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Chapter 11. Thunderstorms and Tornadoes. Figure CO: Chapter 11, Thunderstorms and Tornadoes. © Dobresum/ShutterStock, Inc. What is a thunderstorm?. A cloud or a cluster of clouds that produces thunder, lightning, heavy rain, and sometimes hail and tornadoes Tall cumulonimbus clouds

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chapter 11

Chapter 11

Thunderstorms and Tornadoes

what is a thunderstorm
What is a thunderstorm?
  • A cloud or a cluster of clouds that produces thunder, lightning, heavy rain, and sometimes hail and tornadoes
  • Tall cumulonimbus clouds
    • Form when air rises or is lifted from the surface or near the surface
    • Energy is released when saturated air condenses moisture
    • Air rises rapidly and to great heights—the anvil cloud
      • Overshooting tops briefly overshoot the tropopause
      • Mammatus cloud may form beneath the anvil

Figure 01: Photo of thunderstorm

Courtesy of Dr. John R. Mecikalski


Figure 02A: Visible satellite image of thunderstorms.

Courtesy of CIMSS/SSEC/University of Wisconsin-Madison


Figure 02B: Infrared image of thunderstorms.

Courtesy of CIMSS/SSEC/University of Wisconsin-Madison


Figure 03: World thunderstorm climatology.

Adapted from WMO (World Meteorological Organization), 1956: World Distribution of Thunderstorm Days. WMO Publ. No. 21, TP. 21.


Figure 04: U.S. thunderstorm climatology

Courtesy of Oklahoma Climatological Survey

conditions for thunderstorm formation
Conditions for Thunderstorm Formation
  • A warm moist air mass at or just above the surface
    • High dew-point temperature
    • Maritime tropical air mass
  • A deep layer of conditional instability
    • Saturated air parcels rise freely
    • In the tropics this is the average condition
    • Stability of an air mass can change
conditions for thunderstorm formation continued
Conditions for thunderstorm formation (continued)
  • How stability changes (becomes less stable or more unstable)
    • Warm air advection at low levels
    • Cold air advection at upper levels
    • Lifting of a stable layer that is humid at its base and dry at its top (convective or potential instability)
      • Also described a blowing a capping inversion
    • Surface heating
conditions for thunderstorm formation continued1
Conditions for thunderstorm formation (continued)
  • Lifting mechanisms (more than one may be active) to get air parcels saturated
    • Free convection from buoyancy due to surface heating
      • Common in warm season
      • Thermals, then cumulus clouds
    • Forced lifting from topography
      • Many thunderstorms occur on the upwind side of mountains
conditions for thunderstorm formation continued2
Conditions for thunderstorm formation (continued)
  • Frontal lifting
    • Especially cold fronts
    • Also drylines
  • Convergence
    • Sea breezes converging in central Florida
    • Low pressure centers in the tropics
    • Hurricane is the largest collection of thunderstorms
  • Nocturnal low-level jet (just above the surface)
    • An important ingredient in severe weather
    • Supplies moisture and energy at low levels
lifted index
Lifted Index
  • Is a way to describe stability with one number
  • Is not a perfect measure of stability
  • Is useful for forecasting, but not the only criterion forecasters use
  • Is a difference in temperature at 500mb
  • The parcel temperature is determined by raising a surface parcel to saturation at the DALR, and to 500mb at the SALR
  • LI = T(environment) – T(air parcel)
  • Negative values are unstable

Figure 05: Satellite image of lifted index and severe weather reports

Courtesy of CIMSS/SSEC/University of Wisconsin-Madison

conditions for thunderstorm formation continued3
Conditions for thunderstorm formation (continued)
  • For the more severe thunderstorms, vertical shear of the horizontal wind
    • The westerly (west to east) part of the wind increasing as height increases
    • Clockwise turning of the direction from which the wind blows as height increases
      • For example, southeasterlies at the surface, southerlies at 850mb, southwesterlies at 700mb, and westerlies at 500mb

Figure 06: Severe weather schematic

Adapted from Athrens, C.D. Meteorology Today, Ninth edition. Brooks Cole, 2009


Figure 07: LANDSAT image of tornado destruction

Courtesy of USGS EROS Data Center with processing by Environmental Remote/Landsat-7

