1. Chapter 10: Thunderstorms and Tornadoes
Tornadic thunderstorms
Severe weather and doppler radar

2. Thunderstorms

3. Thunderstorms What do we already know about thunderstorms?
Can produce intense rain, hail, wind, lightning
Can make tornadoes
Formed by rising air from:
Uneven surface heating
Uplift along a frontal boundary
Topographical uplift 3

4. Thunderstorms Definitions:
Supercell thunderstorms – large updraft storms that can produce flash floods, severe weather, tornadoes
Severe Thunderstorm – has to have one of following:
Hail greater than or equal to ¾ inch
A tornado
Wind gusts 50 mph 4

5. Ordinary Cell Thunderstorms
Form in region of low wind shear
Can form due to surface air convergence
Cumulus stage (growth stage) – due to rising warm, humid air condensing into cumulus cloud
Latent heat release keeps cloud warm and unstable
Grows quickly to towering cumulus
Usually no precipitation because of updrafts
No lightning or thunder 5

6. Ordinary Cell Thunderstorms6

7. Ordinary Cell Thunderstorms
Mature stage – marked by considerable downdrafts
Cloud particles grow larger, and begin to fall
Entrainment - Dry air is sucked into cloud, causing evaporation and cooling
Heavier and cooler air descends…downdrafts
Formation of updraft and downdraft cells
Most intense time of thunderstorm
Storm may grow as high as stratosphere (anvil)
Heavy rain, lightning, small hail 7

8. Ordinary Cell Thunderstorms
Mature stage
Cloud top may overshoot into stratosphere 8

9. Ordinary Cell Thunderstorms
Mature stage
Cloud top may overshoot into stratosphere
Downdraft reaches ground and spreads along surface as a gust front
Sometimes rain may not reach ground, but cold air does 9

10. Ordinary Cell Thunderstorms10

11. Ordinary Cell Thunderstorms
Dissipating stage – thunderstorm weakens
Updrafts begin to weaken after 15 to 30 min.
Gust front moves too far from the storm, so updrafts have to weaken
Light precipitation falls
Only anvil remains
All three stages in less than an hour 11

12. Ordinary Cell Thunderstorms12

13. Figure 10.1: Simplified model depicting the life cycle of an ordinary thunderstorm that is nearly stationary. (Arrows show vertical air currents. Dashed line represents freezing level, 0°C isotherm.)
Watch this Active Figure on ThomsonNow website at
Stepped Art
Fig. 10-1, p. 265

14. Severe Thunderstorms and the Supercell
So ordinary cell storms weaken because the updraft weakens
What if the updraft doesn’t weaken quickly?
If moderate wind shear pushes downdraft downwind, updraft is not cut off
If downdraft then undercuts updraft, a multicell storm forms

15. Severe Thunderstorms and the Supercell15

16. Severe Thunderstorms and the Supercell
Multicell storms
Cells of varying age co-mingling
Top of cloud well into stratosphere
Updrafts allow hail to grow large 16

17. Severe Thunderstorms and the Supercell
Supercells
If wind shear is strong and changes direction with height, downdraft may not undercut updraft
Updraft remains strong for long time (> 1 hour)
One rotating vertical column
Can create tornadoes
Hail size of grapefruit 17

18. Severe Thunderstorms and the Supercell
At surface, open wave with warm, humid air ahead of cold front
Overrunning warm air just above surface
Dry air at 700 mb level
Trough of low pressure at 500 mb
Divergence at 300 mb 18

19. Severe Thunderstorms and the Supercell
Severe thunderstorms form in light green area because:
Warm air below cold air is conditionally unstable
Strong wind shear creates severe thunderstorms
In morning, atmos is stable, no thunderstorms
Surface heating creates thunderstorms 19

20. Squall Lines and Mesoscale Convective Complexes
Squall line – line of multicell t-storms extending many kilometers
Mesoscale convective complex- cluster of storms in one spot

21. Squall Lines and Mesoscale Convective Complexes
Squall lines often form ahead of cold fronts (pre-frontal squall-line thunderstorms)
MCCs are long-lasting and provide a lot of rain, and a ton of severe weather
Form during summer 21

22. Dryline Thunderstorms
Dryline – narrow zone with a sharp moisture contrast. Where do the thunderstorms form?

23. Gust Fronts, Microbursts and Derechoes
Gust front – leading edge of the thunderstorm downdraft
Passage can feel like a cold front passage
Shelf cloud – warm air rising above gust front creates cloud
Roll cloud – cloud just behind the gust front that spins horizontally
Outflow boundary – a huge gust front formed by numerous thunderstorm gust front
Downbursts – localized downdraft like water from a tap
Microbursts – downbursts that extend 4 km or less

24. Gust Fronts, Microbursts and Derechoes24

25. Gust Fronts, Microbursts and Derechoes25

26. Figure 10.11: 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.
Fig , p. 271

27. Figure 10.11: 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. 27

28. Figure 10.11: 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. 28

29. Microburst Microbursts can be dangerous to aircrafts
Plane encounters uplift at (a), and pilot puts nose down
Unexpected downdraft crashes plane
Has caused several airplane crashes
Some airports have microburst detection 29

30. Bow Echo and Derecho
Bow echo – a line of thunderstorms often form a bow echo on radar
Derecho – winds associated with downdrafts that exceed 104 mph and can cause a lot of damage…20 derechoes each year in U.S. 30

31. Floods and Flash Floods
Flash floods – floods that rise rapidly with little to no advance warning
Thunderstorms cause flash floods in two ways:
Hovering above one area for a long time
Training – thunderstorms keep passing over same area

32. Distribution of Thunderstorms
18 millions thunderstorms worldwide annually
Need to have a combination of moisture and warmth
So do thunderstorms form in the Arctic? ITCZ? Gulf?

