| NATS 101 Lecture 28 Lightning |
| 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 DoswellÕs web site. HeÕs good! |
| Charge Separation |
| Lightning requires the separation of different charges into different regions of a cloud. | |
| How does charge separation in clouds occur? | |
| We donÕt 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 |
| Top of cloud top has a positive charge. | |
| Lower and middle of cloud has a negative charge. | |
| Charge separation in cloud maintains the earthÕs fair weather electric field denoted by the arrow E | |
| E points toward positive polarity |
| 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 |
| 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. |
| Types of Discharges |
| Lightning Safety |
| 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. |
| 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. |
| 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. |
| National Lightning Detection Network |
| Slide 17 |
| Slide 18 |
| Global Lightning
Distribution from Satellite, Take 2 |
| LetÕs Play ÒWho Gets ToastedÓ |
| What is the probability in Tucson of a Cloud-to-Ground lightning stroke hitting within a certain Radius R of you in an ÒaverageÓ year? | |
| Guesses? No peeking! |
| Slide 21 |
| 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 |
| Slide 23 |
| Slide 24 |
| 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 earthÕs fair weather electric field. |
| Assignment for Next Lecture |
| Topic - Tornadoes | |
| Reading - Ahrens, p277-290 | |
| Problems - 10.25, 10.26, 10.29 | |