Severe Weather: Thunderstorms and Tornadoes

LAST MODIFIED:

POSTED: 10 June 2020

• Thunderstorms

• Over 2,000 worldwide at any given time, over 100,000 per year in the U.S.
• intense rain; strong gusty winds; lightning; hail; short lived
Thunderstorms form when
• cold front moves into region of particularly warm air mass (e.g. mid-latitude North America when cold polar air collides with warm Gulf air)
• convective lifting driven by solar radiation in moisture rich regions (e.g. tropical rain forest; Florida)
• orographic lifting (e.g. Rockies, Sierras in late summer)
• formation similar to that of ordinary storm, except that cumulonimbus clouds reach particularly high (can reach the tropopause, and hence disturb even high-flying air traffic)
• clouds spread to form anvil clouds, once tropopause is reached
• updrafts within clouds are particularly strong so that electric charges (H⁺ and OH^- can separate
• thunderstorm is in its mature state once rain starts to fall; downdraft overpowers the updraft, cutting off supply for thunderstorm (dissipation)


• cell: downdraft and updraft within a thunderstorm constitute a (convection) cell
• supercells: an enormous, rotating severe thunderstorm whose up- and downdrafts are nearly in balance, allowing it to maintain itself for several hours. Updrafts can exceed 165 km/h. Rapidly rising air interacts with steering winds aloft in such a way that new updrafts lead to the formation of new cells. It can produce large hail (grapefruit size) and large, long-lasting tornadoes.
• microbursts: strong local downdrafts near the surface, often accompanied by rain and hail; particularly dangerous to aviation, e.g. planes on approach to land may lose lift due to wind shear or get pushed into ground; on the scale of 10s of meters to 1 km
• in the U.S., most thunderstorms happen in central Florida; many thunderstorms also occur along the eastern side of the Rockies and in the Midwest
• dry thunderstorms: when precipitation never hits the ground (i.e. evaporates in the atmosphere or water droplets never become large enough to produce rain)



• Lightning

• all details on cause and mechanisms still a matter of research
• current idea:
  • when updraft within cloud is particularly strong, the rubbing of air, water and ice molecules is strong enough so that electric charges start to separate (H+ goes to top; OH- to the bottom)
  • + charges are attracted at the surface, while air still acts as insulator
  • eventually, difference in charge gets to high, resulting in instantaneous discharge through air (lightning flash)
• the sound of thunder is caused by the lightning flash heating surrounding air instantaneously to T=8,000-33,000°C. The air then expands explosively, creating sound waves that travel at 330m/s (e.g. if you count to 3 between lightning and thunder, the lightning strike was 1 km away).
• separation of charges and subsequent lightning can also occur without water molecules in the atmosphere; in such cases aerosols such as volcanic ash take on the role of water molecules
• dry lightning: lightning strikes occurring without significant precipitation; somewhat of a misnomer because the "dry" refers to precipitation, not the lightning; particularly dangerous because a fire started by lightning is not dosed by subsequent rain
• in the U.S., about as many people get killed by lightning strike (44) as by wind (47) and winter storms (46) (this number is based on the 1997-2006 10-year average; see lecture 1)
• in the U.S., most lightning deaths occur in Florida
• lightning strikes look for the shortest distance to the ground
• bad places to be during a thunderstorm: open field; under a tree; on the water
• also bad: spread out on the ground or in the water; better: put feet together, hunch down; by spreading on the ground, a voltage difference can develop between the extreme points of the body and a deadly current can flow in the body
• relatively safe: in a car; due to the Faraday effect, charges flow along the outside of the car

