No, it is not safe to swim when there is thunder. Hearing thunder means a thunderstorm is close enough for lightning to strike your location, making all water activities extremely dangerous. Lightning seeks the path of least resistance, and the large, flat surface of any body of water, whether a pool or an ocean, becomes a prime target. The risk is not limited to a direct strike, as the electrical current spreads rapidly through the water and surrounding ground.
Understanding Lightning and Water Conductivity
Lightning strikes release an immense surge of electrical energy, averaging around 30,000 amperes and 300 million volts. While pure water is a poor conductor of electricity, natural water sources like lakes, oceans, and swimming pools are highly conductive due to dissolved salts, minerals, and other impurities. This conductivity allows the massive electrical charge from a lightning strike to travel outwards from the point of impact.
When lightning strikes water, the electrical current does not penetrate deeply but disperses horizontally across the surface. This lateral conduction means a swimmer does not need a direct strike to be severely injured or killed. The electrical charge can travel 20 to over 100 meters away from the strike point, creating a wide danger zone.
A significant hazard on land and near water is ground current, which generates a phenomenon called step voltage. Step voltage occurs when the electrical current spreads through the ground, creating a potential difference between two points on the surface. If a person is standing with their feet apart, this voltage difference can drive a lethal current up one leg and down the other, causing severe injury or cardiac arrest.
Assessing Risk in Different Aquatic Settings
The risk profile changes based on the aquatic environment, but the fundamental danger of electrical conduction remains constant across all settings. In open water, such as oceans and lakes, a swimmer’s head often becomes the highest point on the surface, increasing the likelihood of a lightning strike. Even without a direct hit, the current spreads quickly across the expansive surface, which is particularly conductive in the ocean due to high salt content.
Swimming pools present unique risks because of their connection to plumbing and electrical systems. While a direct strike is devastating, an indirect strike to a nearby tree, fence, or pump house can still conduct electricity into the water through metal pipes, filtration systems, or wet concrete decks. Even indoor pools are not automatically safe, as lightning can travel through the metal piping and electrical conduits connected to the water system.
This principle of conduction means that using indoor plumbing can also be hazardous during a thunderstorm. Lightning can strike a house or the ground nearby and travel through metal water pipes, posing a risk to anyone taking a shower or bath. Therefore, it is advisable to avoid all water sources, both indoor and outdoor, during an active thunderstorm.
Essential Evacuation and Safety Guidelines
Immediate evacuation from the water is the only safe response the moment thunder is heard or lightning is seen. Safety organizations recommend following the “30/30 Rule” for managing thunderstorm risk. The first “30” refers to the flash-to-bang time: if the time between seeing lightning and hearing the thunder is 30 seconds or less, the storm is approximately six miles away or closer, and the area must be evacuated immediately.
Once a storm is detected, all individuals must seek safe, enclosed shelter that is fully substantial, such as a large building or a fully enclosed, metal-topped vehicle. Open-sided structures, like picnic shelters, dugouts, or porches, do not offer adequate protection from lightning. Trees should also be avoided, as they are often struck and can transfer ground current.
The second “30” of the rule dictates that no one should return to the water or resume outdoor activity until 30 minutes have passed since the last visible flash of lightning or clap of thunder. Lightning can strike many miles from the center of a storm, often called a “bolt from the blue,” making it necessary to wait until the storm has completely cleared the area.