thunderstorm cells
Thunderstorm Cells
  • A cell is a compact region of a cloud that has a strong vertical updraft
    • Ordinary cells are a few km in diameter, last less than an hour
    • Supercells are larger and can last several hours
      • Account for the vast majority of severe thunderstorm weather
  • Multicell thunderstorms are composed of lines or clusters of thunderstorm cells, ordinary, supercell, or both
ordinary single cell thunderstorm
Ordinary Single-Cell Thunderstorm
  • Life cycle has three distinct stages
    • Cumulus—parcels ascend in the updraft and get saturated at the lifting condensation level or LCL, which marks cloud base
      • Mixing with environment air is called entrainment
    • Mature—begins when precipitation starts to fall
      • Time of most lightning, rain, small hail
      • A downdraft develops with cooling due to evaporating precipitation
    • Dissipating—updraft weakens, downdraft dominates
  • Also known as the air mass thunderstorm
multicell thunderstorms
Multicell Thunderstorms
  • Composed of several individual single-cell storms, each one at a different stage of development
    • Can last several hours
    • A moderate amount of vertical wind shear
      • Updraft and downdraft can coexist
      • Updraft and downdraft meet at the gust front
    • Groups of multicell thunderstorms are called mesoscale convective systems
squall lines
Squall Lines
  • A squall line is composted of individual intense thunderstorm cells arranged in a line or band
    • Occur along a boundary of unstable air
    • Have life spans of 6 to 12 hours or more
    • Extend over several states simultaneously
    • A shelf cloud is often observed above the gust front
    • Often observed ahead of a cold front
    • Divergence aloft and a broad, low-level inflow of moist air favor development of squall lines

Figure 12: Photo of shelf cloud

© Peter Wollinga/

mesoscale convective complex mcc
Mesoscale Convective Complex (MCC)
  • An MCC is a complex of individual storms that covers a large area in an infrared satellite image and lives more than 6 hours
    • Often form in late afternoon and evening
    • In satellite images give the appearance of a large circular storm with cold cloud-top temperatures
    • Often form underneath a ridge of high pressure
      • Because upper-level divergence can occur in a ridge
    • Do not require as much vertical shear as squall lines
    • Can be maintained by the low-level jet

Figure 13: Satellite image of MCC

Courtesy of SSEC and CIMSS, University of Wisconsin-Madison

supercell thunderstorms
Supercell Thunderstorms
  • The supercell thunderstorm is a large single-cell storm, sometimes 32 km or more across, that almost always produces dangerous weather
    • Strong wind gusts, large hail, dangerous lightning and tornadoes
    • Require a very unstable atmosphere
    • Require both directional and speed shear
      • Vertical wind shear causes supercell thunderstorms to rotate about a vertical axis
cyclonic right moving supercell
Cyclonic Right-Moving Supercell
  • The spinning updraft is called a mesocyclone
    • Resembles an extratropical cyclone
    • 5 to 20 km across
    • Narrows and rotates more quickly when it stretches
    • Is too large and too slow in rotation to be a tornado
    • Has an overshooting cloud top
    • Has two downdrafts ahead of (forward flank) and behind (rear flank) the center of the storm
    • Downdrafts caused by precipitation
    • Gust fronts ahead of the downdrafts

Figure 17: The surface conditions associated with a typical supercell thunderstorm

Courtesy of Oklahoma Climatological Survey

  • Microbursts develop when rain falling from a thunderstorm evaporates underneath the cloud, cooling the air beneath
    • Cold heavy air plunges to the surface and splashes against the ground
    • Air then rushes sideways and swirls upward as a result of the pressure gradient between the cold air and the warm surroundings
  • Microbursts can do as much damage as a small tornado

Figure 18ab: Microburst

Courtesy of Mike Smith,


Figure 18cde: Microburst

Courtesy of Mike Smith,


Figure 20: Ronald Reagan landing before a microburst

Adapted from Fujita, T. The Downburst. University of Chicago Press, 1985.

  • Tornadoes are rapidly rotating columns or funnels of high wind that spiral around very narrow regions of low pressure beneath a thunderstorm
    • Visible because of condensation, dust, and debris
    • Nearly always rotate cyclonically, often move to the northeast
    • If the circulation does not reach the ground, called a funnel cloud
    • Usually < 1.6 km across

Figure 21: Girl in front of tornado

Courtesy of Marilee Thomas

tornado formation
Tornado Formation
  • Most form underneath supercell thunderstorms
  • The cloud base underneath the updraft on the rear side of the thunderstorm may lower
    • Forming a rotating wall cloud
  • A rapidly rotating column of air much smaller than the mesocyclone may protrude beneath the wall cloud
  • As water vapor condenses in the air rushing up into this column, a funnel cloud may form and reach the ground, becoming a tornado

Figure 22: The ominous approach of a rotating wall cloud is a sign that a tornado may develop at any moment.

Courtesy of Nolan T. Atkins

tornado life cycle
Tornado Life Cycle
  • Four stages in the life cycle
    • Organizing stage, funnel picks up debris, as it reaches the surface and widens
    • Mature stage, tornado at peak intensity and width
    • Shrinking stage, the funnel narrows
    • Decaying or rope stage, when the funnel thins out to a very narrow ropelike column after which it eventually dissipates

Figure 23: The life cycle of the Union City, Oklahoma, tornado on May 24, 1973

Modified from Golden, J. H., and D. Purcell, Mon. Wea. Rev., 106 [1978]: 3–11.

