科學揭秘“暴風雨前的寧靜”
當我們描述在某個危險來臨或大事件爆發的前夕所出現的那種異樣的寧靜時，我們總會說“暴風雨前的寧靜”，這似乎已經是常識了。不過暴風雨前真的會有寧靜嗎?這個說法有沒有什么科學依據呢?
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!
你可曾有過這樣的經歷：某天下午正在后院燒烤或者打籃球的時候，突然發現一切瞬間變得安靜下來，空氣似乎凝結了，連鳥兒也不再歌唱而轉身躲回窩里去了?
幾分鐘后，你感覺到空氣中有所變化，忽然之間一排烏云出現在地平線上，讓人有種不祥的預感——烏云的樣子好像是在告訴你它們可不只是做做樣子而已。你急沖進屋里，險些被瓢潑大雨前掉下的大雨點砸個正著。此時，你也許會停下來問自己：“為什么暴風雨來臨前是那么寧靜安詳呢?”
人們在幾個世紀前就已經發現這個神秘的現象了，但究竟是什么原因造成暴風雨前的這番寧靜呢?古諺有云“暴風雨前的寧靜”，這通常用在比喻的修辭中，描述熱鬧的活動以及情緒亢奮前的一段安靜時間。根據自身的經驗，我們也知道事實上暴風雨來臨前也確實是寧靜的。但這是什么原因造成的呢?暴風雨前總是平靜么?讓我們來聽聽科學家們的解釋。
答案是：有時如此，有時則不然。在特定的情況下，暴風雨來臨前會出現令人恐懼的或是祥和的寂靜，降臨在你野餐的時候。其他種類的暴風雨則直接跳過寂靜的步驟，猶如在驕傲地宣布自己的存在一般，瞬間風雨大作。
平靜時期出現在一種特定的暴風雨中，即最簡單的暴風雨——單細胞雷暴。在這種類型的雷暴中，通常只有一種主要的上升氣流，即暖濕氣流從地面升起。溫暖濕潤的空氣是暴風雨形成的必要條件，所以通常會把它們從周圍的環境中吸進來。雷暴能夠從周圍任何一個方向吸進自己所需的暖濕空氣——甚至是處在雷暴運動方向上的空氣也被吸了進來。
由于暖濕空氣被吸進雷暴中，雷暴經過之處就形成了低氣壓真空帶。正在上升的暖濕氣流遇到雷暴云層中原本就存在的干燥冷空氣，此時熱空氣的溫度下降，里面的水分凝結成小的水滴——這是形成雨水的前提。這些水滴不斷聚集，包裹住一些大的顆粒(如灰塵)，當水滴凝聚到足夠大時就會形成降雨。
暖濕氣流持續上升，但當穿越云層時經歷氣溫下降水汽凝結的過程，暖濕氣流開始冷卻并變得干燥。當暖濕氣流達到云層頂部時，會被擠出云層，這股氣流會翻過巨大的鐵砧形雷雨云。此時，氣流開始下沉——被其自身上升時所形成的真空帶吸回低處。干熱的氣流相對穩定，一旦它覆蓋了某一個地區，就會使該區域的氣流穩定下來，這就造成了暴風雨前的平靜。
盡管如此，大多數的雷暴來臨前并不是一片寧靜。這是因為它們大多是風向復雜的暴風雨團。在暴風雨團的周圍，有很多氣流頻繁上下流動，這就使得這種類型的暴風雨來臨前毫無平靜可言。相反，這種類型的暴風雨來臨前往往會狂風大作!
當我們描述在某個危險來臨或大事件爆發的前夕所出現的那種異樣的寧靜時，我們總會說“暴風雨前的寧靜”，這似乎已經是常識了。不過暴風雨前真的會有寧靜嗎?這個說法有沒有什么科學依據呢?
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!
你可曾有過這樣的經歷：某天下午正在后院燒烤或者打籃球的時候，突然發現一切瞬間變得安靜下來，空氣似乎凝結了，連鳥兒也不再歌唱而轉身躲回窩里去了?
幾分鐘后，你感覺到空氣中有所變化，忽然之間一排烏云出現在地平線上，讓人有種不祥的預感——烏云的樣子好像是在告訴你它們可不只是做做樣子而已。你急沖進屋里，險些被瓢潑大雨前掉下的大雨點砸個正著。此時，你也許會停下來問自己：“為什么暴風雨來臨前是那么寧靜安詳呢?”
人們在幾個世紀前就已經發現這個神秘的現象了，但究竟是什么原因造成暴風雨前的這番寧靜呢?古諺有云“暴風雨前的寧靜”，這通常用在比喻的修辭中，描述熱鬧的活動以及情緒亢奮前的一段安靜時間。根據自身的經驗，我們也知道事實上暴風雨來臨前也確實是寧靜的。但這是什么原因造成的呢?暴風雨前總是平靜么?讓我們來聽聽科學家們的解釋。
答案是：有時如此，有時則不然。在特定的情況下，暴風雨來臨前會出現令人恐懼的或是祥和的寂靜，降臨在你野餐的時候。其他種類的暴風雨則直接跳過寂靜的步驟，猶如在驕傲地宣布自己的存在一般，瞬間風雨大作。
平靜時期出現在一種特定的暴風雨中，即最簡單的暴風雨——單細胞雷暴。在這種類型的雷暴中，通常只有一種主要的上升氣流，即暖濕氣流從地面升起。溫暖濕潤的空氣是暴風雨形成的必要條件，所以通常會把它們從周圍的環境中吸進來。雷暴能夠從周圍任何一個方向吸進自己所需的暖濕空氣——甚至是處在雷暴運動方向上的空氣也被吸了進來。
由于暖濕空氣被吸進雷暴中，雷暴經過之處就形成了低氣壓真空帶。正在上升的暖濕氣流遇到雷暴云層中原本就存在的干燥冷空氣，此時熱空氣的溫度下降，里面的水分凝結成小的水滴——這是形成雨水的前提。這些水滴不斷聚集，包裹住一些大的顆粒(如灰塵)，當水滴凝聚到足夠大時就會形成降雨。
暖濕氣流持續上升，但當穿越云層時經歷氣溫下降水汽凝結的過程，暖濕氣流開始冷卻并變得干燥。當暖濕氣流達到云層頂部時，會被擠出云層，這股氣流會翻過巨大的鐵砧形雷雨云。此時，氣流開始下沉——被其自身上升時所形成的真空帶吸回低處。干熱的氣流相對穩定，一旦它覆蓋了某一個地區，就會使該區域的氣流穩定下來，這就造成了暴風雨前的平靜。
盡管如此，大多數的雷暴來臨前并不是一片寧靜。這是因為它們大多是風向復雜的暴風雨團。在暴風雨團的周圍，有很多氣流頻繁上下流動，這就使得這種類型的暴風雨來臨前毫無平靜可言。相反，這種類型的暴風雨來臨前往往會狂風大作!
