Severe Thunderstorm
强雷暴
Severe thunderstorms typically produce weather events that cover a wide range of size scales, from a few hundred kilometers down to just a few kilometers, or even smaller. This is because thunderstorms can occur as isolated events or in groups. In the United States, a thunderstorm-related event is considered severe when the wind gusts equal or exceed 25 m/s, or the hailstone diameters exceed 2 cm, or if a tornado is produced.
严重的雷暴通常会产生涵盖广泛尺度范围的天气事件，从几百公里到仅几公里，甚至更小。这是因为雷暴可以作为孤立事件发生，也可以成组出现。在美国，当雷暴相关事件中出现风速等于或超过25米/秒、冰雹直径超过2厘米，或形成龙卷风时，即被视为严重雷暴事件。
A thunderstorm is composed of one or more cells, where a cell is the basic building block of a thunderstorm. Cells, in turn, are viewed as being made up of one group of air parcels being driven upward by positive buoyancy and another being driven downward by negative buoyancy and the presence of precipitation in the air. Positive buoyancy arises in updrafts by the release of latent heat during the condensation of water vapor. This heat release acts like the burner of a hot air balloon, reducing the density of the air in which condensation is occurring and there by causing the air to rise. As the process continues, precipitation can be formed in the updraft. This precipitation can produce downdrafts simply by its accumulating weight dragging downward on the surrounding air. Moreover, when precipitation falls into relatively dry air surrounding a developing storm, the evaporation of that precipitation chills the air; evaporation absorbs latent heat from the air in the same way that condensation releases that heat.
雷暴由一个或多个单元组成，其中单元是雷暴的基本构建块。单元可被视为由两个空气团组成，其中一组空气团由于正浮力而向上移动，另一组则由于负浮力和空气中降水的存在而向下移动。正浮力在上升气流中通过水蒸气凝结过程中释放的潜热产生。这种热释放的作用类似于热气球的燃烧器，降低了凝结发生区域内空气的密度，从而使空气上升。随着这一过程的持续，降水可能会在上升气流中形成。这种降水可以通过其积累的重量向下拉动周围空气，从而产生下沉气流。此外，当降水落入雷暴周围的相对干燥空气中时，降水的蒸发会冷却空气；蒸发会像凝结释放热量一样，从空气中吸收潜热。
When downdrafts caused by thunderstorms reach the surface, they are forced to spread out. This creates an outflow at the surface (often called a downburst), with the outflow winds sometimes reaching the criterion for calling the thunderstorm severe. On some occasions, these outflow winds can exceed 40 m/s.
当雷暴引起的下沉气流到达地面时，它们被迫扩散。这在地面上形成了一个外流（通常称为下击暴风），外流风速有时达到雷暴严重的标准。在某些情况下，这些外流风速可以超过40米/秒。
Under the right circumstances, notably when the updraft is particularly strong, the possibility of hail formation arises. Hailstones develop in the part of the storm where supercooled water and ice crystals are both present; liquid water is said to be supercooled when its temperature is below the melting point (0℃) and the water is not yet frozen. Hailstones can become quite large, exceeding 5cm diameters at times, and be capable of penetrating roofs, shattering windows, and even creating human casualties. Even small hail can cause crop damage, of course.
在适当的情况下，特别是当上升气流特别强烈时，就会出现冰雹形成的可能性。冰雹在暴风雨中超冷水和冰晶同时存在的部分形成；当水的温度低于冰点（0℃）但尚未冻结时，称为超冷水。冰雹有时可以变得相当大，直径超过5厘米，并能够穿透屋顶、打碎窗户，甚至造成人员伤亡。当然，即使是小冰雹也会造成作物损失。
Occasionally, tornadoes form in association with severe thunderstorms. Tornadoes are intense low-pressure vortices that can produce the strongest winds of any storm: at their highest intensity, tornadic wind speeds can approach 140m·s-1. Most tornadoes, however, are not that intense. Tornadoes over bodies of water are called waterspouts. Tornadoes are created in thunderstorms when pre-tornadic, relatively weak circulations are intensified through conservation of angular momentum.
偶尔，龙卷风会与强烈的雷暴有关。龙卷风是强烈的低压涡旋，可以产生任何风暴中最强的风：在其最高强度下，龙卷风的风速可以接近140米每秒。然而，大多数龙卷风并没有那么强烈。水体上的龙卷风被称为水龙卷。龙卷风在雷暴中形成，当龙卷风前的相对较弱的环流通过角动量守恒被加强时。
Isolated events
孤立事件
The most intense form of thunderstorm is the so-called supercell thunderstorm which typically is isolated from surrounding storms. Supercells are rotating thunderstorms that develop their rotation by tapping the vertical wind shear in the storm environment. The vast majority of supercells produce some sort of severe weather: hail, damaging straight-line winds, and/or tornadoes; only about 20% of them are tornadic. The most violent severe weather of all types is almost always associated with supercells, including the majority of strong and violent tornadoes and giant hailstones (exceeding 5 cm in diameter).
