Chapter 21. Lightning Facts. Lightning Facts 4/12/17. Lightning

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1 Chapter 21 Lightning Lightning Facts How many lightning strikes occur each second around the planet? How is it derived? Where do most occur and what regions are most prone to strikes? How many CG strikes occur annually over the U.S.? Nearly half strike the ground at simultaneous locations, how many locations get struck during the year? Lightning Facts A 10-year climatology of lightning showed that on average: 39 people were electrocuted each year 244 injured Large fraction of these statistics were males Guess that means I can use my umbrella in a thunderstorm! 1

2 Lightning Facts Most fatalities occur in Florida $50 million in property damages every year Lightning Stroke Lightning can extend for miles, but is typically 2 3 cm in diameter The electric field prior to a strike rises to over a million volts/meter Current of a bolt can range between 15,000 and 30,000 amperes Temperature of the bolt reaches 30,000 Celsius (54,000 F), 5x hotter than the suns surface Lightning Stroke Flash from the stroke is wider than the actual bolt. Superheated air expands and results in thunder Energy output of a typical bolt is 1 10 billion Joules Typical household uses 1.8 billion Joules per month 2

3 Lightning Development Need to understand charged particles Protons: positive charge, found in the nuclei of atoms Electrons: negative charge, orbit the nuclei of the atoms Neutral charge means atom has = # of protons and electrons What does it mean if it s positively charged? Negatively charged? What is it called when they aren t the same? Lightning Development Electrons are mobile, protons are not Current ALWAYS flows opposite the direction of the flow of electrons. Why? Electric field is present anytime a +/- charge exists Opposites attract! Magnitude of the force of attraction is measured in volts Strength of the electric field is measured in volts/meter Lightning Development Current flows easily through conductors, but not insulators Is the atmosphere a conductor or an insulator? Fair Weather Electric Field exists all the time because the surface has an excess number of electrons and the atmosphere has an excess number of protons. Why? 3

4 Fair Weather Electric Field Charging Mechanisms Interface Charging Also referred to as non-inductive charging Responsible for the INITIAL charge in a t-storm Two conductors may have a neutral charge, but the distribution of electrons on the surface of the atoms varies between conductors Bringing these conductors into contact will result in a brief current as electrons are exchanged 4

5 Exchange of Charges Interface Charging In thunderstorms, graupel and hail grow by collecting and freezing supercooled water droplets Ice crystals form by vapor deposition Ice formed by these processes has different surface properties that result in different conductor surfaces. As the larger falling particles hit the smaller crystals, the larger particles gain electrons Smaller particles get lofted higher and transfer positive charge to the top of the cloud Interface Charging 5

6 Induction Charging Occurs after interface charging Accelerates overall charging process Strong electric field develops in the storm, which impacts the distribution of electrons and protons within an atoms nucleus Electrons gather towards the top of the atom, protons near the bottom Particle collisions result in electron transfers and each additional collision strengthens electric field Earth s Surface Charging Strong negative charge at cloud base repels electrons at the earth s surface, leaving a positive charge (image charge) Regions closest to the thunderstorm base (trees, structures, mountain tops, golfers heads, etc.) become most devoid of electrons Lightning Stroke Intracloud lightning in-cloud lightning CC lightning cloud to cloud lightning CG lightning cloud to ground lightning Over 80% are Intracloud or CC bolts CG begins when the local area electric field reaches 3 million volts/meter Air loses it s ability to insulate Electrons begin to move from cloud base to ground in surges (stepped leader) 6

7 Stepped Leader Each step is meters long Pauses between steps last a few millionths of a second Charge takes path of least resistance, jumping through raindrops (good conductors) Charge can take several paths simultaneously, leading to forked appearance of the bolt Lightning Stroke As stepped leader approaches the ground, electric field becomes so great that positive charges jump off the object to meet the negative charge When they meet, the return stroke occurs Full current is reached and the bolt occurs Flow of electrons is downward what direction does current move? Channel becomes ionized as discharge occurs Lightning Stroke Same process may be repeated in the same channel Subsequent leader is called a dart leader Moves more rapidly due to the additional ionization in the channel Occur very rapidly, within a few hundredths of a second Most CG strokes occur between cloud base and ground, some can occur between anvil and ground 7

8 Stroke Polarities CG strokes between cloud base and ground have what type of polarity? Why? CG strokes between the anvil and ground have what type of polarity? Why? Strike Formation Positive Polarity Strikes Less common than negative polarity Much more dangerous, why? 8

9 Bolt from the Blue CG bolt that exist from the side of a thunderstorm Why polarity are they? Can occur over 6 miles away from the main storm Other Electrical Charge Phenomena Thunderstorm charging leads to other effects besides lightning Phenomena may be below, at or above clouds Thunder Air heated immediate in the vicinity of the lightning bolt expands explosively Creates shockwave in the air Take 3 s econds to travel a km, 5 s econds to travel a mile Close to the bolt, thunder sounds like a bang Further away, it sounds like a bang and a rumble Further still and it just sounds like a rumble 9

10 Thunder Speed of sound in the atmosphere varies depending on temperature Lower temperature, slower sound waves Temperature generally decreases with height Atmosphere acts like a lens and bends sound waves upward Higher frequencies bend more rapidly up, so bang dissipates at the surface more quickly Heat Lightning Misnomer Results from thunderstorms way off in the distance Light from a distant flash is scattered by molecules overhead Appears a light flashes overhead Most originally thought this was due to the summer heat since the distant storm couldn t be seen Bead Lightning Following a stroke, lightning channels may break up briefly into a series of luminous beads Virtually impossible to observe with the eye Occur immediately following a bright flash Typically seen in high-speed photography and laboratory simulations of lightning 10

11 Sheet Lightning Lightning occurring within or behind a cloud may illuminate the clouds exterior uniformly, giving the appearance of a sheet of light St. Elmo s Fire Result of positive charge concentrating on antennas, ice picks of mountain climbers, and other sharp metal objects Too much charge accumulation will res ult in a discharge of small sparks Bright bluish-green halo may appear as continuous sparking occurs Has also been known to occur on ships with tall masts (hence sailors named it after their patron saint) May be a sign of an impending lightning stroke Sprites, Jets and Elves Red Sprites, Blue Jets and elves are optical phenomena that occur between thunderstorm tops and the mesosphere Sprite are red, large, weak flashes that occur simultaneously with lightning strokes Typically occur over anvil portions of thunderstorms Brightest regions typically 65 to 75 km above the surface Faint red glow may extend up to 90km 11

12 Sprites, Jets and Elves Below the bright red region of the Sprites, blue tendril-like filaments may extend downward to 30km Have very low surface brightness Usually only observed with sensitive cameras Can be seen if no background lighting interferes Almost always occur with positive strokes Sprites, Jets and Elves Jets are blue Cannot be detected by the naked eye Extend upward from the cloud in narrow cones Can travel upwards at 60 miles/sec Coincident with active portions of the thunderstorm Elves are disc-shaped regions of light that last < 1/1,000 sec Occur high above energetic CG bolts (positive polarity) Typically located in the ionosphere Lightning Types Above Cloud 12