Geography 103
Weather
Lecture No. : Lightning and Thunder (Quiz 2)

1. Definition of lightning
    A discharge of electricity, a giant spark, which occurs in mature thunderstorms.
2. Lightning directions
    (1). Within clouds: A majority of lightning strikes.
    (2). Between clouds.
    (3). Between clouds and surrounding air.
    (4). Between cloud and ground: 20%.
3. Thunder
    (1). Booming sound wave, a shock wave, caused by the explosive expansion of air due to high temperature of
          30,000 ^oC (54,000 ^oF) in the lightning path.
    (2). Speed of sound wave
          A. 330 m/s or 1100 feet/s.
          B. 3 seconds/km or 5 sec/miles.
          C. Thunder 15 seconds after lightning:
               The lightning stroke is 5 km or 3 miles away.
          D. Crack, clap or bang: The lightning stroke is within 100 m.
          E. Rumbles: The sound eminating from different areas of the stroke.
    (3). Lightning without thunder ( always a thunder in fact)
          A. Sound travels faster in warm air than in cold air.
          B. Unstable air: Sound waves bends (refracts) upward.
          C. Air molecules attenuate the sound energy.
          E. Eddy less than 50 meters in diaimeter scatters the sound wave.
4. Electrification of clouds
    (1). A net transfer of positive ions from the warmer object to the colder object.
          A. Hailstones:
              (A). Warmer due to the freezing of liquid water on the hailstone surface (release of latent heat).
              (B). Negatively charged (Loss of positive ions).
          B. Ice crystals: Colder and positively charged.
    (2). Freezing of supercooled water droplets on a warmer hailstone:
          A. Tiny splinters of positively charged ice break off and carried upward by updraft.
          B. Heavy hailstones fall toward the bottom of the clouds.
    (3). Distribution of electric charged particles:
          A. Upper cloud: +
          B. Middle part of the cloud: -
          C. Cloud bottom: mixed + and -
          D. Falling rain: +
5. The lightning stroke
    (1). Negative charges at the cloud base attract positive charges on the ground beneath the cloud.
    (2). Cloud to ground lightning
          A. Electric potential exceeds 3 million volts per meter along a path 50 meter long.
          B. Stepped leader
              (A). The first discharge of electrons from cloud base toward the ground.
              (B). Discharge of electrons from cloud base toward the ground in a series of steps.
              (C). Each step takes 50 to 10 meters.
              (D). Pause between steps: 50-millionths of a second: invisible to human eye.
          C. Return stroke
              (A). Initial upward positive charges from the ground to meet the stepped leader.
              (B). A current of positive charges moves upward to meet the stepped leader as the electric potential
                     becomes large.
              (C). A large and luminous positive charges several cm in diameter surge upward toward the cloud base
                     along the path of stepped leader.
              (D). Time span: one-ten thousandth of a second.
          D. Dart leader
              (A). The subsequent leader.
              (B). Leader and stroke repeat in the same ionized channel ar intervals of about four hundredths of a
                     second.
              (C). Weaker energy but higher speed(less electric resistance).
          E. Lightning flash
              3 - 4 strokes in less than one second.
    (3). Shape of lightning
           A. Forked lightning
                A dart leader moving toward the ground deviates from the original path taken by the stepped leader.
Crooked or forked shape.
            B. Ribbon lightning
When the wind moves the ionized channel between each return stroke.
            C. Bead lightning
                (A). The lightning channel breaks up like a series of beads tied to a string.
                (B). The lightning stroke is partially obscured by clouds or falling rain.
            D. Ball lightning
A luminous sphere that appears to float in the air or slowly darts about for several seconds
            E. Sheet lightning
                The lightning flash is not seen but the flash causes the clouds to appear as a diffuse luminous white
                sheet.
            F. Heat lightning
Distant lightning that illuminates the sky but is too far away for its thunder to be heard.
            G. St. Elmo’s fire
                (A). A bright electric discharge that is projected from objects when they are in a strong electric field,
                       such as during a thunderstorm.
                (B). A luminous greenish or bluish halo over the top of a ship’s mast.
             H. Lightning rod.
              I. Fulgurite: A rootlike system of tubes resulting from the fusion of sand particles by lightning strike.
    (4). Lightning Detection
          A. Lightning detection finder: radio waves produced by lightning.
          B. Satellite (GOES): Lightning mapper sensor (planning).
          C. Visual observation.
    (5). Damage in USA
          A. 10,000 fires/year, $50 million worth of timber/year.
          C. Death toll: 100/year, Florida suffers most fatalities (100,000 ampers).
          D. Stay indoor, cars, crouch down in the open field.
          E. Lightning about to strike: Skin tingle and clicking sound.
    (6). Lightning suppression
          A. Seeding a Cb cloud with hair-thin pieces of aluminum about 10 cm long.
          B. Corona discharges (tiny sparks) prevent the electric potential in the cloud from building to a point
               where lightning occurs.
          C. Nature: Pine needles act as tiny electric rods.