最强烈的雷暴形式是所谓的超级单体雷暴，它通常与周围的雷暴隔离。超级单体是旋转的雷暴，通过利用雷暴环境中的垂直风切变来发展其旋转。绝大多数超级单体都会产生某种严重天气：冰雹、破坏性的直线风和/或龙卷风；只有约20%的超级单体是龙卷风。几乎所有类型中最强烈的严重天气几乎总是与超级单体相关，包括大多数强大和暴力的龙卷风以及直径超过5厘米的巨型冰雹。
Whereas the typical thunderstorm cell has a lifetime of about 20-30minutes, supercells can persist for many hours. This means that all forms of severe weather from supercells can be prolonged, sometimes leaving long, wide swaths of damage. The organized nature of a supercell, associated with its overall rotation, means that supercells produce a disproportionate share of the damage associated with thunderstorms. Perhaps only about 10% of all thunderstorms are supercells, but they are responsible for the majority of thunderstorm damage in areas where they occur.
典型的雷暴单体寿命大约为20-30分钟，而超级单体可以持续数小时。这意味着来自超级单体的各种严重天气现象可以被延长，有时会留下长而宽的破坏带。超级单体的组织性质与其整体旋转相关，这意味着超级单体造成的破坏占雷暴总破坏的比例过高。或许所有雷暴中只有约10%是超级单体，但它们在发生的地区占雷暴破坏的多数。
Apart from supercells, isolated thunderstorms usually are non-severe and typically do not last very long. On rare occasions, isolated thunderstorms can produce a brief "pulse" of severe weather, usually hail or winds that are only marginally severe.
除了超级单体雷暴，孤立的雷暴通常是非严重的，通常持续时间不长。在少数情况下，孤立的雷暴可能会产生短暂的"脉冲"严重天气，通常是冰雹或仅有边际严重的风。
Aggregations of thunderstorm
雷暴聚集
Thunderstorms do not typically occur as isolated events. Instead, they tend to form in groups, in either lines or clusters of individual cells. The most common grouping is in lines, sometimes referred to as squall lines. When thunderstorm cells form in aggregations, then the collection of storms can live for a much longer time than the individual cells (which usually retain their 20-30 minute life cycles). This means that the hail and wind events produced by such groupings of thunderstorms are intermittent, rather than prolonged (as with supercells), as cells form and decay. Severe weather still can go on in such cases for many hours in this intermittent fashion. The interactions between individual cells in lines and clusters of thunderstorm cells are often complicated and hard to predict, but those interactions are responsible for severe weather.
雷暴通常不会作为孤立事件发生。相反，它们往往成群出现，呈现为线状或单个细胞的簇。最常见的分组方式是线状，有时称为阵风线。当雷暴细胞以聚集的方式形成时，这些风暴的集合可以比单个细胞（通常保持20-30分钟的生命周期）持续更长的时间。这意味着由这种雷暴分组产生的冰雹和风事件是间歇性的，而不是持续的（如超级单体），因为细胞会形成和衰退。在这种情况下，严重天气仍然可以以这种间歇的方式持续数小时。线状和雷暴细胞簇中单个细胞之间的相互作用通常复杂且难以预测，但这些相互作用是导致严重天气的原因。
A particularly dangerous form of thunderstorm aggregation arises when new cells are constantly forming in one place, and tracking over the same region repeatedly, a situation called "training" because the cells are like cars in a train. This means that a particular area experiences rainfall from a succession of thunderstorm cells, which can result in extremely heavy rainfall. This is the process associated with the majority of flash flood events, worldwide. In the United States, heavy rainfall is not considered to be a criterion for what is officially considered to be "severe" in spite of the importance of such rainfall in flooding events. On the other hand, many other nations around the world consider heavy rainfall to be an important form of severe storm.
一种特别危险的雷暴聚集形式出现在新的雷暴单元不断在一个地方形成，并反复经过同一区域的情况下，这种情况被称为"训练"，因为这些单元就像火车上的车厢。这意味着特定区域会经历一系列雷暴单元带来的降雨，这可能导致极其强烈的降雨。这是与全球大多数闪电洪水事件相关的过程。在美国，尽管这种降雨在洪水事件中非常重要，但重降雨并不被视为官方认定的"严重"标准。另一方面，世界上许多其他国家将重降雨视为一种重要的严重风暴形式。