1. Thunderstorms and Tornadoes
Chapter 14
Thunderstorms and Tornadoes

2. Wall Cloud associated with a super cell thunderstorm

3. Thunderstorms
A storm containing lightening and thunder; convective storms may have heavy rain hail
Ordinary Cell Thunderstorms
Air-mass thunderstorms: limited wind sheer
Stages: cumulus, mature, dissipating
Entrainment, downdraft, gust front

4. FIGURE 14.2 Simplified model depicting the life cycle of an ordinary cell thunderstorm that is nearly stationary as it forms in a region
of low wind shear. (Arrows show vertical air currents. Dashed line represents freezing level, 0oC isotherm.)

5. A dissipating thunderstorm
FIGURE 14.3 A dissipating thunderstorm
near Naples, Florida. Most of the cloud
particles in the lower half of the storm have
evaporated.
A dissipating thunderstorm

6. Thunderstorms Multi-cell Thunderstorms
Thunderstorms that contain a number of convection cells, each in a different stage of development, moderate to strong wind shear; tilt, over shooting top

7. Multicell storm

8. Thunderstorms Multi-cell Thunderstorms
Micro-bursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind; wind shear, virga

9. Dust clouds from microbursts

10. Thunderstorms Multi-cell Thunderstorms
Gust Front: leading edge of the cold air out-flowing air; shelf cloud, roll cloud, outflow boundary

11. ACTIVE FIGURE 14.5 A simplified model describing air motions and other features associated with an intense multicell
thunderstorm that has a tilted updraft. The severity depends on the intensity of the storm’s circulation pattern. Visit the Meteorology
Resource Center to view this and other active figures at academic.cengage.com/login

12. Thunderstorms
Severe thunderstorms: one of large hail, wind gusts greater than or equal to 50kts, or tornado, tilted updraft/downdraft

13. Shelf cloud FIGURE 14.7 A dramatic example
of a shelf cloud (or arcus cloud) associated
with an intense thunderstorm. The
photograph was taken in the Philippines
as the thunderstorm approached from the
northwest.
Shelf cloud

14. FIGURE 14.8 A roll cloud forming
behind a gust front.

15. FIGURE 14.9 Radar image of an outflow boundary. As cool
(more-dense) air from inside the severe thunderstorms (red and orange
colors) spreads outward, away from the storms, it comes in contact with
the surrounding warm, humid (less-dense) air, forming a density
boundary (blue line) called an outflow boundary between cool air and
warm air. Along the outflow boundary, new thunderstorms often form.

16. Thunderstorms Multi-cell Thunderstorms
Squall-line thunderstorms; line of multi-cell thunderstorms, pre-frontal squall-line, derecho

17. Pre frontal Squall line
FIGURE A Doppler radar composite showing a prefrontal
squall line extending from Indiana southwestward into Arkansas.
Severe thunderstorms (red and orange colors) associated with the squall
line produced large hail and high winds during October, 2001.
Pre frontal
Squall
line

18. FIGURE 14.13 Pre-frontal squall-line thunderstorms may form
ahead of an advancing cold front as the upper-air flow develops waves
downwind from the cold front.

19. Pre frontal
Squall
line

20. FIGURE 14.14 A model describing
air motions and precipitation associated
with a squall line that has a trailing
stratiform cloud layer.

21. FIGURE 14.15 A side view of the lower
half of a squall-line thunderstorm with the
rear-inflow jet carrying strong winds from
high altitudes down to the surface. These
strong winds push forward along the surface,
causing damaging straight-line winds that may
reach 100 knots. If the high winds extend
horizontally for a considerable distance, the
wind storm is called a derecho.

22. FIGURE 14.16 The red and orange on this Doppler radar image
show an intense squall line moving south southeastward into Kentucky.
The thunderstorms are producing strong straight-line winds called a
derecho. Notice that the line of storms is in the shape of a bow. Such
bow echos are an indicator of strong, damaging surface winds near the
center of the bow. Sometimes the left (usually northern) side of the
bow will develop cyclonic rotation and produce a tornado.
The thunderstorms are producing strong straight-line winds called a derecho

23. Thunderstorms Multi-cell Thunderstorms
Meso-scale Convective Complex: a number of individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system; summer, 10,000km2

24. FIGURE 14.17 An enhanced infrared satellite image showing
the cold cloud tops (dark red and orange colors) of a Mesoscale
Convective Complex extending from central Kansas across western
Missouri. This organized mass of multicell thunderstorms brought
hail, heavy rain, and flooding to this area.

