chapter 7: atmospheric disturbances part ii: t-storms, tornadoes, lightning & forecasting
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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.