Water provides a conductive path for electricity, making understanding the distance at which a storm becomes dangerous paramount to safety. Lightning is a powerful and unpredictable natural phenomenon. The recommended safe distance is determined not by sight, but by sound, using a simple, actionable rule.
Defining the Immediate Threat: The 30/30 Safety Rule
The “30/30 Rule” is the most widely accepted guideline for determining when a thunderstorm is too close. The first “30” refers to the flash-to-bang count, which is the time in seconds between seeing the lightning flash and hearing the resulting thunder. To estimate the storm’s distance in miles, count the seconds and divide that number by five.
If the time between the flash and the thunder is 30 seconds or less, the storm is within six miles and considered too close for safety. This 30-second count signals that the threat is immediate, as lightning can strike up to 10 miles from the main storm cell. If you can hear thunder, outdoor activity must cease at once.
Understanding Lightning’s Path Through Water and Ground
Water acts as an effective conductor for the massive electrical current in a lightning strike. Natural bodies of water, including oceans, lakes, and pools, contain impurities like salts and minerals that make them conductive. The electrical charge spreads rapidly across the surface and penetrates below, making anyone swimming or near the surface highly vulnerable to shock.
Saltwater vs. Freshwater
When lightning strikes water, the current disperses radially from the point of impact. In the ocean, high salt content makes the water extremely conductive, causing the current to spread out quickly and dissipate within 10 feet below the surface. Freshwater bodies, such as lakes and pools, are less conductive, meaning the current does not dissipate as quickly and can travel further.
Pool Hazards
The human body is an excellent conductor of electricity, increasing the danger in any water type. In swimming pools, metal components and plumbing systems can also provide a conductive path for the electrical charge. This risk exists even if the strike occurs some distance away from the pool itself.
Lightning energy can also travel through the ground surrounding the water, creating a phenomenon known as “step potential.” If a person is standing on the shore, the difference in electrical potential between their feet can cause a current to flow through the body. This danger extends beyond the water’s edge, meaning standing on the bank or beach does not make one completely safe.
Evacuation Protocol and Safe Return Times
Immediate evacuation is necessary as soon as the flash-to-bang count is 30 seconds or less, or if thunder is simply heard. Swimmers must exit the water and seek a safe, enclosed shelter. A safe structure is defined as a substantial building with plumbing or wiring that can help ground a strike, such as a home or office building.
Small, open structures do not offer adequate protection from a lightning strike. These include:
- Sheds
- Covered porches
- Gazebos
- Rain shelters
If a substantial building is unavailable, a fully enclosed, hard-topped vehicle is the next safest alternative. The metal frame of the vehicle can conduct the charge around the occupants, but occupants should avoid contact with any metal or electrical equipment inside.
The second “30” of the 30/30 Rule dictates the safe time to return to outdoor activity. No one should re-enter the water or resume swimming until 30 minutes have passed since the last observed flash of lightning or the last sound of thunder. This waiting period is necessary because thunderstorms can produce strikes from their trailing edges. Waiting the full 30 minutes minimizes the risk of a final, unexpected strike.