How does a lightning discharge work? - a more detailed description ⁽⁴⁾
1. elections bounce: ice particles collide so fast that they knock off each other's electrons. Positive charges now hang near the top of the cloud, while negative charges collect in the middle.
2. discharge emerges: electric energy zipping between opposing charges forms a bolt. It speeds through the cloud in a series of cascading branches roughly 160 ft long; a new spur forms every 50-millionths of a second. These can also escape a cloud to eventually form cloud-to-ground lightning.
3. path connect: as negative charges shoot down the cloud, they repel electrons - leaving positively charged columns of air stretching up from the earth and the tops of tall buildings and trees. When one of these connects with a bolt's branch, the merger sets the lightning's path.
4. stroke returns: the meet-up forms a 'wire in the sky' that sends electrons spilling into the ground and carries about 30,000 amps (enough to power 2000 wall sockets). The bolt looks like an instant flash but actually zaps up its path at 1/3 the speed of light.
6. proton punch: much more rarely, a cloud's positively charge upper region manages to connect with negative particles in the ground. Such strikes can carry 300,000 amps, and are so strong, they can make landfall as far as 25 mi from the storm itself.
7. thunder formation: the main stroke heats air to around 50,000°F, causing gases to expand rapidly and create a shock wave. That force weakens to audible sound as it travels, eventually reaching your ear as thunder.
8. seconds pass: you see lightning instantly, but thunder moves at sound speed in air (Mach 1). A familiar rule of thumb: begin counting when you see the flash, and stop when you hear the thunder. For every five seconds, the strike is about a mile away.



	Now read this! ELEVEN people, including eight children, were struck by lightning in the Parc Monceau in Paris park on May 28, 2016 during a child's birthday party while sheltering under a tree. Several sustained life-threatening injuries.3

• Tornadoes

• near-vertical funnel-shaped clouds that form in violent thunderstorms
• rotates extremely rapidly around the axis of the funnel
• extremely low air pressure at the center due to strong updraft (ΔP -100mbar)
• diameter of the base only 5-1500m wide, making tornadoes extremely local but path of destruction can be 1-500km long

Tornadoes are very destructive due to
• strong winds (probably more than 500km/h)
• low pressure at center


• of all weather phenomena, tornadoes have the highest wind speeds; most wind speeds in a tornado are only estimated using satellite imagery; instruments to obtain wind speeds in situ (anemometers) break before the fastest winds go through


• an urban myth: the low pressure in the tornado sucks out the windows of a house and it is best to leave the windows open. Though air pressures are extremely low near the center, it is most likely the high winds that shattered the windows. However, the extremely low pressure at the center helps to attain the high wind speeds around it.
• another urban myth: "tornadoes rotate counterclockwise due to the Coriolis effect, just like big storm systems do." It is true that many, if not most tornadoes in Tornado Alley rotate counter clockwise. But the wind speeds in a tornado are too fast to be influenced by the Coriolis Effect. The reason why these tornadoes rotate counter-clockwise has something to do with where in a storm, in the U.S., they occur.


• in the U.S., tornadoes travel at 0-100km/h from southwest to northeast due to prevailing winds
• in the U.S., most tornadoes occur in the "tornado alley" (see below) and in Florida
• derechos: strong downburst winds associated with a cluster of severe thunderstorms cam form so-called straight-line winds (wind speed > 165 km/h or 104 mph); in contrast to tornadoes no rotational motion; the winds are called derechos when the associated damage extends for several hundreds of kilometers; derechos typically occur at night

• Tornadoes and the Fujita Scale

The Fujita scale categorizes the tornadoes by wind speed and destruction into 6 categories (F0-F5). The wind speeds are estimated as no one has yet succeeded in measuring winds directly in a tornado.

