Cloud Electrification

• Wallace & Hobbs, Ch. 6

• 6.7 (cloud electrification)

• Final homework due Tuesday

Global Lightning Activity from Optical Transient Detector1 (OTD) Data

1 OTD launched in 1995, ran through 2000

Global Lightning Activity from OTD Data

Global Lightning Activity from OTD Data

Global Lightning Distribution

• Most of the lightning is found over land in the Inter-tropical Convergence Zone (ITCZ), a band of convergence of surface winds near the equator.

• Global lightning during the months of December, January, and February 1999 has shifted into the southern hemisphere.

– Lighting at higher latitudes in the northern hemisphere might be associated with storms forming along the polar front.

• The June, July and August map shows greatest activity in the northern hemisphere.

• 78% of the lightning is found between 30 S and 30 N.

• 88% of the lightning is found over continents, islands, and coastal regions. There is much less activity over the oceans.

– Increased lightning activity over land is primarily due to enhanced convection; convection is much stronger over land, resulting in greater ice production and more lightning.

Global Lightning Distribution

• One interesting result from 5 years of OTD lightning data is a new estimate for the global lightning flashing frequency:

44 flashes/sec 5 flashes/sec

• This is about half of the 100 flashes/sec value that was long thought to be true. The 100 flashes/sec value dates back to about 1925.

– Correction to slide 36 from last lecture!

• Movie from NASA's Lightning Imaging Sensor (LIS).

Sprites, Jets, and Elves or Transient Luminous Events

Transient Luminous Events

• Luminous phenomena seen above the tops of thunderstorms from roughly 20 to 100 km altitude

• These are not the high current electrical discharges like we see in lightning.

• They are visual phenomena caused, in most cases, by the electromagnetic radiation from lightning discharges.

• Sprites: luminous flashes lasting from a few to several hundred milliseconds

– Generated when large positive charge is transferred from cloud to ground in lightning.

• Elves stands for Emissions of Light and Very Low frequency Perturbations from Electromagnetic Pulse Sources

– Luminescent rings at 90 km altitude lasting 1 s

• Blue jets: partially ionized blue cones propagating upward from the tops of thunderstorms.

Ball Lightning: an unexplained phenomenon

• Most reports describe it as a luminous ball about the size of a baseball or softball that moves along the ground or through the air with an appearance, sound, and odor that suggest an electrical nature and origin.

• Ball lightning is generally produced during a thunderstorm.

• Wikipedia has several interesting historical accounts and observations of ball lightning and discusses many of the attempts to produce ball lightning in the laboratory.

– http://en.wikipedia.org/wiki/Ball_lightning

Ball Lightning: an unexplained phenomenon

• Perhaps the best description:

– Ball lightning has been seen by 5 to 10% of the population.

– Although it is generally thought that ball lightning is a rare phenomenon, recent research has shown that the numbers of ball lightning observers are not much different from the numbers who have observed lightning impact points.

– The implication is that ball lightning is usually created at or near the lightning channel and that an appreciable fraction of all cloud-to-ground lightning flashes may give birth to ball lightning.

– Since the balls generally last for only a few seconds, they cannot get too far from the mother channel. Thus ball lightning may well be common, but rarely seen."

• Check out “Birth of Ball Lightning” by Lowke et al. [2012] on the web site under Supplemental Reading (sections 1 & 2 are descriptive)

Grade Weighting

Homework: 30 points

Midterm Exam: 20 points

Term Project 30 points (report 20/presentation 10)

Final Exam 20 points

Half of your grade remains!

Last week of class: 10-15 minute presentations on class project.

Report due at the time of final exam, currently scheduled for May 7, from 4:30 p.m. - 7:00 PM in Duane, D318

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Term Project

• Content: 40%

• Organization: 30%

– see recommended structure from notes

• Writing: 30%

Term Projects: Oral Presentations Dates

Tuesday, 29 April

• Peter Cirkovic

• Chris Clack

• Nick Conant

• Max Gilbraith

• James Herod

• Matthew Mauch

Thursday, 1 May

• Paige Mynatt

• Janelle Pacheco

• Lauren Persons

• Emily Stohl

• Erin Zachman

Please email me slides your slides no later than 8:30 AM the morning of your presentation or be here by 9:15 AM with your slides on a memory stick!

Final: 7 May Topics

Atmospheric Optics

The Halo

The Green Flash

Polarization of the Primary Rainbow/Brewster Angle

The Corona

The Glory

Radiative Transfer

Beer-Lambert (Extinction) Law

Extinction coefficient, mass extinction coefficient, extinction cross section, optical thickness

Langley Plot

Cloud Microphysics

Homogeneous nucleation

Kelvin’s Equation

Heterogeneous nucleation

Cloud Condensation Nuclei

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Final Topics Cloud Microphysics (cont.)

Cloud Condensation Nuclei

Size of Rain Drops, Cloud Droplets, and CCN

Köhler Equation and Köhler Curves

Ship tracks and relationship between droplet size and cloud optical thickness and albedo

Warm and cold cloud precipitation processes

Droplet Growth by Condensation Droplet Growth by Collection/Collision Coalescence

Droplet Fall Velocity

Collision Efficiency

Homogeneous ice nucleation/freezing

Heterogeneous ice nucleation

Ice Nuclei (IN): freezing, contact, and deposition

The Bergeron Process

Supersaturation over liquid water and over ice

Cloud Seeding

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Final Topics Atmospheric Electricity

The Global Electrical Circuit

Maintenance of the global electric circuit

Coulomb’s Law

Charge separation in clouds

Charge distribution in cloud

Lightning, various types: intra-cloud, cloud-to-groud, negative and positive charged strokes

Lightning, stages: stepped leader, streamed, return stroke, dart leaders, multiple return strokes

Thunder

Lightning Safety

Global Lightning Distribution

Transient Luminous Events

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