tornado signatures on radar
Tornado Signatures on Radar
  • Hook echo
    • The pattern of heavy rain inside a supercell forms a kind of hook around the region most likely to produce a tornado
  • Tornado vortex signature
    • A couplet of red and green colors on Doppler radar images
  • Mesocyclone signature
    • A couplet of red and green colors on Doppler radar images larger than the tornado vortex signature
tornado winds and the ef scale
Tornado Winds and the EF Scale
  • The Enhanced Fujita (or EF) scale ranges from 0 to 5, with 5 the most damage
  • The scale uses 28 damage indicators, like schools, barns and vegetation and the damage to each helps place the tornado on the scale
  • The higher the scale number, the more severe are the tornado’s wind and damage
  • Some of the most severe tornadoes are multiple vortex tornadoes

Figure 25: Fujita scale pie charts

Courtesy of Tom Grazulis,


Figure 26: Multiple-vortex tornado

Modified from Tom Grazulis, The Tornado: Nature’s Ultimate Windstorm, Oklahoma University Press, 2000, p. 111.


Figure 27: U.S. tornado climatology

Courtesy of Oklahoma Climatological Survey


Figure 28: A climatology of the relative frequency of killer tornado events from 1950 to 2004.

Courtesy of Dr. Walker Ashley, Meteorology Program, Department of Geography, Northern Illinois University


Figure 29: Time of year of maximum tornado risk

Adapted from H. E. Brooks et al., Wea. Forecasting, 4 (2003).


Figure 30: Line graph of tornado deaths per million Americans

Courtesy of Dr. Charles A. Doswell III,


Figure B03: Photo of Parsons Company damage

Courtesy of NOAA/Matt Dayhoff, Peoria Journal Star


Figure 32: Greensburg, KS devastation

Courtesy of Greg Henshall/FEMA


Figure 33: Path of Atlanta tornado in EF scale

Courtesy of National Weather Service Forecast Office, Peachtree City, GA/NOAA


Figure 34: Atlanta skyscraper post-tornado

Courtesy of Bruce Bracey

the waterspout
The Waterspout
  • Waterspouts are narrow spinning funnels of rising air that form underneath clouds
    • Usually shorter cumulus clouds that are not rotating
    • Low pressure at the center sets up a pressure gradient that drives air inward
    • Air rising and cooling condenses, making the funnel visible
    • Strongest waterspouts only as strong as the weakest tornadoes, < 160 km/hr

Figure 35: Waterspout

Courtesy of Dr. Joseph Golden/NOAA

  • Lightning is a huge electrical discharge
  • Lightning caused by rising and sinking air motions that occur in mature thunderstorms
    • Can travel from cloud to cloud, within the same cloud, or from cloud to ground
    • In-cloud discharges by far most common
  • A lightning bolt is actually a series of flashes

Figure 36: Lightning

© Harald Edens,

lightning strikes a tree
Lightning Strikes a Tree
  • First, charge separates in the cloud
    • Collisions of ice crystal with graupel
  • Second, the ground becomes positively charged
    • The base of the cloud is negatively charged, and like charges repel
    • The voltage between cloud and ground builds up
  • Third, lightning formation begins
    • A pilot leader, then dart leaders of negative charge move down from the cloud
  • Fourth, a brilliant flash is observed
    • Current flows upward in the return stroke

Figure 37B: Negative charges build up near the base of the cloud, the ground repels negative charges and changes from its usual negative to a positive charge.


Figure 38: Color graphic of lightning climatology

Adapted from Orville, R., and Huffines, G., Monthly Weather Review, May 2001.


Figure 39: Schematic of upper-atmosphere lightning: elves, sprites, etc.

Courtesy of National Severe Storms Laboratory/NOAA

flash floods and flooding
Flash Floods and Flooding
  • A flood is a substantial rise in water that covers areas not usually submerged
    • Water flows into a region faster than it can be absorbed, stored, or removed into a drainage basin
    • Caused by high-intensity rainfall, prolonged rainfall, or both
    • Great threats to human life
  • A flash flood is a sudden local flood that has a great volume of water and a short duration
    • Key elements: rainfall intensity and duration

Figure 40: Six Flags under water

© Erik S. Lesser/Landov

  • Hail is precipitation in the form of large balls or lumps of ice
  • Hailstones begin as small ice particle
  • Hailstones grow by accretion of supercooled water droplets
  • Dry growth occurs when the drops freeze on contact—little liquid water on the surface
  • Wet growth occurs when the droplet don’t freeze quickly and spread across the surface of the hailstone—a film of liquid water on the surface
producing hailstones
Producing Hailstones
  • Production of large hailstones requires a strong updraft that is tilted and an abundant supply of supercooled water
    • Hail occurs in regions near the strong updraft
    • Supercell thunderstorms often produce the largest hail
    • The curtain of hailstones that falls below cloud base is called the hailshaft
    • The hailswath is the section of the ground covered with hail

Figure 42: Graphic of hail occurrence across U.S.

Hailstorms Across the Nation by S. Channgon, D. Channgnon, and S. Hilbert, Image courtesy of the Midwestern Regional Climate Center, Illinois State Water Survey