NOUS42 KCHS 060935 PNSCHS GAZ087-088-099>101-114>119-137>141-SCZ040-042>045-047>051-211000- PUBLIC INFORMATION STATEMENT NATIONAL WEATHER SERVICE CHARLESTON SC 500 AM EDT TUE JUN 20 2006 ...UNDERSTANDING THE SCIENCE OF THUNDERSTORMS AND LIGHTNING... A THUNDERSTORM FORMS IN AIR THAT HAS THREE COMPONENTS...MOISTURE... INSTABILITY AND SOMETHING SUCH AS A COLD FRONT TO CAUSE THE AIR TO RISE. CONTINUED RISING MOTIONS WITHIN THE STORM MAY BUILD THE CLOUD TO A HEIGHT OF 35 THOUSAND TO 60 THOUSAND FEET ABOVE SEA LEVEL. LIGHTNING IS ASSOCIATED WITH ALL THUNDERSTORMS. 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. CLOUD TO GROUND LIGHTNING CAN BE CATEGORIZED INTO TWO DIFFERENT TYPES...THE NEGATIVE FLASH AND THE POSITIVE FLASH. 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 CHARGES 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 FRACTION OF A SECOND. 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 CHARGES 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 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. THE BEST ADVICE IN ORDER TO MINIMIZE YOUR RISK OF BECOMING A LIGHTNING VICTIM IS TO GET TO A SAFE SHELTER SOONER AND TO STAY THERE LONGER. IN GENERAL...IF YOU CAN HEAR THUNDER YOU ARE WITHIN STRIKING DISTANCE OF THE STORM. LIGHTNING QUESTION OF THE DAY... HOW CAN YOU TELL HOW FAR A FLASH OF LIGHTNING IS AWAY FROM YOU. ANSWER... WHILE YOU SEE THE VISIBLE FLASH OF LIGHTNING ALMOST INSTANTANEOUSLY... THE SOUND OF THE THUNDER TRAVELS AT A SPEED OF ABOUT 1100 FEET PER SECOND OR ABOUT 1 MILE IN 5 SECONDS. FOR EVERY 5 SECONDS BETWEEN THE TIME YOU OBSERVE THE LIGHTNING AND THE TIME YOU HEAR THE THUNDER...THE LIGHTNING FLASH IS APPROXIMATELY 1 MILE AWAY. IF IT TAKES 20 SECONDS BETWEEN THE LIGHTNING FLASH AND THE THUNDER... THE LIGHTNING FLASH WAS 4 MILES AWAY. UNFORTUNATELY...THIS METHOD ONLY WORKS FOR THE PREVIOUS FLASH AND DOES NOT TELL YOU HOW CLOSE THE NEXT LIGHTNING STRIKE WILL BE. GENERALLY...IF YOU HEAR THUNDER...YOU ARE WITHIN STRIKING DISTANCE FOR THE NEXT FLASH OF LIGHTNING. IF YOU ARE NOT IN A SAFE PLACE AT THE TIME, MOVE TO A SAFE PLACE IMMEDIATELY. FOR ADDITIONAL INFORMATION ABOUT LIGHTNING OR LIGHTNING SAFETY... VISIT NOAA'S LIGHTNING SAFETY AWARENESS WEB SITE AT... HTTP://WWW.LIGHTNINGSAFETY.NOAA.GOV $$