Lightning is a massive natural electrical discharge that can release hundreds of millions of volts of electricity. During a thunderstorm, being in or near a body of water introduces an immediate and serious risk of electrocution. The danger is not limited to a direct strike on a swimmer, but extends through the surrounding water and even the ground nearby. Water is an efficient pathway for electrical current to travel, making immediate shelter mandatory during a storm.
Water’s Role in Electrical Conduction
The assumption that water is a perfect conductor of electricity is a common misunderstanding, as pure H2O is actually a poor conductor. However, the water found in swimming pools, lakes, and oceans is never pure, containing dissolved solids and impurities that dramatically change its electrical properties. These impurities, which include salts, minerals, and chemicals like chlorine, break down into mobile, charged ions when dissolved.
These mobile ions are the agents that allow an electrical current to flow through the water. Ocean water, with its high concentration of dissolved salts, is an extremely effective conductor of electricity. Freshwater sources, such as lakes and tap water, contain fewer ions but still possess enough dissolved minerals to become conductive. When lightning strikes, this highly ionized water acts as a readily available path for the immense electrical charge to dissipate across a wide area.
Understanding Ground Current and Water Spread
A lightning strike does not need to hit the water directly to be a threat to a person swimming in it. The tremendous energy of a strike, whether it hits the water or the ground adjacent to the water, creates a voltage gradient that radiates outward from the impact point. This phenomenon is often referred to as ground current. In a body of water, it causes the electrical potential to spread rapidly across the surface and through the water column, decreasing steadily with distance.
The danger stems from the difference in electrical potential between two points on the body. This potential difference, known as step voltage, is created when one part of the body is closer to the strike point than another. If a swimmer’s head and feet are at different points along this voltage gradient, a current will flow through the body, seeking the path of least resistance. Even a voltage difference of a few thousand volts, which can occur from a strike tens of meters away, is enough to cause severe injury or death.
Recognizing Storm Warnings and Seeking Shelter
The window of time to seek shelter is short, as lightning can strike up to sixteen kilometers away from the rain area in a thunderstorm, even when the sky overhead appears clear. A widely recognized safety protocol is the “30/30 Rule,” which provides a simple way to gauge the danger. The first part involves counting the seconds between seeing a lightning flash and hearing the resulting thunder. If this time is 30 seconds or less, the lightning is close enough to pose an immediate threat, and all outdoor activity must stop immediately.
The second part of the rule dictates waiting at least 30 minutes after the last clap of thunder is heard before returning outside. This waiting period is necessary because a storm’s trailing edge can still produce dangerous strikes.
The safest places to seek refuge are fully enclosed, substantial buildings that have plumbing and electrical wiring for proper grounding. Hard-topped vehicles with the windows closed also offer protection, as the metal frame directs the electrical current around the occupants and into the ground. Conversely, seeking shelter under tall, isolated trees, small open-sided shelters, or near metal objects should be strictly avoided, as these locations significantly increase the risk of a strike.