25. Thunderstorms Supercell thunderstorms
Large, long-lasting thunderstorm with a single rotating updraft
Strong vertical wind shear
Outflow never undercuts updraft
Classic, high precipitation and low precipitation supercells
Rain free base

26. A supercell thunderstormwith a tornado sweeps over Texas
FIGURE A supercell thunderstorm
with a tornado sweeps over Texas.

27. FIGURE Some of the features associated with a classic tornado-breeding supercell thunderstorm as viewed from the southeast.
The storm is moving to the northeast.

28. FIGURE 14.20 A wall cloud photographed
southwest of Norman, Oklahoma.

29. Thunderstorms Supercell thunderstorms Strong vertical wind shear
Surface, 850mb, 700mb, 500mb, 300mb conditions
low-level jet

30. FIGURE 14.21 Conditions leading to the formation of severe
thunderstorms, and especially supercells. The area in yellow shows
where supercell thunderstorms are likely to form.

31. Thunderstorms
Supercell thunderstorms
Cap and convective instability

32. FIGURE 14.22 A typical sounding of air temperature and
dew point that frequently precedes the development of supercell
thunderstorms.

33. Thunderstorms Thunderstorms and the Dryline
Sharp, horizontal change in moisture
Thunderstorms form just east of dryline
cP, mT, cT

34. Figure 14.23
Surface conditions that can produce a dryline with intense thunderstorms.
Fig , p. 384

35. Thunderstorms Floods and Flash Floods
Large floods can be created by training of storm systems, Great Flood of 1993
Flash floods rise rapidly with little or no advance warning; many times caused by stalled or slow thunderstorm

36. FIGURE 14.24 The heavy arrow represents the average position
of the upper-level jet stream from mid-June through July, The jet
stream helped fuel thunderstorms that developed in association with a
stationary front that seemed to oscillate back and forth over the region
as an alternating cold front and warm front. The 鏑� marks the center of
a frontal wave that is moving along the front. Many of the thunderstorms
that formed in conjunction with this pattern were severe, and
over a period of weeks produced 典he Great Flood of 1993.� Most of
the counties within the blue-shaded area were declared 電isaster areas�
due to flooding.

37. downtown Des Moines, Iowa, during July, 1993
Figure 14.25
Flooding during the summer of 1993 covered a vast area of the upper Midwest. Here, floodwaters near downtown Des Moines, Iowa, during July, 1993, inundate buildings of the Des Moines waterworks facility. Flood-contaminated water left 250,000 people without drinking water.
downtown Des Moines, Iowa, during July, 1993
Fig , p. 387

38. Thunderstorms Topic: Big Thompson Canyon
July 31, 1976, 12 inches of rain in 4 hours created a flood associated with $35.5million in damage and 135 deaths

39. Figure 1
Weather conditions that led to the development of intense thunderstorms that remained nearly stationary over the Big Thompson Canyon in the Colorado Rockies. The arrows within the thunderstorm represent air motions.
Fig. 1, p. 386

40. Flash Floods
Slow moving or Stalled thunder Storm, especially in canyon areas
135 deaths in 1976 flood
12 inches of rain in 4 hours
(normal ~16 inches /year)

44. Thunderstorms Distribution of Thunderstorms
Most frequent Florida, Gulf Coast, Central Plains
Fewest Pacific coast and Interior valleys
Most frequent hail Central Plains

45. FIGURE 14.26 The average number of
days each year on which thunderstorms are observed
throughout the United States. (Due to the
scarcity of data, the number of thunderstorms is
underestimated in the mountainous far west.)

46. FIGURE 14.27 The average number of
days each year on which hail is observed
throughout the United States.

52. Thunderstorms Lightening and Thunder
Lightening: discharge of electricity in mature storms (within cloud, cloud to cloud, cloud to ground)
Thunder: explosive expansion of air due to heat from lightening
Electrification of Clouds: graupel and hailstones fall through supercooled water, ice crystals become negatively charged
Upper cloud positive, bottom cloud negative

54. FIGURE 14.28 The lightning stroke can travel in a number of
directions. It can occur within a cloud, from one cloud to another
cloud, from a cloud to the air, or from a cloud to the ground. Notice
that the cloud-to-ground lightning can travel out away from the cloud,
then turn downward, striking the ground many miles from the thunderstorm.
When lightning behaves in this manner, it is often described
as a “bolt from the blue.”

