1. The Thunderstorm: Ultimate Instability
A convective rain or snow shower accompanied by thunder
What causes thunder?
Lightning (spell it correctly): a giant spark between two regions of opposite electrical charge. 50,000 deg F, hundreds of thousands of amps
Air is a poor electrical conductor (an insulator), therefore a huge charge difference builds up either in cloud (80% of all lightning) or between the cloud and the ground (20%) (cloud to ground)

2. Lightning I Formation: Precipitation Theory
Collision of ice crystals and hailstones leaves each with opposite charge, lighter ice crystals transport positive charge upward while heavier hail concentrates negative charge at lower levels
Probably more complicated than this, may need supercooled water in cloud in addition to ice, but it’s likely that ice is required to produce lightning
Hurricanes = little lightning
Sprites

3. Lightning II
Thunder: a warning that lightning is near, explosive expansion of superheated air
Computing distance to the lightning
Speed of light: see lightning when it happens
Speed of sound: thunder is delayed 5 sec / mile
All lightning produces thunder
The rumble
“Heat lightning”

4. Lightning Safety Photo Courtesy of Dave Crowley
Don’t
Swim, boat
Stand in an open field
Stand under an especial-ly tall or isolated tree
Use a wall phone
Use the shower Do
Go inside
Get into a non-convertible car
Get to lower elevation
Crouch on balls of feet
Perform CPR if someone is struck

5. Thunderstorm Triggers
Start with an environment in unstable equilibrium
Diurnal heating primes the atmosphere
Localized thunderstorm triggers
Sea breeze (Florida, Gulf Coast)
Elevated convection (eastern Rockies)
Large scale thunderstorm trigger (discussed later)
Convergence into low pressure
Fronts (all types, but cold preferred)

6. Air Mass Thunderstorms
Typical lifespan 30 minutes to an hour
Life Cycle
Cumulus Stage
Instability and adequate moisture required
Building cumulus clouds dominated by updrafts

7. Air Mass Thunderstorms
Life Cycle
Mature Stage
Top of cloud glaciates (freezes)
Precipitation through Bergeron-Findeisen Process
Lightning
Updraft continues and downdraft develops from falling precipitation

8. Air Mass Thunderstorms
Life Cycle
Dissipating Stage
Downdraft dominates
Updraft ceases, convection collapses
Dry air entrainment

9. Thunderstorm Dangers
Flash flooding: more rain falls in a short time than ground can absorb
Hail: “The White Plague”
Microbursts: unusually strong downdrafts
Lightning, of course
Thunderstorm Complexes
Squall Lines
Mesoscale Convective Systems

10. Flash Floods A big U.S. killer (~100 people/yr)
DO NOT DRIVE or walk on flooded streets
Ingredients for flash flooding
Weak upper-level winds
Orography
Training

11. Hail Ingredients for hail Formation
Strong updrafts to support growing chunks of ice
Below-freezing air relatively close to ground
Formation
Ice crystals become coated with supercooled water blown upward within updraft
Hailstones grow as long as updraft can offset gravity
Fall to earth when updraft weakens or hail is thrown out of updraft

12. The Microburst Huge threat to aviation Damaging “straight-line” winds
Rapid loss of lift
Occurs close to ground
Damaging “straight-line” winds

13. Thunderstorm Complexes
Mesoscale Convective Systems
Long-lived bundle of thunderstorms
Greatest threat is flooding
Beneficial to U.S. crops (usually)
Squall Lines
Organized band of thunderstorms, usually well ahead of a cold front
Greatest threat is damaging winds

14. Key Figures
9.5, 9.11, 9.14, 9.16, 9.19, 9.21, 9.32, 9.53