
Although we seem to have passed through the biggest barrage of thunderstorms of this spring storm season, with notably sunnier weather ahead, there are always risks of thunder and lightning in Maryland in the summertime. So, here are some lightning facts to know and use, courtesy of the National Weather Service in observance of Lightning Safety Awareness Week:
What are thunder and lightning, and how are they produced?
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 50,000
DEGREES FAHRENHEIT, ABOUT 5 TIMES 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).
So how is that giant electrical spark created?
AS A THUNDERSTORM CLOUD GROWS, PRECIPITATION FORMS WITHIN THE CLOUD
WITH MOSTLY SMALL ICE CRYSTALS IN THE UPPER LEVELS OF THE CLOUD, A
MIXTURE OF SMALL ICE CRYSTALS AND SMALL HAIL (GRAUPEL) 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 BECOME POSITIVELY CHARGED AND ARE CARRIED UPWARD INTO THE
UPPER PART OF THE STORM BY THE UPDRAFT. THE HEAVIER HAIL BECOMES
NEGATIVELY CHARGED IS SUSPENDED BY THE UPDRAFT OR FALLS TOWARD THE
LOWER PART OF THE STORM. THE END RESULT IS THAT THE TOP OF THE
CLOUD BECOMES POSITIVELY CHARGED AND THE MIDDLE AND LOWER PART OF
THE STORM BECOMES NEGATIVELY CHARGED.
NORMALLY, THE EARTH`S SURFACE HAS A SLIGHT NEGATIVE CHARGE;
HOWEVER, AS THE NEGATIVE CHARGES BUILD UP IN THE LOWER AND MIDDLE
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, THE INSULATING CAPACITY OF THE AIR BREAKS DOWN
AND THERE IS A DISCHARGE OF ELECTRICITY THAT WE KNOW AS
LIGHTNING.
(SUN PHOTOS by Karl Merton Ferron 2004)
Do all lightning bolts reach the ground?
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 MIDDLE OR BOTTOM OF THE CLOUD. CLOUD-
TO-GROUND LIGHTNING OCCURS BETWEEN CHARGES IN THE CLOUD AND
CHARGES ON THE GROUND. LIGHTNING CAN ALSO OCCUR BETWEEN CLOUDS.
Are all bolts that reach the ground alike?
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.
Do these distinctions make any difference to people on the ground?
WHILE BOTH NEGATIVE AND POSITIVE FLASHES OF LIGHTNING CAN BE DEADLY,
POSITIVE FLASHES 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.
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.
What's the smartest course of action when a thunderstorm approaches?
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. REMEMBER...WHEN THUNDER
ROARS, GO INDOORS!
Is there a way to track a thunderstorm's movements without radar or venturing outside to watch?
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 1 MILE
AWAY. IF YOU COUNT 10 SECONDS BETWEEN THE LIGHTNING FLASH AND
THE CORRESPONDING THUNDER, THE LIGHTNING FLASH WAS 2 MILES AWAY.
FOR 15 SECONDS, THE FLASH WOULD BE THREE 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.
(SUN PHOTOS by Karl Merton Ferron 2004)