WFO Denver/Boulder

PUBLIC INFORMATION STATEMENT 
NATIONAL WEATHER SERVICE BOULDER CO
600 AM MDT TUE Jun 20 2006

COLORADO LIGHTNING AND WILDFIRE PREPAREDNESS WEEK CONTINUES THROUGH THIS
SATURDAY.  A PUBLIC INFORMATION STATEMENT WILL BE ISSUED EACH DAY ON A
DIFFERENT LIGHTNING TOPIC.  TODAY WE DISCUSS THE SCIENCE OF LIGHTNING.  

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

Every thunderstorm produces lightning.  Lightning is a giant spark that
moves within the cloud…between clouds…or between the cloud and the
ground.  As lightning passes through the air…it heats the air rapidly to
a temperature of about 50,000 degrees Fahrenheit…which causes a rapid
expansion of the air near the lightning channel.  This rapid expansion
causes a shock wave that we hear as thunder. 

Thunderstorms will form if there is enough moisture and instability in
the atmosphere. As the sun warms the air near the ground...pockets of
warmer air begin to rise and cool.  Condensation of water vapor causes
cumulus clouds to form.  Continued heating can cause these clouds to
continue to grow upward into the atmosphere.  These "towering cumulus"
clouds may be one of the first indications of a developing thunder-
storm.  The mature thunderstorm has both an updraft of rising motion and
a downdraft of sinking cool air accompanied by rain and sometimes hail.   

Thunderstorms grow tens of thousands of feet into the atmosphere. In the
cloud...precipitation forms as ice crystals...hail...and rain.  
Collisions between ice particles causes a charge separation...and
positively charged ice crystals are carried by the updraft high in the 
thunderstorm. The heavier hail gathers a negative charge and falls toward
the lower part of the storm.  The top of the cloud becomes positively
charged and the lower part of the storm becomes negatively charged.  

Normally, the earth's surface has a slight negative charge.   As the
negative charges build up in the lower part of the storm, the ground near
the thunderstorm 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…a
stronger negative charge may be induced.

Air normally acts as an insulator.  When the electrical potential 
between the positive and negative charges becomes too great…there is a
discharge of electricity that we know as lightning.

Cloud-to-ground lightning can either be a negatively charged flash or a
positively charged 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 invisible, negatively-charged
step leader forms near the cloud base and surges downward toward the
ground.  As this "step leader" approaches the ground, streamers of
positive charge move 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 place in fractions of a second.  If you are under a
thunderstorm and your hair rises...you are in an area where the positive
charges are rising up objects towards the storm. It is a dangerous
location...because lightning may be about to strike.    

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 the
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. Positive flashes are infrequent and may strike the
ground miles from the main part of the storm.  The positive flashes may
involve the exchange of a much greater charge and are usually more
destructive.  Positive flashes also strike well beyond the area where
rain is falling...and away from the bulk of the lightning. 
Consequently…many victims of positive lightning strikes 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 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.  For
example…if it takes 10 seconds between the lightning flash and the
thunder…the lightning flash was 2 miles away. 

For additional information about lightning or lightning safety…visit the
NOAA Lightning Safety Awareness web site at (in lower case):

     http://www.lightningsafety.noaa.gov  (All in lower case)



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