1. Lecture 17 Thunderstorms & Supercells, Intro to Thunder and Lightning

2. Learning Goals for Part 1 of Chapter 10
Be able to describe what a THUNDERSTORM is and the general stages of growth for an AIRMASS THUNDERSTORM.
Be able to describe what a SEVERE THUNDERSTORM is (how it differs from a regular one) and be able to identify specific parts of a SUPERCELL THUNDERSTORM.
Be able to describe what LIGHTNING is and how it occurs, also be able to describe what THUNDER is and how it is related to lightning.
You can just read this slide.

3. Thunderstorm Introduction
A storm that generates lightning and thunder.
Are characterized by strong “up” and “down” motions
Frequently produces gusty winds, heavy rain and hail.
They can form on their own or they can form in conjunction with mid-latitude cyclone on the edge of a Cold-Front
They can form in conjunction with hurricanes
Locations in the continental US
Florida has the most thunderstorms (100s per year)
Commonly occur in the foothills of the Rocky Mountains
Associated with Tornado Alley in the middle of the country
Associated with drylines and air with different humidities

4. Thunderstorm Formation Introduction
Form when warm, humid air rises in a conditionally unstable or unstable environment
There are two categories:
Air Mass Thunderstorms – formed by unequal heating of the Earth’s surface within a maritime tropical (mT) air mass.
Severe Thunderstorms – formed by unequal heating & lifting of warm air along a front or mountain.

5. Air Mass Thunderstorms
Happens inside an air mass (usually mT)
Usually in spring and summer
Usually in mid-afternoon
Not associated with a front
Due to local differences in temperature

6. Air Mass Thunderstorm Life Cycle
Like mid-latitude cyclones T-Storms have a “Life Cycle”
Stages of Development
Stage 1: Cumulus Stage
Stage 2: Mature Stage
Stage 3: Dissipating Stage

7. Air Mass Thunderstorm Life Cycle
Stage 1
Stage 2
Stage 3

8. Stage 1: The Cumulus Stage
Rising air only
Makes a cloud
Cloud gets bigger and bigger
Moisture is being added to higher and higher altitudes
Needs a continuous supply of moisture to survive

9. Stage 2: The Mature Stage
Precipitation forms
Mainly by the Bergeron Process
Rain begins to fall
Associated with the downdraft (air going down)
Most active stage
Gusty winds, lightning, heavy rain, hail
Updrafts and downdrafts exist side by side

10. Stage 2: The Mature Stage
Downdrafts form for two reasons
Entrainment – mixing of dry air with cloud air at the edge of the cloud
Causes the cloud drops to evaporate
Energy from evaporation comes from the temperature of the air parcel
By evaporating drops, you use up the energy and end up cooling the air parcel
Temperature drops, parcel cools
Cooler air sinks to the surface
Drag – Air is dragged downwards as precipitation falls.

11. Stage 3: Dissipating Stage
Cooling effect of falling precipitation and influx of colder air up top mark it’s END!
Downdraft cuts off updraft
Cloud stops growing
You’re left with weakly descending air parcels
The cloud basically KILLS itself

12. Severe Thunderstorms
Heavy downpours, flooding, gusty straight-line winds, large hail, lightning and tornadoes!
NOT Common on the Hawaiian and Pacific Islands.
To be classified as Severe:
Must have winds in excess of 93 km/h or 58 mph
or hail 0.75 inches
or a confirmed tornado

13. Severe Thunderstorm

14. Severe Thunderstorm Overshooting Top Persist for many hours
Vertical wind sheer causes precipitation to fall in downdraft, allowing the updraft to retain strength.
Sometimes the updraft is so strong you get overshooting tops and anvils!
Cold air of the downdrafts spread out making:
“mini cold front”
Called a GUST FRONT
Called an Outflow Boundary
Can form a Roll Cloud
Anvil

15. Supercell Thunderstorm
Causes dangerous weather
Large, very powerful, up to 20 km in height
Last many hours
20-50 km in diameter
Vertical wind profile may cause the updraft to ROTATE!
MESOCYCLONE – usually spawns Tornadoes!
Need a lot of Latent Heat
Requires moist troposphere
An inversion a couple of km above the surface (caps moisture)
Unstable air can break through the inversion by “eroding” it

16. Supercell Diagram

17. Lightning and Thunder Lightning is ulia in Hawaiian
Thunder is hekili in Hawaiian
A storm is classified as a thunderstorm only after thunder is heard!!
Because thunder is produced by lightning, lightning must also be present!
Lightning is a discharge of electrical energy. 
Essentially a giant “spark” between regions of positive (+) and negative (-) charge.

18. Lightning
May occur:
Between cells in the same storm
inter-cloud lightning
Within a cloud
intra-cloud lightning
Cloud to air
Cloud to ground (CG)
Lightning forms when a charge separation occurs in a cloud
The earth is trying to equalize the electrical difference
Negative charges want to flow to the ground.

19. Lightning Formation – Charge Separation
We don’t exactly know why it happens
One theory:
Hail stones tend to have a warmer surface than ice crystals
When warm hail collides with colder ice, electrons transfer from ice to hail
Hail (-) is bigger and heavier and settles toward the bottom of the cloud
Smaller (+) ice crystals are lofted to the top.

