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Graupel Or Hailstone), Some Negative Ions Transferred to the Colder Object What Causes the Charge Distribution? Part 2

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Graupel Or Hailstone), Some Negative Ions Transferred to the Colder Object What Causes the Charge Distribution? Part 2 ATM 10 Severe and Unusual Weather Prof. Richard Grotjahn L 15/16 http://canvas.ucdavis.edu/ Lecture topics: • Lightning – Formation of charge conditions – Demonstrations – Various types – Climatology – Damage – Safety do’s and don’ts – videos • Hail – Formation – Locations – Damage Short Video Segments • Lightning – Daytime lightning & sound (10 sec, .mov) – Night, slow crawlers (18 sec, mpg) – Night, city (10 sec, mpg) What is lightning? • A very large spark • Traveling through a poor conductor (air) What is thunder? • Lightning instantly heats air to ~30,000 C (~54,000 F)! • Heated air expands (ideal gas law) compressing adjacent air • Sound wave (thunder) is created Perceiving Thunder • Air attenuates (absorbs) sound, removing high frequencies sooner than low • You hear high pitch “Crack!” (from that part of lightning close to you) • followed by low rumbles for that part of lightning furthest away • Sound waves (like light) refract (bend) towards cooler air (recall mirages) • If you are too far away (~15km) you may not hear it, though you see a flash “heat lightning” • Sound travels • 1 km in 3 seconds • 1 mile in 5 seconds bolt traveling horizontally towards camera, then dipping to the ground Lightning Bolt = Giant Spark 1. Negative charge in cloud base attracts positive charge on ground (esp. high pts) 2. "Stepped leader" steps downward 50-100m at a time -- creates the jagged shape. Each step ~ 50/1,000,000 second 3. When step leader nears the ground, ground leaders may rise from taller objects 4. << BOOM!>> -- positive charge rushes up: this is much brighter RETURN STROKE. A short circuit. The stepped leader made the air conducting along that path and the return stroke follows that easier path. Why are there multiple strokes? • After first return stroke can get multiple strokes: up to 26 times • Each is led by dart leader (heads down faster than step leader) • Each return with a brighter return stroke What causes the charge distribution? • Earth has a background electric field 1. Generally 100 Volts per m 2. More positive charge in upper atmosphere (ionosphere) 3. More negative charge at surface of the earth What causes the charge distribution? Part 1 Charge separation (above) inside cloud still unclear. 2 theories. Interface charging (initial charging) 1. When warmer object (snowflake, riming droplets that shatter) touches a colder one (graupel or hailstone), some negative ions transferred to the colder object What causes the charge distribution? Part 2 Charge separation (above) inside cloud still unclear. 2 theories. Induction Charging (further charging) 2. Earth’s electric field makes uneven charge on falling graupel or hail. Falling graupel or hail collide with smaller items on bottom side of the hail. The smaller objects pick up the positive charge and sweep it upwards (in updraft) Simple Charge Demonstrations • Rabbit fur & PVC: – Make objects move! – Measure ES field before/after • Balloon – & hair – Light CFL! – Measure ES field before/after • These make sparks, but do they demonstrate charge separation? Interface Charging Demonstration • True interface charging: need cold object to strike warmer object. • Equipment: – CO2 fire extinguisher – PVC ‘snowflake’ – Electrostatic VM • Activity: – Extinguisher spits out solid bits of very cold CO2 ice that brush over surface of the warmer ‘plastic snowflake’ – Did charge separate? measure Polarity • Most (~90%) cloud-to-ground (CG) lighting is “negative” polarity: • the stepped leader carried negative charge down • Some CG lightning (~10%) is “positive” polarity: the stepped leader has positive charge • typically from anvil can be understood from charge separation in cloud • anvil positive charge causing negative to accumulate at ground beneath • often has higher current & so causes more damage Lightning orientation • Most lightning (~80%) is cloud-to-cloud (CC) but the ratio of CC vs CG varies. Red: CC=10xCG; Blue: CC ~ CG Review: what are the main types? • Cloud to cloud (within the cloud) • Cloud to ground (negative) • Cloud to ground (positive) • Red (+ charge) • Blue (- charge) Other types • Many unusual forms • Ball • Ribbon • Spider • Etc. • Also have discharges from top towards ionosphere Other types • Many unusual forms • Ball • Ribbon • Spider • Recoil, bead, etc. • Also have discharges from top towards ionosphere Other types • Many unusual forms • Ball • Ribbon • Spider • Etc. …. • Also have discharges from top towards ionosphere Videos of the day: stepped leader and return stroke: • Ultra high-speed videos – 7,200 fps – slowed down by 240x – Tom Warner (former UCD student in the Atm. Sci. program) Videos