the United States, and on every continent, with the exception of Antarctica (even there, a tornado occurrence is not impossible). Wherever the atmospheric conditions are exactly right, the occurrence of a tornadic storm is possible.

• However, some parts of the world are much more prone to tornadoes than others. Globally, the middle latitudes, between about 30° and 50° North or South, provide the most favorable environment for tornadogenesis. This is the region where cold, polar air meets against warmer, subtropical air, often generating convective precipitation along the collision boundaries. In addition, air in the mid-lattitudes often flows at different speeds and directions at different levels of the troposphere, facilitating the development of rotation within a storm cell.

• Interestingly, the places that receive the most frequent tornadoes are also considered the most fertile agricultural zones of the world. This is due in part to the high number of convective storms delivering needed precipitation to these areas.

Regions of the world with increased likelihood of experiencing tornadoes.

Florida and “Tornado Alley”

• In the United States, there are two regions with a disproportionately high frequency of tornadoes. Florida is one and "Tornado Alley" in the south-central U.S. is the other. Florida has numerous tornadoes simply due to the high frequency of almost daily thunderstorms. In addition, several tropical storms or hurricanes often impact the Florida peninsula each year. When these tropical systems move ashore, the embedded convective storms in the rain bands often produce tornadoes.

• Tornado Alley is a nickname given to an area in the southern plains of the central U.S. that consistently experiences a high frequency of tornadoes each year. Tornadoes in this region typically happen in late Spring and occasionally the early fall. The Gulf Coast area has a separate tornado maximum nicknamed "Dixie Alley" with a relatively high frequency of tornadoes occurring in the late fall (October through December). Dixie Alley

Tornado Alley

When are tornadoes most likely?

• In the southernUnited States thepeak occurrenceof tornadoes is March through May - but tornadoes can occur any time of year.

Most tornadoesoccur between 3and 9 PM - buttornadoes can occur at any time of day or night

Hail• Hail is a form of solid precipitation which consists of balls

or irregular lumps of ice, that are individually called hail stones.

• Hail stones consist mostly of water ice and measure between 5 millimetres (0.20 in) and 150 millimetres (5.9 in) in diameter, with the larger stones coming from severe thunderstorms.

• Hail is possible with most thunderstorms as it is produced by cumulonimbi (thunderclouds), usually at the leading edge of a severe storm system.

• Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics. The largest recorded hailstone in the

United States by diameter (8 inches) and weight (1.93 pounds). The hailstone fell in Vivian, South Dakota on July 23, 2010.

Hail Structure

• Unlike ice pellets, hail stones are layered and can be irregular and clumped together. Hail is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 millimeter (0.039 in) thick, which are deposited upon the hail stones.

• What causes hail to have this structure?

Hail stone that has been cut in half

Hail Formation• Inside of a thunderstorm are strong updrafts of warm air

and downdrafts of cold air. • If a water droplet is picked up by the updrafts it can be

carried well above the freezing level. With temperatures below 32F the water droplet freezes.

• As the frozen droplet begins to fall, carried by cold downdrafts, it may thaw as it moves into warmer air toward the bottom of the thunderstorm

• But, this small half-frozen droplet may also get picked up again by another updraft, carrying it back into very cold air and re-freezing it. With each trip above and below the freezing level the frozen droplet adds another layer of ice.

• Finally, the frozen water droplet., with many layers of ice - much like the rings in a tree, falls to the ground - as hail!

Is there a way to estimate hail size...or do I have to go outside and measure it?

• It's often difficult to get an accurate measurement of hail diameter - especially when it's falling

• The table to the right helps observers estimate the size of hail based on average diameters of common items

• When in doubt - play it safe and wait until the thunderstorm has moved away before going outside to measure the size of hail.

Estimating Hail Size

Pea = 1/4 inch diameter Marble/mothball = 1/2 inch diameter Dime/Penny = 3/4 inch diameter - hail penny size or larger is considered severe Nickel = 7/8 inch Quarter = 1 inch Ping-Pong Ball = 1 1/2 inch Golf Ball = 1 3/4 inches Tennis Ball = 2 1/2 inches Baseball = 2 3/4 inches Tea cup = 3 inches Grapefruit = 4 inches Softball = 4 1/2 inches

Detecting Hail in a storm• There are methods available to detect

hail-producing thunderstorms using weather satellites and radar imagery.

• When large hail is present in a thunderstorm a signature can be seen on a radar image called a Three Body Scatter Spike or TBSS. This is caused by the radar beam being reflected off of the hail causing it to take longer to be returned to the radar. This extra time causes radar reflectivity to be plotted beyond the main reflectivity of the storm, seen as a spike on the radar screen.

Radar reflectivity during the time of baseball size hail in downtown Wichita.

Detecting Hail in a storm

The 2238Z composite reflectivity from Maxwell Air Force Base (KMXX) on February 18, 2009. This supercell in west-central Alabama had split into two cells, while a spurious hail spike flared to the west of the storm.

Is there a way to simply look at athunderstorm and tell if it will produce hail at

the ground?

• There is no positive way to look at a thunderstorm in the distance and tell if it will produce hail reaching the ground.

• Meteorologists use weather radar to "look" inside a thunderstorm. Since hail reflects more energy back to the radar than raindrops it often shows up in red shades.

• The WSR-88D Doppler Radar can also estimate size of the hail based on the amount of energy reflected back.

Should I be concerned about tornadoes when hail is observed?

• The presence of large hail indicates very strong updrafts and downdrafts within the thunderstorm. These are also possible indicators of tornadic activity. Often large hail is observed immediately north of a tornado track - but the presence of hail doesn't always mean a tornado and the absence of hail doesn't always mean there isn't a risk of tornadoes.

Hail Frequency

• Large hail results in nearly $1 billion in damage annually to property and crops in the U.S.

Today in the Lab

• Today in the lab we will be using upper air soundings to detect severe weather.

• Upper air soundings are plotted data from balloons. • The balloon records temperature, humidity and

winds and these are plotted versus pressure/height to give details on the vertical structure of the atmosphere.

• The type of plot is called a SkewT and a set of SkewTs are available for most US stations. These are updated once every 12 hours at roughly 10:00 EST.

Loaded Gun Sounding• Severe weather sounding

(large CAPE, very unstable LI)Large hydrolapse in mid-levels (mT air in boundary layer and capped by cT air).

• There must be an inversion above mT air.

• Most common in Great Plains, Midwest and SE US.

• Most common severe weather: Large hail, tornadoes, convective wind gusts of 58mph or greater. If speed /directional wind shear and strong low-level jet are present on sounding, severe weather chances are enhanced.

Homework

• Study for final quiz. Labs covered:– Lab 6: Global Cloud Cover Trends– Lab 8:El Niño– Lab 10: Convection in the Atmosphere and Oceans