是什么导致夏天冰雹？

Hail, the irregularly shaped lumps of ice that tumble out of the sky during thunderstorms, is a puzzling precipitation type. It's made of ice and common during the spring and summer months, yet it resembles winter's sleet and graupel. The explanation of how this is possible lies overhead: Although outdoor temperatures might be 70, 80, or 90 degrees F outside your door, tens of thousands of feet aloft, temperatures are typically freezing, 32 degrees F and below.

Although most convective thunderstorms produce hail, not all thunderstorms drop hailstones on the ground, according to NOAA’s National Severe Storms Laboratory (NSSL).尽管如此，在过去的十年中，美国每年造成的冰雹每年造成8至140亿美元的财产损失。

How Does Hail Form?

Hail is born deep inside the belly of towering cumulonimbus clouds, which can extend 40,000 to 60,000 feet into the atmosphere. (To get a feeling for how high that is, most commercial airliners cruise at altitudes of31,000 to 38,000 feet。。但是，他们的中区通常是发现冰点水平的地方。雷暴内部的上升气流可以将雨滴吹入冰点区域，从而导致它们变成冰晶。然后，这些冰种子与邻近的冰晶和过冷的云液滴相撞，将其冻结到其表面上，从而生长成冰雹。

随着每次碰撞发生在云层的冰点水平上方，在迷你冰雹中添加了新的冰涂层，从而扩大了其尺寸。如果温度接近冰点标记，则水会在生长的冰雹周围缓慢冻结。这使空气气泡逃脱，并一层透明的冰结果。但是，如果环境beplay体育官网电脑是亚冻结的，那么超冷的水滴几乎立即冻结在生长的冰雹上，将气泡捕获到位并产生多云的冰。（如果您曾经仔细观察冰雹并看到类似洋葱层的条纹，这就是原因。）

Lift a hailstone up too high—to the top levels of a thunderstorm where cloud temperatures can easily measure around minus 60 degrees F, for example—and it won't grow. That’s because at temperatures that cold, all liquid water, even supercooled water, will have frozen into ice. And hail needs liquid water or a water-ice blend in order to aggregate.

A hailstone’s collision-accumulation cycle can repeat multiple times, but typically not for longer than 30 minutes, since thunderstorms typically don’t live much longer than this.

At What Speed Does Hail Fall?

一旦冰雹的质量变得太重，无法提升，重力就胜出了，冰块的一块掉落了。

How fast a hailstone falls varies depending on the hailstone’s size and shape, the frictional force between it and the surrounding air, the degree of melting during its journey, and local wind conditions. According to NSSL, hail’s terminal fall speed (the maximum speed it can reach before gravity’s acceleration balances air resistance) ranges from about 10 mph for very small hailstones to 100 mph for baseball-size and larger hailstones.

What Determines Hailstone Size?

冰雹的尺寸最终取决于其父母雷暴的上升强度。上升气流越强，冰雹的时间越长，将悬浮在风暴云中，从而经历多次碰撞，从而增长。

According to the National Weather Service, updraft speeds of around 24 mph are needed to sustain even some of Mother Nature’s smallest hailstones, such as pea-size hail. As for the 8-inch diameter, 1.93-pound hailstone that fell in Vivian, South Dakota, in June 2010, and ranks as the widest and heaviest hailstone in the United States, meteorologists estimate it was supported by a 160 to 180 mph-strong updraft.

熔化在确定冰雹撞击一旦将是什么尺寸的角色发挥作用。一旦冰雹降至云的冰点水平以下（每年的云，每年的时间和地理位置）就会开始融化。According to the National Weather Service office in Louisville, Kentucky, if a hailstone falls through a layer of warm air that's 11,000 feet or thicker, it typically won’t survive its journey to the ground and will instead arrive at the surface as what it began: a raindrop.