55. FIGURE 14.29 When the tiny colder ice crystals come in
contact with the much larger and warmer hailstone (or graupel), the ice
crystal becomes positively charged and the hailstone negatively charged.
Updrafts carry the tiny positively charged ice crystal into the upper
reaches of the cloud, while the heavier hailstone falls through the
updraft toward the lower region of the cloud.

56. FIGURE 14.30 The generalized charge distribution in a mature
thunderstorm.

57. Thunderstorms
Observations: Elves
Blue jets, red sprite, ELVES

58. Figure 2
Various electrical phenomena observed in the upper atmosphere.
Fig. 2, p. 390

59. Thunderstorms The Lightening Stroke
Positive charge typically on ground, cloud to ground lightening
Stepped leader, ground stroke, forked lightening, ribbon lightening, bead lightening, corona discharge

60. ACTIVE FIGURE 14. 31 The development of a lightning stroke
ACTIVE FIGURE The development of a lightning stroke. (a) When the negative charge near the bottom of the
cloud becomes large enough to overcome the air’s resistance, a flow of electrons — the stepped leader — rushes toward the earth.
(b) As the electrons approach the ground, a region of positive charge moves up into the air through any conducting object, such
as trees, buildings, and even humans. (c) When the downward flow of electrons meets the upward surge of positive charge, a
strong electric current — a bright return stroke — carries positive charge upward into the cloud. Visit the Meteorology Resource
Center to view this and other active figures at academic.cengage.com/login

61. Thunderstorms Observation: Apple tree
DO NOT seek shelter during a thunderstorm under an isolated tree.
Lightening Detection and Suppression
Lightening direction finder detects radiowaves produced by lightening, spherics
National Lightening Detection Network
Suppression: seed clouds with aluminum

62. ACTIVE FIGURE 14. 31 The development of a lightning stroke
ACTIVE FIGURE The development of a lightning stroke. (a) When the negative charge near the bottom of the
cloud becomes large enough to overcome the air’s resistance, a flow of electrons — the stepped leader — rushes toward the earth.
(b) As the electrons approach the ground, a region of positive charge moves up into the air through any conducting object, such
as trees, buildings, and even humans. (c) When the downward flow of electrons meets the upward surge of positive charge, a
strong electric current — a bright return stroke — carries positive charge upward into the cloud. Visit the Meteorology Resource
Center to view this and other active figures at academic.cengage.com/login

63. Figure 3
A cloud-to-ground lightning flash hitting a 65-foot sycamore tree. It should be apparent why one should not seek shelter under a tree during a thunderstorm.
Fig. 3, p. 395

64. Figure 14.32
Time exposure of an evening thunderstorm with an intense lightning display near Denver, Colorado. The bright flashes are return strokes. The lighter forked flashes are probably stepped leaders that did not make it to the ground.
Fig , p. 392

65. Figure 14.33
The lightning rod extends above the building, increasing the likelihood that lightning will strike the rod rather than some other part of the structure. After lightning strikes the metal rod, it follows an insulated conducting wire harmlessly into the ground.
Fig , p. 393

67. Figure 14.34
A fulgurite that formed by lightning fusing sand particles.
fulgurite
Fig , p. 393

68. Figure 14.35
The four marks on the road surface represent areas where lightning, after striking a car traveling along south Florida’s Sunshine State Parkway, entered the roadway through the tires. Lightning flattened three of the car’s tires and slightly damaged the radio antenna. The driver and a six-year-old passenger were taken to a nearby hospital, treated for shock, and released.
Fig , p. 394

69. Figure 14.36
Cloud-to-ground lightning strikes in the vicinity of Chicago, Illinois, as detected by the National Lightning Detection Network.
sferics
Fig , p. 394

70. FIGURE 14.26 The average number of
days each year on which thunderstorms are observed
throughout the United States. (Due to the
scarcity of data, the number of thunderstorms is
underestimated in the mountainous far west.)