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)

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

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”

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

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)

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

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

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

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

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

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

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

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

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