When a thunderstorm approaches, the danger lightning poses to anyone in or near water—whether a pool, lake, or ocean—is severe. The sheer power of a lightning strike, which can carry hundreds of millions of volts, makes any body of water a high-risk location. Water’s ability to conduct electricity means that a strike nearby, not just a direct hit, can be deadly to swimmers. Clear, actionable safety rules are necessary to know precisely when to evacuate and when it is safe to return to the water.
Determining the Evacuation Zone
A simple and effective method for gauging the proximity of a thunderstorm is the “Flash-to-Bang” technique. This involves counting the seconds between seeing a flash of lightning and hearing the subsequent clap of thunder. This count estimates the storm’s distance because sound travels through the air much slower than light.
To perform the calculation, every five seconds counted represents approximately one mile of distance from the strike location. For example, a count of 15 seconds means the lightning struck about three miles away. Safety officials recommend that all water activities cease immediately and the area be evacuated if the time between the flash and the bang is 30 seconds or less.
A 30-second count corresponds to a distance of roughly six miles, which is the accepted minimum distance for a safe evacuation threshold. If the storm is closer than this six-mile radius, the risk of a strike is too high to remain in the water or near its edge. This measurement signals that immediate action is necessary to move to a fully enclosed, substantial shelter.
How Lightning Interacts with Water
The danger in the water stems from electricity’s tendency to spread out across the surface when it strikes. Lightning current travels horizontally along the water’s surface, making a wide area hazardous for anyone swimming or wading. This horizontal spreading means a direct hit is not required to cause serious injury or death.
The composition of the water influences the danger zone. Freshwater, like that found in lakes and pools, is a relatively poor conductor compared to saltwater. Paradoxically, this lower conductivity means the electrical energy does not dissipate as quickly, potentially creating a higher voltage gradient across the surface. This effect can make the immediate vicinity of a strike in a freshwater body dangerous.
In pools, the danger is amplified by the presence of metal pipes, plumbing, and electrical wiring connected to the water. A lightning strike to the ground anywhere on this metallic network can transmit a lethal charge to the water and surrounding pool deck structures. A particularly hazardous phenomenon is step potential, which is the voltage difference between a person’s two feet, causing current to flow through the body.
The Post-Storm Waiting Protocol
Once a storm has passed, a specific waiting period is required before re-entering the water. This widely accepted safety guideline dictates waiting a minimum of 30 minutes after the last sign of storm activity. The 30-minute rule is a safeguard because electrical charges and associated lightning activity can linger in storm clouds even after the heavy rain and thunder have stopped.
Thunderstorms can be highly mobile, and a storm that seems to be moving away can quickly change direction or regenerate. The danger persists even if the sky appears clear or the sun is out. To maintain safety, the 30-minute clock must be reset every time a new flash of lightning is seen or a clap of thunder is heard.
This conservative waiting period ensures the storm system has truly moved out of the six-mile danger radius and that the likelihood of a residual strike is minimized. Rushing back into the water prematurely voids the safety provided by the initial evacuation. Safe re-entry is only possible after the full 30-minute period has elapsed without any further evidence of the storm.