為什么天空有時會變成紫色?

為什么天空有時會變成紫色?

A blue sky is a good sky. It's a reassuring sight that promises clear weather and bright days.

藍色的天空是一個好天氣。這是令人安心的景象，預示著晴朗的天氣和晴朗的日子。

When you see purple skies, it's because light has scattered in a different way than usual. (Photo: JhonnierAndrey/Wikimedia Commons)

But what in the world does a purple sky mean? It's not uncommon to spot purple skies during sunsets or sunrises. Sometimes they even appear after hurricanes, as they did in Florida following Hurricane Michael.

但是紫色的天空到底是什么意思呢?在日落或日出時看到紫色的天空并不罕見。有時，它們甚至會在颶風過后出現(xiàn)，就像在颶風邁克爾之后的佛羅里達州一樣。

But what causes them?

是什么引起的呢?

Seeing Blue

藍色的天空

That's certainly a good view. (Photo: Juanedc/Wikimedia Commons)

To understand why the sky is sometimes purple, it's helpful to first understand why the sky is blue.

要理解為什么天空有時是紫色的，首先理解為什么天空是藍色的是有幫助的。

Light is white, but when you put it through a prism, you see all the different colored light waves in the spectrum: red, orange, yellow, green, blue, indigo, and violet. Light travels in waves, sometimes in short dippy ones and other times in long lines with lots of peaks.

光是白色的，但是當你把它通過棱鏡時，你會看到光譜中所有不同顏色的光波:紅、橙、黃、綠、藍、靛、紫。光以波的形式傳播，有時是短而模糊的，有時則是有許多峰的長波。

Light will travel in a straight line unless something gets in its way, like a prism or molecules in the atmosphere.

光會直線傳播，除非有東西擋住它，比如棱鏡或大氣中的分子。

Gases and particles in the atmosphere scatter light, and since blue travels on shorter, smaller waves, it causes charged particles to move faster, scattering more light. So we see more blue than we do red because blue gets particles worked up more than the other colors. Our eyes are also a bit more sensitive to blue light, which helps.

大氣中的氣體和粒子會散射光，由于藍色的傳播波更短更小，它會導致帶電粒子運動得更快，散射出更多的光。所以我們看到的藍色比紅色更多，因為藍色比其他顏色更能激發(fā)粒子。我們的眼睛對藍光更敏感，這也有幫助。

All About the Angles

都是關于角度

Even the landing lights of Osaka Kansai International Airport can't detract from that red-orange sunset. (Photo: Ken H/Wikimedia Commons)

Violet is always there, but we don't detect it as well as we do blue, so blue wins the light wave battle for our eyes. The right conditions have to be met for purple, or violet, to be visible.

紫羅蘭色一直存在，但我們對它的探測不如對藍色的探測，所以藍色贏得了我們眼睛的光波大戰(zhàn)。要想看到紫色，必須滿足適當?shù)臈l件。

In some cases, it's a matter of where the sun is coming in at a certain angle. Some of those colors that get blocked out by blue, like yellow, red and orange, appear a lot during sunrises and sunsets for this reason.

在某些情況下，這是太陽以特定角度從哪里進來的問題。因此，在日出和日落的時候，會經(jīng)常出現(xiàn)一些被藍色遮擋的顏色，比如黃色、紅色和橙色。

Purple Skies

紫色的天空

That's some purple power in Ludwigsburg, Germany. (Photo: Sascha Schuermann/AFP/Getty Images)

But other factors can come into play that can jumble up the light waves and the particles even more.

但其他因素也會起作用，使光波和粒子更加混亂。

According to Sarah Keith-Lucas from BBC Weather, "dust, pollution, water droplets and cloud formations" can influence the colors of the sky, too. Occasionally, pink and purple will appear more often than red and orange due in part to "the optical illusion of the pink wavelengths lighting up the base of the cloud (due to the low angle of the sun's rays), and these pink clouds superimposed on a dark blue sky. The combination of pink and dark blue can make the sky appear a deep purple."

據(jù)BBC氣象頻道的莎拉·基斯-盧卡斯說，“灰塵、污染、水滴和云的形成”也會影響天空的顏色。偶爾，粉色和紫色會比紅色和橙色出現(xiàn)得更頻繁，部分原因是“粉色波長照亮云的底部的光學錯覺(由于太陽光線的低角度)，這些粉紅色的云疊加在深藍色的天空上。”粉色和深藍色的組合可以讓天空呈現(xiàn)出深紫色。”

In the case of Hurricane Michael, water droplets, a setting sun and low cloud cover played a part in creating a purple sky after Michael had passed.

以“邁克爾”颶風為例，水滴、落日和低云層在“邁克爾”颶風經(jīng)過后形成了一片紫色的天空。

In the end, getting those purple hues is about having just the right conditions happen at the right time.

總之，要想獲得這些紫色的色調(diào)，需要在合適的時間、合適的條件下。