Average annual # of days with thunderstorms

Deaths from Natural Hazards in the US

Air mass thunderstorms: (90% of all t-storms are of this type) —occur as mT air masses are heated from below by warm land surfaces --usually are of short duration and do not produce hail, damaging winds, or tornadoes --isolated distribution

Severe thunderstorms (10% of all t-storms are of this type) —may involve convection due to surface heating, but uplift is enhanced by frontal activity or orography --typically produce winds + 58 mph, and hail --storms may be aligned along a front

Life Cycle of a Thunderstorm 1. Cumulus stage—water vapor is moved aloft by updrafts; vertical cumulus development

2.Mature stage—once the cloud reaches the freezing level precipitation forms via the Bergeron process; falling precipitation produces a drag on the surrounding air causing cold downdrafts; Entrainment—cold air from outside the storm aloft is pulled into the storm and moves down as part of the downdraft

3. Dissipating stage—downdrafts dominate and moisture input from surface ceases

“Anvil head”

Help me!

A roll cloud may signal the advance of a gust front

In some areas, the break up of a temperature inversion in the afternoon can lead to thunderstorm development

One thunderstorm can spawn other ‘daughter’ cells through the formation of a gust front

In summer, cT air masses from the arid SW US can be drawn into low-pressure systems in the Great Plains states. This creates a dryline boundary (between cT air and less dense mT air) where violent thunderstorms may develop.

if a dryline exists, storm development along the cold front will be minimal squall line: line of intense thunderstorms

Mesoscale Convective Complex— --a very large (~40,000 mi 2 ) organized cluster of severe thunderstorms

MCC over Oklahoma

MCC

Imbedded in a squall line or an MCC, large, rotating thunderstorms called mesocylcones may form (1/2 of all mesocyclones produce tornadoes)

Mesocyclone: (typically miles wide and 70,000 feet tall!)

Thunderstorm hazards

Hail forms as ice crystals are swept aloft numerous time by updrafts

Concentric rings of a hailstone

Mircroburst—a narrow, high speed downdraft (may exceed 100 mph)

Downbursts can create straight-line winds called derechos (may be +100mph) derechos are often mistaken for tornadoes

Destruction path of a derecho

Derecho damage

Lightning –electrical stroke between oppositely charged particles charge separation is caused by updrafts, downdrafts

Lightning heats the air to 50,000º F

if you feel static electricity like this person get to a low area immediately

Lightning Safety

Thunder—sound of air expanding upon heating by lightning strike (sound waves travel about 1000 ft/sec)