莱了｜冬天⾥的霓虹

谁在冬天⻅到过彩虹吗? 谁在冬天⾥⻅到过双彩虹吗? 反正我不曾有过, 直到2024 年年初。

腊⽉初三,  周六下午(01/13/2024),  准备到波⼠顿⻄南⻆的千禧公园⾛路健身。到⼭顶才⾛了三分钟, 蚕⾖⼤的⾬点就劈头盖脸地落下来了。放弃执着, 及时返转, 躲进⻋⾥。

回头看, ⻄边的天, ⼀束强烈的⾦光穿过乌⿊的云层, 光束来⾃离地平线较低的部位。头顶上, 遮天蔽⽇的乌云像⼀⽚厚实⽆⽐的棉被在移动 着, 泼下如注的暴⾬之同时, ⼜吞⻝了东⽅的天空。⼏分钟内, 整个公园⼈迹尽矣。

隐隐地, 我感觉有好“戏” 可看, 便坐在⻋⾥等待, 眼睛就没有离开过东⽅。果然, ⼗多分钟后, 夕阳冲破云层, ⾦红⾊的阳光从我们背后斜射过来, 照亮了东⽅, 两轮彩虹⻜跃上⾬幕。冒着⾬冲出⻋⻔, 我⽤⼿机(Google Pixel 8 Pro) 拍下了这难得的美景。眼前的彩虹这么饱满, 这么鲜艳! 特别是靠近地⾯的那⼀段, 红, 橙,⻩, 绿, 蓝, 靛, 紫, 七⾊⻬全。近在眼前的彩虹, 似乎触⼿可及。如此奇景, 令⼈⼼动。

忽然⽿边传来⼀串银铃般的叫声: “Rainbow! Rainbow! Rainbow!!!” ⼀个两三岁的⼩⼥孩, 边跳边跑, 张开双臂向彩虹奔去。想必是她第⼀次看⻅彩虹, 她好兴奋, 我也好感动。

以前从来没有在冬天看到过彩虹, 那么到底冬天看到彩虹的⼏率有多⼤呢?

怀着好奇⼼, 上⽹⼀番查寻: “从季节上讲, ⼀般冬天的⽓温较低,  空中不容易存在⼩⽔滴,  下⾬的机会也少, 所以冬天⼀般不会有彩虹出现。” (来源:   科技⽇报, 摘⾃新华⽹2020-09-08)

我们所在的北美地区,  冬季温度常常在冰点以下, 雪多⻛⼤, 彩虹的形成想必更加难得。

     综合我在⽹上学到的知识, 试图做些解释: 彩虹的形成是因为阳光以40-42°的⻆度照射到空中的⼩⽔滴, 阳光进⼊⽔滴, 先折射⼀次, 在⽔滴的背⾯形成反射, 最后离开⽔滴时再折射⼀次,总共经过⼀次反射两次折射。因为⽔对光有⾊散的作⽤, 不同波⻓的光的折射率有所不同, 因此形成我们⾁眼可⻅的七种⾊带, 称之为“虹” 。

      ⽽“霓” 是阳光经由⼩⾬滴内两次反射和两次折射⽽产⽣的, 出现的⻆度在50–53°。因为霓是⽔滴经两次反射⽽产⽣的结果, 所以霓的⾊彩暗淡很多, 通常不容易被观察到。只有在⾬量充沛, ⾬滴较⼤, 阳光⻆度亮度合适的情况下, 霓的⾊彩才会鲜艳, 我们才能即⻅到虹也能⻅到霓。⽽⻅到形态完整的环形双彩虹, 更是难能可贵的。在冬季⻅到双彩虹必定是难上加难。

      我们在合适的时间段, 合适的观景点, 合适的⽓候条件, 却是在不合适的季节⾥, 遇⻅这美丽⽆⽐的霓虹奇景, 岂不是幸运?!

A rainbow is the result of light passing through a spectrum – in most cases, a collection of raindrops – and, therefore, having that light broken up and refracted into individual colors.

In the winter months, however, temperatures in the upper atmosphere drop to freezing, causing the raindrops to freeze into snow. This blocks the light from passing through the drop (or the snowflake) and prevents the appearance of a rainbow.

A double rainbow appears when light is reflected twice in a raindrop, and thus, you can see two distinct reflections that are coming from two different angles.

Sometimes we see two rainbows at once. What causes this? We have followed the path of a ray of light as it enters and is reflected inside the raindrop. But, not all of the energy of the ray escapes the raindrop after it is reflected once. A part of the ray is reflected again, and travels along a different path inside the drop to emerge from the drop at a different angle. The rainbow we normally see is called the primary rainbow and is produced from one internal reflection. The secondary rainbow arises from two internal reflections and the rays exit the drop the second time at an angle of around 50o, rather than the 42o for the primary rainbow. This effect produces

the secondary rainbow, with the colors reversed from the primary rainbow. It is possible for light to be reflected more than twice within a raindrop, but these additional rainbows are typically never seen under normal circumstances.