Review: Thunderstorms • A cumulonimbus with lightning and thunder! • Deep layer of conditionally unstable air is necessary to produce a thunderstorm. • Several types of thunderstorms. Single Cell, Multicell, Squall Line, Mesoscale Convective Complexes, Supercells • Pose major hazards to public and economy. Lightning, Hail, Microburst Winds, Flash Flooding, Tornadoes
Lightning Basics • What is lightning? An electric discharge, or spark, that occurs in thunderstorms (usually) 80% occurs within clouds 20% occurs between cloud and ground • Lightning is ubiquitous, with more than 6,000 ground strikes per minute from 40,000 thunderstorms per day worldwide
Lightning Videos • Examples were shown of In-Cloud (IC) Lightning Cloud-to-Cloud (CC) Lightning Forked Lightning IC Lightning Video from MetEd/UCAR • MCC Lightning from Space Shuttle Lightning from Space Video from NASA
Lightning Pictures • Examples were shown of Cloud-to-Ground (CG) Lightning In-Cloud (IC) Lightning Cloud-to-Cloud (CC) Lightning Forked Lightning Chuck Doswell's Lightning Pictures-Very Nice! • Excellent photography tips can be found at Chuck Doswell’s web site. He’s good!
Charge Separation Lightning requires the separation of different charges into different regions of a cloud. How does charge separation in clouds occur? We don’t know for certain, but we observe this: Lightning only occurs in cold clouds with supercooled droplets and temps below 5oF. Thus, the ice crystal processes responsible for precipitation in cold clouds likely plays an critical role in charge separation.
Charge Separation: One Theory Hailstones are covered by a layer of liquid water. The thin layer of liquid is positively charged. When hailstones and ice crystals collide, some of liquid molecules stick to the ice crystals. Along with the mass transfer, positive ions transfer from the hailstones to the ice crystals. The heavier, negative hail falls to cloud bottom. The lighter, positive ice crystals drift to cloud top. Produces negative lower, positive upper cloud.
Charge Separation + Polarization • Top of cloud top has a positive charge. • Lower and middle of cloud has a negative charge. • Charge separation in cloud maintains the earth’s fair weather electric fielddenoted by the arrow E • E points toward positive polarity E – Williams, The Weather Book
Fair Weather Electric Field • An electric potential exists between the ionosphere (positive) and surface (negative) • Potential varies between 200,000 - 500,000 Volts • Average current is 2x10-12 Amps/m2 • Power is 10-6 W/m2 www://thunder.msfc.nasa.gov/primer
Lightning Stroke Cloud-Ground Sequence 1) Downward stepped leader. Stepped leader is invisible. 2) Upward return stroke. 3) Downward dart leaders. 4) Upward return strokes. Dart leaders-return strokes: up to 25 cycles, 3-4 usually. Ground strikes are usually negative, that is electrons flow from cloud to ground. Williams, The Weather Book
Types of Discharges www://thunder.msfc.nasa.gov/
Lightning Safety Williams, The Weather Book
Thunder What Causes Thunder? • Lightning rapidly heats air to more than 30,000oC. • The intense heating causes the air to expand rapidly. • The expanding air cools, then contracts rapidly. • The expansion-contraction generates sound waves. Williams, The Weather Book
How Far Away Is It? • We see lightning instantly. • But sound travels 1,000 ft every second. If you hear thunder 10 seconds after seeing lightning, the bolt is 2 miles (~10,000 ft) away. • We hear thunder from closest part of flash first, farthest part last. This causes the rumble sound. Williams, The Weather Book
Why Thunder Rumbles? • Assume that you are one mile away from a a one mile long bolt. • You hear thunder from the lower part of flash in 5 seconds, from the upper part of flash 7 seconds. 1.4 miles 7 seconds 1 mile 1 mile 5 seconds
National Lightning Detection Network thunder.msfc.nasa.gov/primer
Gallup Flagstaff Phoenix Tucson www.nssl.noaa.gov/western/kaney
Phoenix Tucson www.nssl.noaa.gov/western/kaney
Global Lightning Distribution from Satellite, Take 2 www.msfc.nasa.gov
Let’s Play “Who Gets Toasted” • What is the probability in Tucson of a Cloud-to-Ground lightning stroke hitting within a certain Radius R of you in an “average” year? • Guesses? No peeking!
CG Lightning over Tucson(2000-2002) • 65,000 flashes in 80 km 80 km over 3 years ~3.3/km2 per year • Much higher during monsoon ~12/km2 per year Courtesy Prof. E.P. Krider Dept. of Atmospheric Sciences
Summary: Key Points • Lightning -electric discharge in thunderstorms 80% within clouds, 20% cloud to ground • Lightning is ubiquitous, with more than 6,000 cloud-to-ground strikes per minute from more than 40,000 thunderstorms per day worldwide. • Lightning requires the separation of different charges into different regions of cloud. • Charge separation maintains the earth’s fair weather electric field.
Assignment for Next Lecture • Topic -Tornadoes • Reading -Ahrens, p277-290 • Problems -10.25, 10.26, 10.29