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The science of thunderstorms and lightning

Tuesday, June 20th, 2017 4:00am MDT

Lightning produced by thunderstorms presents a very real danger to Colorado residents. Image courtesy VORTEX2 / NSSL.

Thunderstorms and their associated hazards frequently make appearances in Colorado during the spring and summer months.  Lightning presents one of the damaging – and deadly – aspects of these storms and knowing more about the science behind the phenomena helps us to better protect ourselves.

As part of Lightning and Wildfire Safety Awareness Week, this installment from the National Weather Service gives more information on the science behind these bolts of electricity.

From the National Weather Service:

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE BOULDER CO
600 AM MDT TUE Jun 20 2017

…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 fifty thousand degrees Fahrenheit. This 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. Towering cumulus clouds may be one of the first indications of a developing thunderstorm. 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 cause 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.

Due to the pool of negative charges in the lower part of the storm…a pool of positive charges will develop along the ground and follow the cloud like a shadow. Farther away from the cloud base…but under the positively charged anvil…a strong negative charge may be induced.

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 completed…and the result is 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 as they typically deliver more overall electrical charge to the ground…and they remain in contact with the ground surface for a longer period of time as compared to a negative cloud-to-ground flash.

The best advice in order to minimize your risk of becoming a lightning strike victim is to get indoors into a substantial shelter the second you hear thunder…and to remain there for at least 30 minutes after the last rumble of thunder.  In general…when thunder roars…go indoors. An enclosed hard topped automobile is also a safe place to be during thunderstorms.

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

http://www.lightningsafety.noaa.gov

Lightning Safety and Wildfire Awareness Series:

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