How Lightning Strikes on a Cloudless Day

There’s a fearsome killer, dangerous and lethal, that strikes silently, swiftly and viciously. This killer happens to be a particularly fierce form of lightning that erupts unexpectedly from a clear sky – a veritable “bolt from the blue” – which strikes with such ferocity that it must surely have been hurled earthward by Zeus himself.

The expression, “bolt from the blue” (also called a “positive giant”), was originally coined to describe an aggressive and unpredictable lightning strike that ignites the air when the sky appears cloudless and blue. This extremely hazardous type of lightning originates as a bolt within a thundercloud before escaping to wreak its havoc. It can travel many more miles than thunder, to areas of clear blue sky that are distant from a storm. Because of this phenomenon, you can be outside, under a blue sky devoid of clouds or thunder, and still get struck by lightning without any forewarning.

Bolts from the blue have such infamy that they’ve sizzled as an idiom in speech and in literature. In Jack London’s “Jerry of the Islands,” he writes, “Above him, falling upon him like a bolt from the blue, was a winged hawk unthinkably vaster than the one he had encountered.” And in Gilbert K. Chesterton’s “The Innocence of Father Brown,” is the sentence, “In an instant like a bolt from the blue, like a thunderbolt from nowhere, that beautiful and defiant body had been dashed down the open well of the lift to death at the bottom.” 

At its most basic, lightning is an electrical current. Inside a thundercloud, small pieces of ice collide, creating an electric charge. This is like the static electricity that builds up in your body when you shuffle across a carpet, and is released when you touch something metallic. Eventually, the cloud is teeming with these electrical charges. Positive charges accumulate at the top of the cloud, and negative charges fill its underside. Opposites attract, causing positive charges to gather on the ground underneath the cloud. These positive charges gravitate toward anything that’s upright, including trees, light poles, mountains – and people. The charge from the ground eventually unites with the charge from the cloud, causing a lightning strike. Lightning is also very versatile – it can strike from one cloud to another, within a cloud, from the ground to a cloud or from a cloud to the ground. It can also be deadly – lightning kills approximately 55 people in the United States per year.

Most lightning commonly carries a negative electrical charge, and is released from the base of a thunderstorm cloud, so it can be anticipated and planned for accordingly. Bolts from the blue, however, form atop a thunderhead, carry a positive charge, pack more than 10 times the electrical current of a regular bolt, are hotter and last 10 times longer. Additionally, they move horizontally, away from the storm – miles farther than conventional lightning – then finally curve crackling down to the ground.

Protecting yourself from these insidious, unpredictable fair-weather bolts can be tricky. There are few warnings on a deceptively pleasant day that lightning is lurking and ready to strike, capable of causing serious injury or even death. Although they only contribute to 5% of all lightning strikes, they’re not to be taken lightly. Keep in mind that even jets cannot withstand a strike from a positively charged bolt, and that these bolts often travel 25 miles from a thunderhead. And there have been unconfirmed reports of this type of lightning striking as far as 50 miles from its source. As a rule of thumb, even if you can only faintly hear a thunderstorm rumbling in the distance, stay indoors. Wait at least 30 minutes before going out. There’s no safe place outside during an electrical storm. Where there’s thunder, you can be sure there are also lightning strikes.