Most people know that you shouldn't stand under a large tree during a thunderstorm. Searching online for similar questions often yields answers like "stay away from isolated large trees" or "the farther away from trees, the better," as these types of accidents are most common. However, these statements are not entirely accurate and are somewhat one-sided.

In open areas, isolated large trees act like lightning rods, making them the most vulnerable to lightning strikes. If lightning strikes a large tree while someone is standing beneath it, it can cause side flashover and step voltage damage, so it's a good idea to stay away from large trees during thunderstorms.

But how far away from trees should one stay in open areas? Is farther the better? Actually, not really. The protection range of a lightning rod is limited. If a person stands too far from a tree, they are outside the protective range of the tree, acting as a "lightning rod," and could be struck directly by lightning (the human body becomes the "lightning rod"). In reality, accidents involving direct lightning strikes on farmland are common, such as fatal lightning strikes on farmland where workers have been directly struck by lightning. There are two main reasons for this: 1) Farmland contains water, which is a relatively good conductor of electricity. 2) People are usually at the highest point in farmland.

So how far away should we be from the tree? We can use the rolling ball method to calculate. Suppose we are in an open field, the tree is 10 meters tall, the rolling ball has a radius of 30 meters, and the person is 2 meters tall.

According to the model calculation, a person with a height of 2 meters is out of the protection range of the tree when the distance from the tree exceeds 22.4-10.8=11.6 meters (the green diagonal part in the figure).

We also need to maintain a safe distance from trees. Some say at least 2 meters, others 3 or 5 meters. Here, we'll take the most conservative 5 meters. (In practice, we need to consider not only the trunk but also the branches. If the distance from the trunk is large enough, and a person's head is very close to the overhanging branches, it can still be dangerous. The model uses the distance from the trunk for simplified calculations.) Therefore, in this model, the safe distance between a person and a tree is 5-11.6 meters.