Thunderstorms

Thunderstorms & Tornadoes

Objectives:

Key Terms:

Notes: (20-3)

Thunderstorms

Thunderstorms form from cumulonimbus clouds in the presence of warm moist air. They generally occur around noon as the ground heats up forcing unstable air up into the lower atmosphere. Once the atmosphere becomes unstable (supersaturated) a trigger starts the event. Triggers may include:

Cold front (frontal storm)

forms squall lines preceded by strong winds (updraft)

(www.msc-smc.ec.gc.ca/.../ cumulonimbus_formation.gif)

Thunderstorm

(www.ucar.edu/news/images/ thunderstorm.jpg)

movement of air up a mountainside forming convection cells

	supercells may form as the cells connect to one another from the updraft
	supercells may form into tornadoes if the conditions are right

Lightning

	formed as the lower becomes negatively charged relative to the upper cloud levels
	the discharge of electricity causes superheated air to rapidly expand & contract creating thunder

(www.windows.ucar.edu/.../images/ tstorm_charges.jpg)

(i.timeinc.net/time/photoessays/ deadlystorms/tornado.jpg)

Tornadoes

Thunderstorms are perhaps the most powerful of Earth's weather systems. Tornadoes form as supercells from thunderstorms begin to spin as a large system called a mesocyclone.

(www.ianr.unl.edu/snrs/amet351/ dodge/supercell.jpg)

this updraft is created by the rapidly cooling air in the cumulonimbus cloud

if the mesocyclone has enough strength, the formation of a wall cloud accompanies the rotating cloud 10 - 20 minutes before the tornado becomes visible

the funnel-shaped cloud descends from the wall cloud to become a tornado

	results from the very low pressure at the center of the cloud system
	can appear anywhere in the U.S. but usually are common in tornado alley (Texas to N.Dakota) in the summer months

tornado alley map

(whyfiles.org/013tornado/ images/alleymap.gif)

tornado with helix circulation around small funnel

(www.chaseday.com/PHOTOSHP/ 2JUL76/11-tornado.JPG)

Last modified: January 09, 2004