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 can be rapid, now have hailstone

Figure 8.23 (Ahrens)

Graupel has irregular, rounded shape with many tiny air pockets. 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. • If cold enough, graupel may collect supercooled water • that supercooled water freezes forming dense, clear layer • growth can be rapid => hailstone

Figure 8.23 (Ahrens)

Figure 8.12 Read: Hail Global Climatology • Hail needs: – Very deep cloud: thunderstorm – Supercooled water conditions in lower part of cloud Hail accumulation, Bogota Columbia, – more common for © UC Press storms forming over high altitude ground

• But worst hail is from the most severe thunderstorms

Figure 15.21 (Ahrens) Days with any hail type

• Observed frequency (above) in hail differs from the pattern of thunderstorm occurrence (lower right) Figure 15.20 (Ahrens) • Why? Thunderstorm days • thunderstorms in South have thick layer of warm air that melts hail before reaching the ground. Idea consistent with: • high frequency of graupel in mountain thunderstorms • high cloud base of thunderstorms in high plains. • thunderstorms in central plains are often more intense – stronger updrafts suspend larger hailstones • damaging hail (>2” size) more common in central plains Hail formation – multiple layers

• Large stones have multiple layers due to these steps (upper fig.): 1. Hailstone fell into main updraft is being swept upward (clear ice layer) 2. Leaves region of supercooled water at some point. Now grows by riming and collisions with ice bits – cloudy layer. Gains mass so starts to fall again. 3. Reaches the main updraft and carried upward again in supercooled water – clear layer. 4. Finally so large (or misses the strongest updrafts) and falls to earth. • Hailstone in picture sliced in half >5 inches across (1.7 lbs)! How does hail form?

1. Collisions and riming create a graupel particle. Graupel is filled with air pockets and appears ‘cloudy’. It gains mass and may start to fall relative to the updraft. 2. At the top of the vault it grows by collisions with supercooled water droplets. They flow over the surface and freeze making clear ice. 3. If the stone follows a path back into stronger part of updraft it is drawn back upward 4. When hailstone can no longer be supported by the updraft it falls to the earth. The stronger the updraft, the larger the hailstones that can be produced by the thunderstorm. 5. Not shown, sometimes a hailstone drifts out of the main updraft and collides with ice crystals forming another layer of ‘cloudy’ ice.

• How strong does the updraft need to be for the various sizes of hail? • This table provides the approximate speed for each size. • ‘Hairdryer’ demonstration! Hailstone Lobes

Photos courtesy C. Knight

4 Stones from Coffeyville KN, 1970. C. Knight

• Except very early on (during the graupel stage) hailstones don’t grow by other stones sticking to them. • The lobes seen on some, usually very large stones are protrusions that grew from supercooled water that hit the broad (bottom) side of the stone then streamed off the top side as the stone fell Can Hail be detected? • Visually: – ‘Greenish’ cloud – White or ‘Streaky’ precip. shaft • By RADAR – TBSS: ‘three body scatter spike’

– Radar pulse reflects off Direction of Large stones, then Radar beam ground, then off stones back towards radar – Radar makes longer time = further distance along path of radar beam – ‘spike’ appears on radar where there is no actual precip. Read: Which one is biggest? Circumference (so far… in USA)

Hailstone fell near Aurora Nebraska 22 June 2003. 17.8 cm (7”) diameter, 47.6 cm (18.75”) circumference It weighs about 1.5 lbs! Yikes! “Divits” up to 14” in diameter, 3” deep found

Was accompanied by other large hail (photo at right) $1.5M in property & crop damage from that storm’s large hail

Which one is biggest? By Weight (so far… in the USA) Vivian SD (2010)

• Hailstone fell near Vivian SD on 23 July 2010. – 20.3 cm (8”) diameter, 47.3 cm (18.62”) circumference – It weighs 1.9375 lbs! Yikes! – Updraft estimate: 160+ mph! – Fell with other large hail • Is it the world’s heaviest? (no) – Bangladesh 14 April 1986 – 1.02 kg (~2.25 lbs)! – 92 fatalities Read: Hail: oddities • Hail often forms on a soft hail (graupel) core. • This one formed by water freezing on a stalk of chaff (bit of grass or cornstalk) tossed high into a cloud by a tornado (Pampa, TX, 16 Nov. 2015).

• Crop damage:

Read: Hail: what does it do?

• House damage:

Read: Hail: what does it do?

• Other damage:

Fort Worth, 28-March-2000, window damage Lecture Summary: • Hail – formation & growth • riming on snowflake creates graupel or “soft hail” • graupel (or rime-coated frozen drop) is seed for hailstone • hail grows by: – further riming (making a bubbly whitish layer) or – by freezing supercooled liquid water (making clear layer) – growing hail found at top of vault (or WER) in updraft – when updraft can’t support stone, it falls out of cloud, usually at leading edge of downdraft – more common in: high plains (intense storms) & mountains (cold cloud base) – Detectable sometimes as ‘spike’ on radar. May be visible as streaky precipitation falling from cloud base – Various kinds of damage from impacts, including injuries (next time Hurricanes, part 1-1.5)

End of lecture 15/16