...UNDERSTANDING THE SCIENCE OF THUNDERSTORMS AND
LIGHTNING...


BY DEFINITION...ALL THUNDERSTORMS CONTAIN LIGHTNING. LIGHTNING IS
A GIANT SPARK OF ELECTRICITY THAT OCCURS WITHIN THE ATMOSPHERE OR
BETWEEN THE ATMOSPHERE AND THE GROUND.  AS LIGHTNING PASSES THROUGH
THE AIR...IT HEATS THE AIR RAPIDLY TO A TEMPERATURE OF ABOUT 50000
DEGREES FAHRENHEIT...CONSIDERABLY HOTTER THAN THE SURFACE OF THE
SUN. DURING A LIGHTNING DISCHARGE...THE SUDDEN HEATING OF THE AIR
CAUSES IT TO EXPAND RAPIDLY.   AFTER THE DISCHARGE...THE AIR
CONTRACTS QUICKLY AS IT COOLS BACK TO A NORMAL TEMPERATURE.  THIS
RAPID EXPANSION AND CONTRACTION OF THE AIR CAUSES A SHOCK WAVE THAT
WE HEAR AS THUNDER...THIS SHOCK WAVE CAN DAMAGE WALLS AND BREAK
GLASS.

ALL THUNDERSTORMS GO THROUGH VARIOUS STAGES OF GROWTH...
DEVELOPMENT...ELECTRIFICATION AND DISSIPATION.  THE PROCESS OF
THUNDERSTORM DEVELOPMENT OFTEN BEGINS EARLY IN THE DAY WHEN THE SUN
HEATS THE AIR NEAR THE GROUND AND POCKETS OF WARMER AIR START TO RISE
IN THE ATMOSPHERE.  WHEN THESE POCKETS REACH A CERTAIN LEVEL IN THE
ATMOSPHERE...CUMULUS CLOUDS START TO FORM.  CONTINUED HEATING CAN
CAUSE THESE CLOUDS TO GROW VERTICALLY UPWARD INTO THE ATMOSPHERE.
THESE TOWERING CUMULUS CLOUDS MAY BE ONE OF THE FIRST INDICATIONS
OF A DEVELOPING THUNDERSTORM.  THE FINAL STAGE OF DEVELOPMENT OCCURS
AS THE TOP OF THE CLOUD BECOMES ANVIL SHAPED.

AS THE CLOUD GROWS...PRECIPITATION FORMS WITHIN THE CLOUD WITH
MOSTLY ICE CRYSTALS IN THE UPPER LEVELS OF THE CLOUD...A MIXTURE OF
ICE CRYSTALS AND SMALL HAIL IN THE MIDDLE LEVELS OF THE CLOUD...AND
A MIXTURE OF RAIN AND MELTING HAIL IN THE LOWER LEVELS OF THE CLOUD.
DUE TO AIR MOVEMENTS AND COLLISIONS BETWEEN THE PRECIPITATION
PARTICLES NEAR THE MIDDLE OF THE CLOUD...THE VARIOUS PRECIPITATION
PARTICLES BECOME CHARGED. THE LIGHTER ICE CRYSTALS GATHER A POSITIVE
CHARGE AND ARE CARRIED UPWARD INTO THE UPPER PART OF THE STORM BY THE
UPDRAFT.  THE HEAVIER HAIL GATHERS A NEGATIVE CHARGE AND FALLS TOWARD
THE LOWER PART OF THE STORM.  THE END RESULT IS THAT THE TOP OF THE
CLOUD BECOMES POSITIVELY CHARGED AND THE MIDDLE TO LOWER PART OF THE
STORM BECOMES NEGATIVELY CHARGED.

NORMALLY...THE EARTHS SURFACE HAS A SLIGHT NEGATIVE CHARGE
HOWEVER...AS THE NEGATIVE CHARGES BUILD UP IN THE LOWER PART OF THE
STORM...THE GROUND BENEATH THE BASE OF THE CLOUD AND IN THE AREA
IMMEDIATELY SURROUNDING THE CLOUD BECOMES POSITIVELY CHARGED. AS THE
CLOUD MOVES...THESE INDUCED POSITIVE CHARGES ON THE GROUND FOLLOW
THE CLOUD LIKE A SHADOW. FARTHER AWAY FROM THE CLOUD BASE...BUT
UNDER THE POSITIVELY CHARGED ANVIL...THE NEGATIVE CHARGE MAY BE
FURTHER INDUCED.

IN THE INITIAL STAGES OF DEVELOPMENT...AIR ACTS AS AN INSULATOR
BETWEEN THE POSITIVE AND NEGATIVE CHARGES. HOWEVER...WHEN THE
ELECTRICAL POTENTIAL BETWEEN THE POSITIVE AND NEGATIVE CHARGES
BECOMES TOO GREAT...THERE IS A DISCHARGE OF ELECTRICITY THAT WE
KNOW AS LIGHTNING.

