Behind the Forecast: Bolts from the blue

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Published: Jun. 21, 2019 at 9:25 AM EDT
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LOUISVILLE, Ky. (WAVE) - “When Thunder Roars, Go Indoors.” It’s phrase meteorologists repeat consistently.

If you can hear thunder then you are close enough to be struck by lightning. This means a storm may be miles away but lightning can still strike your area.

According to the National Weather Service, there are 25 million cloud-to-ground lightning strikes each year in the United States. On average, more than 500 people are injured each year and around 47 people die.

The term “bolt from the blue” usually refers to something unforeseen. The phrase was first used in the early 1800s.

There are two main types of lightning: cloud-to-ground lightning or intra-cloud lightning.

Intra-cloud lightning is defined as an electrical discharge between oppositely charged areas within a thunderstorm.

When opposite charge between the cloud and the ground leads to an electrical discharge then that's called cloud-to-ground lightning. If there is a positive charge on the ground and a negative charge in the cloud then that's defined as a negative flash. If there's a positive charge in the cloud and a negative ground charge then that's a positive flash. Each cloud-to-ground strike is made of one or more leaders and return strokes. The leader is defined as the initial step in a lightning flash that forms the channel that the lightning will take.


Positive flashes are much more rare (only 10% of cloud-to-ground lightning is originates from positively charged leaders) and are more likely to be the dreaded "bolt from the blue."

Positive leaders begin their life at the top of cloud which is typically positively charged.

The tops of clouds tend to be positive because ice crystals (which form in the colder temperatures of the upper atmosphere) take on a positive charge while liquid water has a negative charge.

Usually the Earth's surface has no interaction with the positive part of the storm because of the center of the storm's negative charge. When low-level winds are weaker then the upper-level winds, the storm tilts and clouds at the top of the storm are dragged away from the updraft taking the positive charge with them.

When the differences of these charges become large enough, a positive leader may form. A larger charge is needed to *initiate* a positive lightning flash since the distance is greater to the surface. The flash lasts long and can have a charge 10 times greater than that of a negative cloud-to ground lightning strike; up to 300,000 amps and 1 billion volts.

Since the tops of the clouds can be dragged a significant distance away from the thunderstorm’s rain, positive lightning can occur miles away from the storm itself. According to the National Weather Service, these types of bolts have been documented more than 25 miles away from a thunderstorm.

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