Popular Science - USA (2019-10)

by Rob Verger

MOST LIGHTNING JUST

bounces around within its cloud

of origin. But the rogue charges

that escape are enough to daz-

zle observers, inspire awe and

fear (which is fair—the zaps do

kill a few dozen people in the

US every year), and make some

magnificent thunder. A typi-

cal cloud- to- ground strike lasts

for just a fraction of a second.

Here’s how the short but brilliant

lifetime of a bolt goes down.

OVERVIEW

POPSCI.COM • WINTER 2019 23

it shot

me down

1 /ELECTRONS

BOUNCE

The show starts in a

thunder head, where ice

particles collide so fast,

they knock off each other’s

electrons. That leaves

posi tive charges hanging

near the top of the cloud,

while negative ones

accumu late in the middle.

2 /DISCHARGE

EMERGES

Energy zipping between

opposing charges forms

bolts. Given enough

buildup, electricity es-

capes the cloud in a series

of cascading branches

roughly 160 feet long; a

new spur forms every

50-millionths of a second.

5 /PROTONS PUNCH

Much more rarely, a

cloud’s positively charged

upper region manages to

connect with negative

particles on the ground.

Such strikes can carry

300,000 amps, and are so

strong, they can make

landfall as far as 25 miles

from the storm itself.

3 /PATHS CONNECT

As negative charges shoot

down from the cloud, they

repel electrons—leaving

positively charged columns

of air stretching up from

the earth and the tops of

buildings and trees. When

one of these connects with

a bolt’s branch, the merger

sets the lightning’s path.

6 /CHARGE

FORMATION

Here’s the boom: The main

stroke heats air to around

50,000 degrees Fahren-

heit, causing gases to

expand and create a shock

wave. That force weakens

to audible sound as it trav-

els, eventually reaching

your ears as thunder.

7 /SECONDS PASS

You see lightning instantly,

but thunder moves at— you

guessed it— Mach 1. That

familiar trick for calculat-

ing distance really works:

Begin counting when you

see the flash, and stop

when you hear the crash.

For every five seconds, the

strike is about a mile away.

4 /STROKE RETURNS

The meetup forms a wire

in the sky that sends elec-

trons spilling into the

ground and carries about

30,000 amps (enough to

power 2,000 wall sock-

ets). The bolt looks like an

instant flash but actually

zaps up its path at one-

third the speed of light.

SO
UR
CE

: (^) M
AR
TIN
(^) A
. (^) U
MA
N,
(^) U
NI
VE
RS
IT
Y (^) O
F (^) F
LO
RID
A