LIGHTNING CAN OCCUR COMPLETELY WITHIN THE THUNDERSTORM CLOUD OR
BETWEEN THE CLOUD AND THE GROUND. IN CLOUD LIGHTNING GENERALLY
OCCURS BETWEEN POSITIVE CHARGES NEAR THE TOP OF THE CLOUD AND
NEGATIVE CHARGES NEAR THE BOTTOM OF THE CLOUD. CLOUD TO GROUND
LIGHTNING OCCURS BETWEEN THE CLOUD AND THE GROUND AND IS...OF
COURSE...THE MOST DANGEROUS.  LIGHTNING CAN ALSO OCCUR BETWEEN
CLOUDS.

CLOUD TO GROUND LIGHTNING CAN BE CATEGORIZED INTO TWO DIFFERENT
TYPES...THE NEGATIVE FLASH AND THE POSITIVE FLASH.  THE NEGATIVE
FLASH USUALLY OCCURS BETWEEN THE NEGATIVE CHARGES IN THE LOWER PART
OF THE STORM AND THE POSITIVE CHARGES ON THE GROUND UNDER AND NEAR
THE CLOUD BASE.  POSITIVE FLASHES USUALLY OCCUR BETWEEN THE
POSITIVELY CHARGED UPPER LEVELS OF THE STORM AND THE NEGATIVELY
CHARGED AREA SURROUNDING THE STORM.

IN THE NEGATIVE CLOUD TO GROUND FLASH...AN ALMOST INVISIBLE
NEGATIVELY CHARGED CHANNEL OF AIR FORMS NEAR THE CLOUD BASE AND
SURGES DOWNWARD TOWARD THE GROUND. AS THIS STEP LEADER APPROACHES
THE GROUND...STREAMERS OF POSITIVE CHARGE SHOOT UPWARD FROM TREES
BUILDINGS AND OTHER OBJECTS ON THE GROUND.  WHEN THESE STREAMERS
MEET THE STEP LEADER...THE CONNECTION IS COMPLETE...AND A SURGE OF
ELECTRICAL CURRENT MOVES FROM THE GROUND TO THE CLOUD CAUSING THE
VISIBLE RETURN STROKE THAT WE CALL LIGHTNING.  THE ENTIRE PROCESS
TAKES ONLY A MATTER OF A SECONDS.

THE PROCESS FOR A POSITIVE FLASH IS SIMILAR EXCEPT THAT A POSITIVE
CHANNEL USUALLY ORIGINATES IN THE ANVIL OF THE STORM AND SURGES
DOWNWARD.  IN THIS CASE...STREAMERS OF NEGATIVE CHARGE SHOOT UP TO
MEET POSITIVELY CHARGED CHANNEL AS IT APPROACHES THE GROUND. WHEN A
CONNECTION IS MADE...A POSITIVE FLASH OF LIGHTNING OCCURS.

WHILE BOTH NEGATIVE AND POSITIVE FLASHES OF LIGHTNING CAN BE DEADLY.
THE POSITIVE FLASHES GENERALLY ARE MORE DESTRUCTIVE AND ARE MORE APT
TO CATCH PEOPLE BY SURPRISE.  BECAUSE THE DISTANCE BETWEEN THE GROUND
AND ANVIL IS MUCH GREATER THAN THE DISTANCE BETWEEN THE GROUND AND
THE CLOUD BASE...A MUCH LARGER ELECTRIC POTENTIAL IS NEEDED TO
INITIATE A POSITIVE FLASH OF LIGHTNING.  FOR THE SAME REASON
POSITIVE FLASHES ARE INFREQUENT AND WIDELY SCATTERED AROUND THE
STORM...BUT GENERALLY INVOLVE THE EXCHANGE OF A MUCH GREATER CHARGE
AND ARE USUALLY MORE DESTRUCTIVE.

THE GREATEST DANGER ASSOCIATED WITH THE POSITIVE FLASHES...HOWEVER
IS THAT THEY STRIKE IN AREAS WHERE MOST PEOPLE THINK THEY ARE SAFE
FROM THE STORM.  THEY GENERALLY STRIKE WELL BEYOND THE AREA WHERE
RAIN IS FALLING AND WELL BEYOND THE MAIN AREA WHERE MOST OF THE
LIGHTNING...NEGATIVE FLASHES AND THUNDER IS OCCURRING.
CONSEQUENTLY...MANY VICTIMS ARE CAUGHT COMPLETELY OFF GUARD.