21 Thunderstorms

Thunderstorms are one of nature's most common phenomena, events that just about everyone has experienced. Since they are also potentially one of nature's most deadly occurrences, it is a good idea to recognize the conditions that cause them to develop.

For a thunderstorm to form, the atmosphere in an area must have become unstable, usually because of heat. The developing storm on the ground will be warm or because of a sudden influx of cold air. The first result of this instability is that warm air will ascend and cool air will descend. As the warm air goes up, the water vapor within it condenses, and becomes larger and larger, until an enormous cumulonimbus cloud—the kind that generates the thunderstorm—is produced.

Soon the rain begins. When it does, the downdrafting air that it mixes with the updrafting warm air. One result of the turbulence this causes is that the air within the storm—sometimes even the air a few miles away—takes on an electric charge. Conditions are now ripe for lightning to develop. The more charged up the air, the more lightning there will be.

How can you determine how close to you a flash of lightning has struck? The amount of time it takes for a thunderclap to follow is thought to be a good indicator. Where you see the lightning begin counting slowly. If you can count to five before you hear thunder, the lightning probably struck about five miles away. If you can't even count to three, take cover immediately.

21 雷暴

雷暴是自然界最常见的现象之一，几乎每个人都经历过。由于它们也可能是自然界最致命的事件之一，识别引发雷暴的条件是个不错的主意。

雷暴形成的前提是某一区域的大气变得不稳定，通常是因为热量。地面上正在发展的风暴区域会变暖，或是因为冷空气的突然涌入。这种不稳定的第一个结果是暖空气上升、冷空气下沉。随着暖空气上升，其中的水蒸气凝结，体积越来越大，直到形成巨大的积雨云——也就是产生雷暴的云。

很快开始下雨。降雨时，下沉气流与上升的暖空气混合。这种湍流产生的一个结果是，风暴内部的空气（有时甚至是几英里外的空气）会带上电荷。此时的条件已适合闪电形成。空气的电荷越多，闪电就会越多。

如何判断闪电击中的位置离你有多近？雷声滞后的时间被认为是一个不错的指标。看到闪电时开始慢慢数数。如果数到五才听到雷声，闪电可能击中了约五英里外的地方。如果连数到三都做不到，要立即寻找遮蔽处。