Swimming during a thunderstorm is highly hazardous and strongly discouraged by safety experts. Lightning is an electrical discharge that can strike the ground or water surface miles away from the main storm cell, making its trajectory unpredictable. Water significantly increases the danger, providing an effective pathway for the electrical current to travel rapidly over a wide area.
How Lightning Interacts with Water
Water acts as a conductor for electricity, allowing a lightning strike’s massive electrical charge to spread outward. When a lightning bolt connects with a body of water, the current immediately disperses across the surface in all directions. This rapid spread creates a dangerous condition known as Ground Potential Rise, or step voltage, which can affect swimmers even if they are not at the precise strike point.
The current and voltage are highest at the point of impact, declining exponentially the further a swimmer is from the strike zone. A person in the water can be subjected to a difference in electrical potential across their body, forcing current through their internal systems. This effect can cause severe injury or death by inducing cardiac arrest or paralyzing respiratory muscles.
The electrical charge from a lightning strike tends to travel across the surface of the water rather than deeply penetrate it. This concentrates the electrical energy near the top layer, putting a swimmer with their head above the surface at heightened risk. While a direct strike is the most dangerous event, an indirect strike can still impart a lethal shock across a wide radius.
Varying Risk Levels Based on Location
The specific risks of a lightning strike differ depending on the swimming environment, though all outdoor water activities become unsafe when a storm approaches. In open water, such as lakes or oceans, a swimmer’s head becomes the tallest object on the flat expanse, making it a preferential target for a direct strike. The vast surface area also means that an indirect strike can travel a significant distance, endangering swimmers far from the point of impact.
There is a slight difference in conductivity between freshwater and saltwater, with ocean water being a better electrical conductor. However, this difference does not make freshwater significantly safer, as both water types effectively transmit a dangerous electrical current over a large area. The primary safety concern in open water remains the inability to quickly reach safe shelter when a storm is detected.
Swimming pools, whether private or public, introduce unique electrical pathways that increase risk. Lightning can strike the water directly, or it can travel through the pool’s metal filtration and circulation systems, including pumps, heaters, and ladders. Even if a lightning strike occurs nearby, the electrical energy can follow the plumbing or wiring into the pool water.
Indoor pools are only safe if they are housed within a fully enclosed, grounded building. Swimmers should still avoid contact with the pool water and plumbing during a thunderstorm. Activities like showering or bathing should be postponed, as lightning can travel through a building’s metal or even modern plastic plumbing system. This current can pose a risk even when the strike is not direct.
Essential Safety Protocols
The most important guideline for staying safe near water during a thunderstorm is the “30-30 Rule.” The first part states that if the time between seeing a lightning flash and hearing the thunder is 30 seconds or less, the storm is close enough to be a danger, and all outdoor activities must stop immediately. A 30-second count indicates the storm is approximately six miles away, which is within striking distance.
The second part of the rule requires waiting a minimum of 30 minutes after the last observed flash of lightning or clap of thunder before returning to the water. Lightning can strike from the edge of a storm cloud, often called a “bolt from the blue,” even when the sky appears clear overhead. Waiting the full 30 minutes ensures the entire electrical threat has passed over the area.
Immediate action upon hearing thunder is to exit the water quickly and move away from the shoreline. Swimmers should not seek shelter under open-sided structures, such as picnic shelters, gazebos, or isolated trees, as these offer no protection from a lightning strike. These structures are often tall, isolated targets and do not provide the necessary enclosure.
A safe shelter is a sturdy, fully enclosed structure, like a home, school, or office building, which provides grounding through its foundation and utility systems. If no building is immediately available, a hard-topped vehicle with the windows fully rolled up is the next best option. The metal frame of the vehicle can safely conduct the current around the occupants. While inside, avoid contact with any electrical equipment or plumbing fixtures.