• Definition: a storm containing lightning & thunder; it may produce gusty winds with heavy rain & hail; Severe T'storms may spawn tornadoes.

• the storm itself may be a single cumulonimbus cloud, a cluster of them, or even a line of clouds stretching for more than 100 km (62 miles)

• T' storms develop: within an airmass, at/ahead of fronts (particularly cold fronts), along the ITCZ, and at mountainous locations

• the birth of a T'storm occurs when warm humid air rises in an unstable environment

• in North America most t'storms occur in areas dominated by mT air masses

Life Cycle of an Air-Mass T'Storm

Lightning: flashes of light resulting from electrical discharges within the cloud (the majority of strikes), between the cloud and the ground (~20% of strikes), from one cloud to another, or from a cloud to the surrounding air

• lightning is created when there is a buildup of electrical energy between areas within the cloud, or between the cloud and the ground

• for lightning to occur, separate regions containing opposite electrical charges must exist in a cumulonimbus cloud

"Hail" - forms within cumulonimbus clouds when raindrops are repeatedly carried above and below the freezing level - layers of ice are added until it becomes too heavy and can no longer be supported by the updrafts

B. Tornadoes

• are rapidly rotating winds that blow around a small area of intense low pressure

• are spawned from severe T'storms

• "funnel cloud" - the visible swirling circulation extending from the parent cumulonimbus cloud

• "tornado" - a funnel cloud that extends all the way to the ground

• tornado diameter: a few meters to a few hundred meters

• typical forward speed: ~20 to 40 knots

• most last only a few minutes & have an average path length of ~ 7 km (4 mi)

• horizontal wind speeds in tornadoes: can exceed 300 mph - most have lesser wind speeds

• 65% of tornadoes are weak (wind speed < 112 mph)
• 33% of tornadoes are strong (wind speed:113-206 mph)
• 2% of tornadoes are violent (wind speed >207mph)

• all 50 states have experienced tornadoes

• North America is by far the biggest recipient of Tornadoes (avg. 800 annually)

• in particular, the greatest number occur in the "tornado belt" (alley) of the Central Plains - from Texas to Nebraska - because of its unique geographical location with regard to contrasting air masses

• 3/4 of tornadoes occur from March- July; May has greatest #; most frequent in late afternoon (4-6 pm)

Tornado Formation:

• in order for a t'storm to spawn a tornado, the updraft (& the t'storm itself) must rotate

• this rising, spinning column of air (5-10 km across) is called a "mesocyclone"

inside the mesocyclone a spinning vortex (tornado) may appear near the mid-level of the cloud and gradually extend downward to the cloud base; and then perhaps down to the ground