When lightning strikes a body of water, the danger is extremely high. Many people incorrectly assume that a direct hit is the only threat. In reality, the peril extends far beyond the immediate strike point, creating a widespread electrical field across the water’s surface that can be instantly lethal. Understanding how electrical energy moves through water is paramount to recognizing the immense danger posed by thunderstorms near lakes, rivers, or oceans.
How Lightning Energy Spreads Across Water
When cloud-to-ground lightning connects with the water’s surface, the electrical current does not immediately plunge deep into the depths. Instead, it travels primarily along the surface, a phenomenon known as the “skin effect.” The electricity spreads radially outward in all directions, creating a dangerous and rapidly expanding electrical field. This means the lethal energy remains concentrated in the top few inches of the water.
The primary mechanism for injury or death to anyone in the water is the creation of a “step potential” or “step voltage.” This occurs because the electrical voltage is highest at the strike point and drops off dramatically with distance. A person swimming or floating bridges the distance between two points of different electrical potential, causing the current to flow through the body. Even a short distance between a swimmer’s two hands or between the head and feet can provide a lethal pathway for the current.
This surface conduction creates a significant danger zone that can extend outward for dozens of yards. The electrical jolt within 20 to 30 meters of a strike is powerful enough to cause instant cardiac arrest, severe burns, or muscle contractions. Since the current is mostly contained on the surface, marine life like fish are often spared if they are swimming just a few feet below. For a person, being anywhere near the strike zone presents a severe risk.
The Role of Conductivity: Fresh vs. Saltwater Danger Zones
The specific danger zone is heavily influenced by the water’s conductivity, which differs greatly between freshwater and saltwater. Saltwater is a far better conductor of electricity due to its high concentration of dissolved ions. This high conductivity allows the lightning’s charge to dissipate more quickly, but it spreads the current over a much wider area.
In the ocean, the danger radius is generally larger, potentially extending the hazard to a greater number of people or boats over a vast surface area. Conversely, freshwater has much lower conductivity, making it a poorer electrical pathway. This resistance means the current does not spread out as readily, resulting in a more intense, highly concentrated electrical field near the strike point.
The highly localized intensity in freshwater can make the immediate strike area more violent, though the overall danger zone may be slightly smaller than in saltwater. In both environments, anyone on the surface is at risk, but the density and reach of the current are dictated by the water’s mineral content.
Essential Safety Rules When Near Bodies of Water
The moment thunder is heard, or lightning is seen, all water-based activities must cease immediately. The most important action is to exit the water and move away from the shoreline as quickly as possible.
A safe location is a substantial, fully enclosed building or an enclosed metal-topped vehicle with the windows rolled up. Small, open-sided structures like gazebos, picnic shelters, or dugouts offer no protection from lightning. If you are on a small craft or boat, you should immediately head to shore; if that is not possible, drop anchor, move to the lowest point of the vessel, and stay away from metal objects.
The “30/30 Rule” helps determine when to seek shelter and when it is safe to return. The first “30” suggests that if the time between seeing lightning and hearing thunder is 30 seconds or less, the storm is close enough to be dangerous, and you must seek shelter. The second “30” states that you should wait at least 30 minutes after the last sound of thunder before resuming any outdoor activities. Staying clear of all plumbing and corded electrical devices inside a shelter is also recommended, as metal pipes can conduct electricity.