Category	Descriptor	Wind Speed	Path Length	Typical Damage
		km/h (mph)	and width
F0	weak	64-116 (40-72)	0-1.6 km; 0-17m	branches and windows broken
F1	moderate	117-180 (73-112)	1.6-5.0 km; 18-55m	trees broken; shingles peeled off; mobiles homes moved off their foundation
F2	strong	181-253 (113-157)	5-16 km; 56-175m	large tree broken; mobiles homes destroyed; roofs torn off
F3	severe	254-332 (158-206)	16-50 km; 176-556m	trees uprooted; cars overturned; well-constructed roofs and walls removed
F4	devastating	333-418 (207-260)	50-160 km; 560-1500m	strong houses destroyed; buildings torn off foundations; cars thrown; trees carried away
F5	incredible	419-512 (261-318)	160-500 km; 1500-5000m	cars and trucks carried more than 90m; strong houses disintegrated; bark stripped off trees; asphalt peeled off roads

• Tornadoes and Thunderstorms

• typically near the rear end of violent thunderstorms, where winds, updraft and downdraft are strongest
• the most violent tornadoes occur in supercell thunderstorms, but single cell thunderstorms can also have (weaker) tornadoes
• a violent thunderstorm can spawn several tornadoes
• only 15% of supercell thunderstorms actually spawn tornadoes


• the ultimate rotation direction (clockwise or counterclockwise) of the tornado depends on where the cold front is coming from (west or east) relative to the location of the updraft and downdraft in the thunderstorm

• Conditions for Tornadoes to Form

• conditionally unstable atmosphere is essential for tornado development (cold air above warm air)
• strong wind shear: tornadoes therefore tend to form in intense thunderstorms
• strong, rotating updraft (mesocyclone), driven by jet stream blowing directly above unstable atmosphere

• Tornado Alley

• most tornadoes form in North America:
• in U.S., in last 30 years: 420-1100 tornadoes per year with 80 casualties, though a single F5 tornado may kills hundreds
• "tornado alley": band extending from Texas north through Kansas, then east to Indiana
• conditions for tornadoes to form in Tornado Alley
  In spring and early summer, cold polar air from Canada can move south and collide with warm tropical air moving in from the Gulf of Mexico. The location and movement of the jet stream also is instrumental for tornado formation. The jet stream high aloft blows in yet another direction, over a low pressure trough that sits to the east of the collision of warm and cold air below.
  
  So the three conditions that increase the chance of violent thunderstorms and the formation of tornadoes in "Tornado Alley" (starting from the lowest altitude and moving upward):
  1. a low-altitude, northerly flow of marine tropical air from the Gulf of Mexico (at about 5000ft) (southeast winds)
  2. a mid-altitude, cold and dry air mass moving down from Canada or east from the Rockies (at about 10,000ft) (strong westerlies)
  3. high-altitude jet-stream winds (at about 30,000ft) racing (north)eastward, on the eastern side ALONG a cold front
  these winds blow in different directions and provide a "conditionally unstable atmosphere" in which tornado-spawning thunderstorms can form. In addition to this, several other factors play a role:
  4. at the surface, a cold front chases a warm front near the center of a wave cyclone, i.e. a low-pressure system (see Lecture 18); the most likely place for violent tornadoes is then near the "low" and in front of the cold front
  5. a trough of low pressure exists to the west of the surface low at an altitude of 18,000ft, just below the jet stream


• tornado season: tornadoes can occur year-round but most likely from March through September; in Tornado Alley, the majority of tornadoes occur Apr-Jul.
  On the other hand, most fatalities from tornadoes occur in Mar-May, perhaps indicating that tornadoes earlier in the year are more violent (unlikely?) or that, after a long winter, people are initially not prepared for tornadoes.
• following the influx of marine tropical air from the Gulf of Mexico,
  • tornadoes in the spring tend to occur in the south
  • tornadoes in summer tend to occur in the north
  
  
• Prediction: meteorologists watch out for tornado-suspicious weather conditions on a large scale, using Doppler Radar and other satellite methods; warning are issued as general measures; but locally, there is currently only little warning time (15 min max.?). The exact paths of tornadoes currently can't be predicted but there is a tell-tale sign in the Doppler Radar images. The updraft in the mesocyclone is typically so strong that precipitation (rain and hail) cannot fall. Southwesterly winds aloft blow the precipitation northeastward, in front of the mesocyclone. If the mesocyclone lives long enough, it can swing the precipitation around it in a counterclockwise direction. While the mesocyclone itself does not show up in a Doppler Radar image, the hook-like swirl of precipitation (also called hook echo) is a sign that a mesocyclone may be present.

