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 mile Figure 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.