of the day: stepped leader, ground leader from a tower and return stroke: • Ultra high-speed videos – 9,000 fps – slowed down by 300x – Tom Warner (former UCD student in the Atm. Sci. program) Lightning: how often does it occur? • 40 every second; 3 million every day! Lightning Climatology • Climatology shown: 5 years of satellite data • Lightning favors land areas • Largest values are in the tropical land areas. • Little over tropical oceans except for storms moving off adjacent lands • Ocean exception: Gulf stream & similar currents Movie: space shuttle mission #48 Individual thunderstorms lit up • Absolute max in central Africa Annual average flashes per square km: 4/1995 – 3/2000 Average Thunderstorm Days & Lightning Density Figure 15.20 (Ahrens) • Observed density of lightning similar to pattern of thunderstorm occurrence (of course!) • Secondary maxima: central plains, Ohio R. valley. Why? • More intense thunderstorms produce more lightning Lightning Damage • Power systems • Structures • Forests • People Ville de Geneve © C. Suarez Read: Lightning damage – electric power • Electric power – Power surges that lead to outages – Spot fires – Damaged equipment Read: Lightning damage – structures • Physical damage • Fire • Lightning rods Read: Lightning – trees & forests • Strikes on trees – Split bark – ‘Top’ the tree • Forest fires – Much of west is arid in summer – Fewer thunderstorms, but the dry conditions cause more fires – Lightning strikes (black dots) during 1 day (24 August 2000) – Subsequent fires (red dots) Read: Lightning - Hazard to people • Lightning amperage varies widely, can be 100,000 amps! • Lightning causes 50-70 fatalities/year • Most people struck by lightning survive but often with permanent injuries Leg & clothing burns from lightning Read: Lightning Safety - General • Trees are not safe! • Avoid being the “tallest object around” • Avoid “open” places • Beware of a “bolt from the blue”. Lighting can travel several miles from the storm Read: Lightning Safety – Camping & Climbing • Lightning “safety” position • Camping • Mountain climbing – Grand Teton 21 July 2010 • 17 climbers struck in one storm. Most indirectly. 83 © W. Faidley rescuers + helicopter NOAA photo © J. Reed Lightning Safety – ‘Rules’ • Stay indoors, avoid metal connected to outdoors, e.g. TV antenna cable • Wait until well after storm has passed Lecture Summary: • Lightning – Many different types, 3 are most common. Odd types fascinating – ~20% are cloud-to-ground (CG); ~80% cloud-to-cloud (CC) – Charge separated in cloud • positive (+) charge drawn upper cloud, negative in lower cloud • theories based on collision of small object with falling hailstone: – collision transfers + charge to warmer object (which is smaller and dragged higher in cloud) (Interface charging) – collision on bottom side of hailstone (where charge is more +) transferring + charge to smaller object (Induction charging) • attracts opposite charge on ground underneath – CG (cloud to ground): • growth by step leader (down) making ion channel, then return stroke (up) • often followed by: dart leader then multiple strokes in ion channel • most are negative (downward current) from primary rain area • ~10% are positive, often from the Anvil, usually more powerful – lightning most common in tropical land areas. – Over US: Florida has most lightning; more lightning happens in severe storms (central US) – safety: • avoid open areas, tall objects, & conductors. minimize ground contact • stay in protected building until storm well past (30/30 rule) Sound: 5sec/mile Hail 1. What is it? 2. how does it form? (creation conditions) 3. where does it occur? (climatology) 4. what does it do? (examples) Hail: Is it snow? • No. – Much more dense – Produced in a different way • But it can accumulate like snow Hail: What is it? • Ice in a generally rounded or ball-like or conic structure. (Unusual shapes with lobes occur for rare extremely large stones) • Hailstone is generally dense, making the stone heavy for its size. Large snow flakes have air pockets – much lighter • There are several intermediate forms… Hail: how does it form? • Different forms of ice are created in clouds • Ice crystal type depends on the temperature (flakes versus needle or plate shapes) • Frozen precipitation reaching the ground may be snow that broke apart, recombined, melted and refroze, or collected liquid water that froze to it. Ice & snow forms and soft hail • ice plates and crystals falling through a cloud collect smaller material: • tiny ice crystals adhere to surface of big flake = “riming” • tumbling spreads small crystals into rounded layer of rimed ice covering the large crystal. Process leaves air bubbles in the rime ice • that rounded object is called “graupel” = “soft hail” • graupel may melt to form large raindrop. • graupel may collect supercooled water • that supercooled water freezes forming dense, clear layer • growth
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