Most hikers know that thunderstorms pose a serious threat. We’re taught to be wary of lightning strikes, and to get off high peaks and seek shelter when we hear the rumble of thunder. You may have been taught the old counting rule: Count the seconds between a flash of lightning and a rumble of thunder. Each second you count represents a fifth of a mile between you and the storm (15 seconds means the storm is 3 miles away, 5 seconds means the storm is a mile away, and so on). While this rule is a very good indicator of when to take cover, there can be exceptions. This is because lightning can travel much further than you might think, and you won’t always get fair warning.
“Lightning can travel ahead of and behind storms, sometimes at significant distances,” says Ryan Knapp, weather observer and meteorologist at New Hampshire’s Mt. Washington Observatory. Lighting can occur “out of the blue,” when a bolt travels away from a storm cloud and reaches the ground under fair skies. Instead of lightning striking directly down from a cloud, imagine a bolt traveling diagonally; you may be standing in sunshine with stormclouds off in the distance, but you could still be in the danger zone. Such strikes can travel 10 or 15 miles, and in rare cases, more than 25 miles from a stormcloud.
Plus, you might not always hear it coming. “Thunder is typically only heard 10 to 15 miles from the lightning causing it,” says Knapp. But certain conditions or locations can make it even harder to hear thunder: Hazy, humid conditions, thickly forested areas, or certain mountainous terrain can inhibit the sound from traveling even 10 miles, giving you less warning that a storm is on its way.
If storms are in the forecast, stay on your toes, even if the weather above you looks nice. When lighting strikes out of the blue, “you won’t be able to hear thunder, you will likely not be able to see the lightning, and you might not even see the anvil cloud tops—but you’re still in the [potential] strike zone,” says Knapp. While you may be unable to predict rogue lightning strikes, the principles of lightning safety still apply. Stay on your toes when storms may be nearby. Especially in the summer, take note of thunderstorm forecasts before heading out. Plan to be on summits and exposed ridgelines early in the day when storms are less common, and always have a plan to reach shelter in the event of lightning.
Want to learn more about reading the weather and staying safe? Check out our Wilderness Weather Fundamentals online course.