33. Lightning and Thunder
Lightning – simply a discharge of electricity, usually in mature thunderstorms
Heats air up to 54,000F (5x hotter that surface of sun
Thunder – the explosive expansion of the hot air creates a sound wave that travels in all direction
According to the Guinness Book of World Records, Roy Sullivan holds the world’s record for being struck by lightning seven times between 1942 and his death in 1983.

34. Lightning and Thunder
Light travels faster than sound, so we see the lightning before we hear the thunder
Sound takes 5 seconds to travel 1 mile
So if we hear thunder 5 seconds after we see the lightning, the stroke was 1 mile away

35. Lightning and Thunder

36. Electrification of Clouds
Must have charging of the cloud to have lightning
Electrification of clouds not fully understood
Theory 1: Supercooled droplets fall through cloud and collide with warm hailstone. Latent heat is released (warm or cold?)

37. Electrification of Clouds
Net transfer of positive ions from warm to cold object
Falling hailstone is negatively charged, falls to bottom of cloud
Light positively charge particle is lifted to top of cloud
Small area of positively charge particles near melting level 37

38. Electrification of Clouds
Theory 2: When precip forms, it has a neg in the upper portion and pos charge in the lower
As droplets collide, the large droplets become neg and fall
The small droplets are lifted by updrafts and rise 38

39. The Lightning Stroke Basics of lightning Opposite charges attract
Positive charges on ground follow negative charges at base of cloud
Electrical current will not flow because air is good insulator
Charge must be large (< 1million volts per meter) to create a lightning bolt

40. The Lightning Stroke Cloud-to-ground lightning
Discharge of electrons from cloud to ground as a stepped leader (many times)
Positive charges race back up from elevated object as a return stroke 40

41. The Lightning Stroke Cloud-to-ground lightning
Many electrons flow to ground and a stronger return stoke follows (this is what you see)
1/10,000 of a second so it looks like one continuous flash 41

42. The Lightning Stroke
Dart leader – subsequent initial stroke that follows same path as initial stepped leader
Causes the multiple flash of the lightning 42

43. Types of Lightning
Forked lightning – dart leader takes different path than stepped leader
Ribbon lightning – wind blows charges into ribbon-like lightning
Dry lightning – lightning that occurs in a dry thunderstorm

44. Types of Lightning
Heat lightning – lightning that is seen by not heard (can be orange)
St. Elmo’s Fire – luminous green or blue halo around the top of pointed objects (antennas, masts of ships). Lightning may occur after this is seen. 44

45. Lightning Detection and Suppression
Lightning direction-finder

46. Tornado Life Cycles
Tornado – rotating column of air blowing around a small low pressure that reaches the ground

47. Tornado Life Cycles Funnel cloud
Tornado that hasn’t reached the ground
Dust-whirl stage
Swirling dust at the ground marks the tornadoes circulation
Mature stage
Funnel at greatest width, most intense damage. Often vertical
Decay stage
Funnel shrinks, damage becomes less, tornado becomes stretched 47

48. Tornado Outbreaks
Tornado families – tornadoes spawned by the same thunderstorms
Tornado outbreaks – many tornadoes that form over same region
Much ground-breaking research on tornadoes was conducted by Professor Ted Fujita of the University of Chicago. The “F-scale” of tornado intensity was named after him.

49. Tornado Occurrence
Tornado alley – part of the Central Plains from Texas through Nebraska
Time of day – most often in the afternoon
Times of year – most often in Spring, lease often in Winter

50. Tornado Winds Multi-vortex tornadoes Suction vortices
A single tornado with multiple rotating columns within it
Suction vortices
The small rotating columns within multi-vortex tornadoes

51. Seeking Shelter
Tornado watch – tornadoes are likely to develop within the next few hours
Tornado warning – issued once a tornado is spotted
It’s always a good idea to know what to do if a tornado watch or warning is issued for your area.

52. The Fujita Scale
Tornado classification based on damage

53. Supercell Tornadoes
Mesocyclones – Rising, rotating column on the south side of a supercell. Acts to increase the updraft
Bounded weak echo region – area inside t-storm where radar does not pick up precip
Hook echo – Radar representation of a tornado (looks like a hook)
Rotating clouds – First sign tornado is about to form
Wall cloud – rotating clouds that have lowered below the base of a supercell
A rotating wall cloud is an unforgettable sight - just ask a successful storm chaser.

54. Figure 10.35: Some of the features associated with a tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast.
Fig , p. 290

55. Figure 10.35: Some of the features associated with a tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast. 55

56. Nonsupercell Tornadoes
Gustnadoes – tornadoes that flow along a gust front
Landspouts – similar to water spouts and form due to developing cumulus congestus

57. Severe Weather and Doppler Radar
Doppler shift – Change in frequency in sound waves for moving objects
Tornado vortex signature – rapidly changing wind direction in mesocyclone
NEXRAD – Network of 150 radar stations in U.S.
Waterspout – Tornado that forms over water