Chapter 7: Atmospheric Disturbances

Part II: T-storms, Tornadoes, Lightning & Forecasting

• Nature of T-storms

– Violent– Thunder & lightning– Hail– Strong updrafts & downdrafts

• T-storm formation stages

– Cumulus stage– Mature stage– Dissipating stage

Localized Severe Weather

Figures 7-28 & 7-25

Localized Severe Weather

• Thunderstorms– Atmospheric conditions

for t-storm formation• High temperatures• High humidity• High instability

Days with t-stormsby latitude: Figure 7-26

Days per year with t-storms in U.S.: Figure 7-26

6-13-2004 Supercell

What it looked like under the supercell on 6-13-2004

Localized Severe Weather

• Tornadoes– Deep low pressure vortex– Winds so violent, vacuum

develops inside funnel• May be in excess of 300 mph• Most extreme pressure

gradients known

– Don’t know exact mechanism of formation

• Warm, moist, unstable air• Spring & summer = most

favorable• Most occur in mid-

afternoon – Time of max heating

Figure 7-30

Localized Severe Weather

• Tornado formation– Vertical wind

shear creates horizontal rotation

– Horizontal rotation tilted vertical by updrafts

– Mesocyclone & tornado development

Figure 7-31

Localized Severe Weather

Over 90% of reported tornadoes worldwide occur in the U.S.

Figure 7-33Figure 7-32

It’s all a matter of statistics: Who has the worst tornadoes?

8-28-1884 Howard, SD

• Safest location in a building• Opening windows to reduce damage• Using highway overpasses as shelter• Escaping a tornado in a vehicle• Tornadoes “skip” houses• Association of size with intensity• Appearing to reach the ground• Direction of travel• Geographical scope• Near various landscape features• Attraction to mobile homes and/or

trailer parks• Downtown areas• During winter

Localized Severe Weather: Tornado Myths

April 2011 Tornado Outbreak

Hallam, NE (2004)

Omaha’s 1913 Easter Tornado

Localized Severe Weather

• Lightning– 8.5 million bolts daily– Electric discharge in t-storms– Lightning types

• Cloud to ground• Cloud to cloud• Within cloud

• Thunder– Expansion of air from heat of

lightning

• Time lightning distance – Speed of sound vs. speed of

light• Every 5 seconds equals about 1

mileFigure 7-29

• Oklahoma – most tornadoes worldwide• Vertical wind shear

– Change in wind direction from storm’s bottom to top – Mesocyclones

• 50% develop into tornadoes

• NEXRAD– Doppler effect

• Vertical wind profiler– Dryline: boundary

between mT & cT air

• OK Mesonetwork: – 111 solar-powered

weather stations

Localized Severe Weather: Forecasting

Tornado warning issued

Localized Severe Weather: Forecasting with NEXRAD

Figure 7-E: Reflectivity = Storm intensity

Figure 7-F: Motion detection = Funnel I.D.