20. Lightning Formation Excess electrons cause the air to ionize
Rip molecules apart (N2 or O2 become N+, O -…)
Air is normally very INSULATING
Ionized air is very conducting
like a metal wire

21. NOTE: There still hasn’t been a flash yet!!!
Lightning Formation
Ionized air forms tubes of ~50 m in length (150 ft) and 10 cm in diameter (4 in)
Called a LEADER
A bunch of leaders that are connected is called a STEP-LEADER.
NOTE: There still hasn’t been a flash yet!!!

22. Lightning Formation Each electron contains LOTS of energy
When it reaches the surface the energy is RELEASED
This energy is RELEASED as HEAT!!
Electrons are drained at the surface first so the FLASH starts at the ground!!!
Can have several (3-4) in rapid succession (1/10 of a second apart). (lead by a DART LEADER)

23. Lightning Formation
AGAIN: Electrons are drained at the surface first so the FLASH starts at the ground!!!
This is why lightning is said to GO UP, rather than down
The movement is STILL from CLOUD to GROUND
But the “heating” happens at the surface FIRST.
The electrons closest to Earth are “discharged” first
Heat that is released causes air around a step-leader to reach 30,000°C (54,000°F)
The warmer, the shorter the shorter the wavelength
30,000 K
8,000 K

25. Lightning Occurrence on the Mainland

26. Global Lightning Occurrence

27. Real-time Lightning Websites
World Wide Lightning Location Network
Blitzortung.org - Oceana

28. Thunder When air is heated quickly a shockwave forms  causes thunder
Similar to planes crossing the sound barrier, firecrackers and gun shots
Sound travels at 330 m/s or 1000ft/s
If thunder takes 3 seconds to happen after observing lightning then the storm is:
3 s * 1000 ft/s = 3000 ft away (6/10 of a mile away).

29. Key Information 1
Be able to describe what a THUNDERSTORM is and the general stages of growth for an AIRMASS THUNDERSTORM.
Generates lightning and thunder.
Characterized by strong “up” and “down” motions
Frequently produces gusty winds, heavy rain and hail.
They can form on their own or they can form in conjunction with mid-latitude cyclone on the edge of a Cold-Front
They can form in conjunction with hurricanes
Air Mass Thunderstorms – formed by unequal heating of the Earth’s surface within a maritime tropical (mT) air mass.
You can just read this slide.
● Rising air only – Makes cloud
● Moisture is being added to higher and higher altitudes
● Needs a continuous supply of moisture to survive
● Both Updrafts and Downdrafts are present, and exist side by side.
● Most active time with heavy rain, hail, and strong winds.
● Downdrafts form for 2reasons
Entrainment – mixing of dry air with cloud air at the edge of the cloud
Drag – Air is dragged downwards as precipitation falls.
● Cooling effect of falling precipitation and influx of colder air up top mark it’s END!
● Downdraft cuts off updraft
● Cloud stops growing
● You’re left with weakly descending air parcels
● The cloud basically KILLS itself

30. Key Information 2
Be able to describe what a SEVERE THUNDERSTORM is (how it differs from a regular one) and be able to identify specific parts of a SUPERCELL THUNDERSTORM.
SUPERCELLS
Large, very powerful, upt o 20 km in height and km in diameter. They last for MANY HOURS.
Vertical wind profile may cause the updraft to ROTATE forming a MESOCYCLOE (which usually spawn TORNADOES).
These storms need a lot of LATENT HEAT, a moist troposphere, and ideally an inversion a couple of km above to surface to cap the moisture.
SEVERE THUNDERTSORMS
Heavy downpours, flooding, gusty straight-line winds, large hail, lightning and tornadoes!
Persist for many hours
Vertical wind sheer causes precipitation to fall in downdraft, allowing the updraft to retain strength.
Sometimes the updraft is so strong you get overshooting tops and anvils!
To be classified as Severe:
Winds in excess of 93 km/h or 58 mph
or hail 0.75 inches
or a confirmed tornado
Cold air of the downdrafts spread out making:
“mini cold front”
Called a GUST FRONT
Called an Outflow Boundary
Can form a Roll Cloud
You can just read this slide.

31. Key Information 3
Be able to describe what LIGHTNING is and how it occurs, also be able to describe what THUNDER is and how it is related to lightning.
LIGHTNING
May occur in several different ways.
Between cells in the same storm (inter-cloud lightning)
Within a cloud (intra-cloud lightning)
Cloud to air
Cloud to ground (CG)
Formation – charge separation occurs, STEP-LEADERS ionize the air forming a narrow conductive path to the surface, the bright RETURN STROKE allows negative charge to deposit on the surface (the flash starts at the surface and goes up), a DART-LEADER can re-ionize the conductive path bringing more negative ions down and starting another flash, after several strokes there are no more negative charges.
THUNDER
When air is heated quickly a shockwave forms
Similar to planes crossing the sound barrier, firecrackers and gun shots
Sound travels at 330 m/s or 1000ft/s
You can just read this slide.