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 calmeven the birds stop singing and quickly return to their nests.你可曾有过这样的经历：某天下午正在后院小食或者打篮球的时候，忽然找到一切瞬间显得安静下来，空气或许凝固了，连鸟儿也仍然歌唱而上前躲回窝里去了？After a few minutes, you feel a change in the air, and suddenly a line of clouds ominously appears on the horizonclouds with a look that tells you they arent 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?几分钟后，你感觉到空气中有所变化，突然之间一排乌云经常出现在地平线上，让人有种不祥的预感乌云的样子样子是在告诉他你它们可不只是做做样子而已。你急冲入屋里，差点被瓢泼大雨前丢弃下的大雨点扔个正着。此时，你或许不会停下问自己：为什么暴风雨到来前是那么宁静安静呢？Its 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 meaninga 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? Lets hear what scientists have to say.人们在几个世纪前就早已找到这个谜样的现象了，但到底是什么原因导致暴风雨前的这番宁静呢？古谚有云暴风雨前的宁静，这一般来说用在比喻的思辨中，叙述繁华的活动以及情绪烦躁前的一段安静时间。

根据自身的经验，我们也告诉事实上暴风雨到来前也显然是宁静的。但这是什么原因导致的呢？暴风雨前总是安静么？让我们来讲出科学家们的说明。The answer is sometimes there is; sometimes there isnt. 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.答案是：有时如此，有时则不然。在特定的情况下，暴风雨到来前会经常出现令人不安的或是祥和的宁静，复活在你野餐的时候。

其他种类的暴风雨则必要跳过宁静的步骤，有如在骄傲地宣告自己的不存在一般，瞬间风雨大作。A period of calm happens in a particular kind of storm, the simplest kind of storma single-cell thunderstorm . 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 directionseven from the direction in which the storm is traveling.安静时期经常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，一般来说只有一种主要的下降气流，即暖湿气流从地面升起。

寒冷湿润的空气是暴风雨构成的必要条件，所以一般来说不会把它们从周围的环境中吸进来。雷暴需要从周围任何一个方向拉入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食了进去。As the warm, moist air is pulled into a storm system, it leaves a low-pressure vacuum 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 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.由于暖湿空气被拉入雷暴中，雷暴经过之处就构成了低气压真空带上。正在下降的暖湿气流遇上雷暴云层中原本就不存在的潮湿冷空气，此时热空气的温度上升，里面的水分凝固成小的水滴这是构成雨水的前提。

这些水滴大大挤满，包覆寄居一些大的颗粒（如灰尘），当水滴汇聚到充足大时就不会构成降雨。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 shaped head of the thunderclouds. From there, the air descendsdrawn 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.暖湿气流持续上升，但当穿过云层时经历气温上升水汽凝固的过程，暖湿气流开始加热并显得潮湿。

当暖湿气流超过云层顶部时，不会被吸管云层，这股气流不会翻越极大的铁砧形雷雨云。此时，气流开始沉降被其自身上升时所构成的真空带上吸回较低处。干燥的气流比较平稳，一旦它覆盖面积了某一个地区，就不会使该区域的气流稳定下来，这就造成了暴风雨前的安静。

Most thunderstorms, though, dont start with a calm. Thats because most are actually clusters of storms with complex wind patterns. Theres so much air moving up and down in the vicinity of these storm clusters that the calm before the storm never happens. And instead, before the storm, it might be really windy!尽管如此，大多数的雷暴到来前并不是一片宁静。这是因为它们大多是风向简单的暴风雨团。在暴风雨团的周围，有很多气流频密上下流动，这就使得这种类型的暴风雨到来前没什么安静可言。

忽略，这种类型的暴风雨到来前往往不会狂风大作。

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