Lightning is a massive, rapid discharge of electrical energy in the atmosphere, often reaching temperatures hotter than the surface of the sun. This powerful natural phenomenon can deliver hundreds of millions of volts and poses a significant threat during a thunderstorm. The danger zone extends far beyond the immediate point of a strike, meaning injury or fatality can occur even when the lightning is miles away. Understanding the proximity limits of this hazard is a foundational element of outdoor safety. Assessing the storm’s distance is the first step in mitigating this risk and knowing precisely when to seek shelter.
The 30/30 Rule for Calculating Distance
The primary method for determining a storm’s dangerous proximity is the widely accepted 30/30 rule. This guideline provides an actionable measurement for when to halt outdoor activity and when it is safe to resume. The first “30” refers to counting the seconds between seeing the lightning flash and hearing the resulting thunderclap, known as the “Flash-to-Bang” method.
Sound travels approximately one mile every five seconds. Dividing the measured time in seconds by five yields the approximate distance of the strike in miles. If the time count is 30 seconds or less, the storm is close enough to pose a direct threat, and you must seek immediate shelter. A count of 30 seconds means the storm is roughly six miles away, which is within the hazardous range.
The second “30” mandates waiting 30 minutes after the last clap of thunder before returning to outdoor activities. This waiting period accounts for the fact that lightning can strike from the edge of a storm cloud, often traveling up to 10 miles horizontally from the main storm cell. Waiting 30 minutes ensures that the trailing part of the storm has passed completely.
Understanding Ground Current and Indirect Strikes
While a direct strike is the most powerful mechanism of injury, most lightning-related casualties are caused by indirect strikes, particularly ground current. When lightning strikes the earth, the massive electrical charge dissipates radially outward through the ground’s surface. This dispersion creates a voltage gradient, meaning the electrical potential is highest at the strike point and decreases with distance.
This phenomenon is known as “step voltage” or ground current, which is hazardous to anyone standing near the strike point. If a person’s feet are apart, a significant voltage difference is created between the two points of contact on the ground. The electrical current flows through the body, potentially causing cardiac arrest, nerve damage, or severe burns.
Another indirect mechanism is “side flash,” where the current jumps from a struck object, such as a tree or pole, to a nearby person. This occurs when the object cannot fully conduct the charge to the ground, causing a portion of the current to “splash” sideways to a closer conductor.
“Contact voltage” is also a risk, happening when a person touches an object—such as a fence, metal railing, or corded phone—that is part of the lightning’s electrical path. The greatest defense against these indirect mechanisms is ensuring you are in a fully enclosed, protected space when the 30/30 rule indicates danger.
Identifying Safe and Unsafe Shelters
Once the 30/30 rule signals a threat, the priority is moving to a safe shelter—a structure designed to handle a lightning strike. A safe shelter is a large, fully enclosed building with plumbing and electrical wiring, such as a home, school, or office building. These structures safely conduct the electrical current through their systems and into the ground, protecting the occupants.
A hard-topped, fully enclosed vehicle with the windows rolled up is the second-best option. The metal shell of the car directs the current around the occupants and into the ground, a principle known as the Faraday cage effect. The metal frame provides safety; the tires’ rubber offers no protection.
Conversely, small, open structures offer little protection and are considered unsafe shelters. These include picnic pavilions, gazebos, bus stops, tents, or baseball dugouts. They are often not grounded and may increase the risk of a side flash or ground current injury.
If no safe shelter is immediately available, the last resort is to crouch down low in the open. Keep your feet together and minimize contact with the ground to reduce the risk of step voltage.