科学揭秘“暴风雨前的宁静”

当我们描述在某个危险来临或大事件爆发的前夕所出现的那种异样的宁静时，我们总会说“暴风雨前的宁静”，这似乎已经是常识了。不过暴风雨前真的会有宁静吗?这个说法有没有什么科学依据呢?

Have you ever spent an afternoon in the backyard, maybe grilling or enjoying a basketball game, when suddenly you notice that everything goes quiet? The air seems still and calm—even the birds stop singing and quickly return to their nests.

After a few minutes, you feel a change in the air, and suddenly a line of clouds ominously appears on the horizon—clouds with a look that tells you they aren’t fooling around. You quickly dash in the house and narrowly miss the first fat raindrops that fall right before the downpour. At this moment, you might stop and ask yourself, “Why was it so calm and peaceful right before the storm hit?”

It’s an intriguing phenomenon that people have recognized for centuries, but what on earth causes this calm? And there is an old phrase “calm before the storm”, often used in its figurative meaning—a quiet period just before a great activity or excitement. According to our own experience, we know there is literally calm before the storm. But what causes this calm? And is it always calm before the storm? Let’s hear what scientists have to say.

The answer is sometimes there is; sometimes there isn’t. Under the right conditions, an eerie or peaceful calm will befall your picnic before a storm moves in. Other storms skip the calm and proudly announce their presence by instantly unleashing bad weather.

A period of calm happens in a particular kind of storm, the simplest kind of storm—a single-cell thunderstorm[1]. In this type of thunderstorm, there is usually only one main updraft, which is warm, humid air and drawn from places near the ground. Storms need warm and moist air as fuel, so they typically draw that air in from surrounding environment. Storms can draw in the air that fit their need from all directions—even from the direction in which the storm is traveling.

As the warm, moist air is pulled into a storm system, it leaves a low-pressure vacuum[2] in its wake. The rising air meets the cold dry air that has already existed in the storm clouds, thus the temperature of the warm air drops, and the water vapor in it condenses[3] into tiny droplets that are a precondition of rain. These droplets clump up and build on larger particles like dust, until they grow large enough to form raindrops.

This warm, moist air keeps moving upwards, but it becomes colder and drier during its trip through cloud which involves cooling and condensation. When it reaches the top of the cloud mass, the air gets spit out at the top. This air is sent rolling out over the big, anvil[4]-shaped head of the thunderclouds. From there, the air descends—drawn back toward lower altitudes by the very vacuum its departure created in the first place. Warm and dry air is relatively stable, and once it blankets a region, it stabilizes that air in turn. This causes the calm before a storm.

Most thunderstorms, though, don’t start with a calm. That’s because most are actually clusters of storms with complex wind patterns. There’s so much air moving up and down in the vicinity[5] of these storm clusters that the calm before the storm never happens. And instead, before the storm, it might be really windy!

你可曾有过这样的经历：某天下午正在后院烧烤或者打篮球的时候，突然发现一切瞬间变得安静下来，空气似乎凝结了，连鸟儿也不再歌唱而转身躲回窝里去了?

几分钟后，你感觉到空气中有所变化，忽然之间一排乌云出现在地平线上，让人有种不祥的预感——乌云的样子好像是在告诉你它们可不只是做做样子而已。你急冲进屋里，险些被瓢泼大雨前掉下的大雨点砸个正着。此时，你也许会停下来问自己：“为什么暴风雨来临前是那么宁静安详呢?”

人们在几个世纪前就已经发现这个神秘的现象了，但究竟是什么原因造成暴风雨前的这番宁静呢?古谚有云“暴风雨前的宁静”，这通常用在比喻的修辞中，描述热闹的活动以及情绪亢奋前的一段安静时间。根据自身的经验，我们也知道事实上暴风雨来临前也确实是宁静的。但这是什么原因造成的呢?暴风雨前总是平静么?让我们来听听科学家们的解释。

答案是：有时如此，有时则不然。在特定的情况下，暴风雨来临前会出现令人恐惧的或是祥和的寂静，降临在你野餐的时候。其他种类的暴风雨则直接跳过寂静的步骤，犹如在骄傲地宣布自己的存在一般，瞬间风雨大作。

平静时期出现在一种特定的暴风雨中，即最简单的暴风雨——单细胞雷暴。在这种类型的雷暴中，通常只有一种主要的上升气流，即暖湿气流从地面升起。温暖湿润的空气是暴风雨形成的必要条件，所以通常会把它们从周围的环境中吸进来。雷暴能够从周围任何一个方向吸进自己所需的暖湿空气——甚至是处在雷暴运动方向上的空气也被吸了进来。

由于暖湿空气被吸进雷暴中，雷暴经过之处就形成了低气压真空带。正在上升的暖湿气流遇到雷暴云层中原本就存在的干燥冷空气，此时热空气的温度下降，里面的水分凝结成小的水滴——这是形成雨水的前提。这些水滴不断聚集，包裹住一些大的颗粒(如灰尘)，当水滴凝聚到足够大时就会形成降雨。

暖湿气流持续上升，但当穿越云层时经历气温下降水汽凝结的过程，暖湿气流开始冷却并变得干燥。当暖湿气流达到云层顶部时，会被挤出云层，这股气流会翻过巨大的铁砧形雷雨云。此时，气流开始下沉——被其自身上升时所形成的真空带吸回低处。干热的气流相对稳定，一旦它覆盖了某一个地区，就会使该区域的气流稳定下来，这就造成了暴风雨前的平静。

尽管如此，大多数的雷暴来临前并不是一片宁静。这是因为它们大多是风向复杂的暴风雨团。在暴风雨团的周围，有很多气流频繁上下流动，这就使得这种类型的暴风雨来临前毫无平静可言。相反，这种类型的暴风雨来临前往往会狂风大作!