• How Do Tornadoes Form? ⁽²⁾

The way tornadoes form is not fully understood. The following stages describe a tornado forming in a large, supercell thunderstorm that involves the formation of a mesocyclone.

• strong wind shear, with winds in lower atmosphere blowing in one direction and aloft in another
• air then starts to rotate in horizontal cylinder; vortex tube
• a mesocyclone forms when strong updraft in a thunderstorm lifts one end of vortex tube up
• the mesocyclone now rotates about a vertical axis
• updraft, swirling precipitation, and surrounding air may all interact to produce a rear flank downdraft. When this downdraft strikes the ground, it may interact with the region of surface inflow into the mesocyclone to produce a supercell tornado (under favorable conditions).
• a first sign that a tornado is about to form is the sight of a wall cloud, a rotating cloud at the base of the storm that will lower toward the ground to form a funnel cloud.

The rapidly rotating funnel cloud will then extend from the wall cloud down to the surface. The steps involved in this process:
• mesocyclone expands vertically and shrinks horizontally; air spirals inward and, due to conservations of angular momentum, it gets faster (like ice skater tugging in arms); spinning air accelerates upward
• air rushes upward into low-pressure core of mesocyclone; air expands and cools; condensation into funnel cloud, if sufficient moisture present
• air beneath the funnel cloud is drawn into core; air cools rapidly and condenses; funnel cloud descends toward surface
• when touchdown, dirt and debris is sucked into funnel to make it look dark
• air along outside of funnel is spiraling upward, while air inside is descending toward extreme low pressure near surface; resulting evaporation during warming makes core free of clouds
• conditions have to be just right; i.e. only 15% of supercell thunderstorms produce tornadoes

• National Geographic Storm Chaser Story

In 2013, several storm chasers died on May 31, when they tried to get in the path of a tornado to collect scientific data. In its November 2013 issue, National Geographic printed a story about Tim Samaras and his team. It reads a bit like the story in the 1996 movie "Twister" where Helen Hunt and Bill Paxton play scientists competing with other teams to chase tornadoes. Here is the link to the online version, including more features on tornadoes and how they form. The story also tries to explain why storm chasers put themselves in harms way, as many scientists often do when they do field work.

• Recommended Reading

• ⁽¹⁾"Extreme Weather" by Christopher C. Burt, 2004, Norton&Comp, ISBN: 0-393-32658-6
• ⁽²⁾"Meteorology Today" by C. Donald Ahrens, 2003, Thomson Brooks/Cole, ISBN: 0-534-39771-9
• ⁽³⁾ Time Magazine, June 13, 2016
• ⁽⁴⁾ modified from: Popular Science, Winter 2019

• Glossary

The following definitions are from the American Meteorological Society Glossary of Meteorology via the NOAA website
• tornado: A violent rotating column of air, in contact with the surface, pendant from a cumuliform cloud, and often (but not always) visible as a funnel cloud.
• landspout: 1. (rare) a tornado. 2. Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer.
• nonsupercell tornado: A tornado that occurs with a parent cloud in its growth stage and with its vorticity originating in the boundary layer. The parent cloud does not contain a preexisting midlevel mesocyclone. The landspout is an example of the nonsupercell tornado.

1. Alaska
2. Texas
3. Oklahoma
4. Florida
5. California

1. cold front moving in
2. strong convective lifting during the summer
3. heat waves
4. strong convective lifting during the winter
5. hurricanes

1. true
2. false

1. low air pressure in center
2. strong winds
3. extensive rain and hail
4. all of the above
5. a) and b)

1. winter
2. spring
3. summer
4. fall

1. true
2. false

1. 5%
2. 15%
3. 50%
4. 85%