Download - Chapter 7: Atmospheric Disturbances
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.