The science behind snowflakes

雪花背后的学问

译锐翻译   2020-12-25   16:13 p.m.

A snowflake is a small weather feature that can pack a big punch.

作为一种气候特点，雪花是微不足道的。但是，雪花背后的科学却会让你大吃一惊。

Many are made up of just one single ice crystal while some, more elaborate snowflakes are comprised of as many as 200 ice crystals fused together.

有许多雪花只由一片冰晶组成。但是，也有一些雪花，一些更加精致的雪花可以由多达200片冰晶融合而成。

All flakes maintain the same six-sided basic shape, though no two snowflakes will have exactly the same arrangement of molecules.

所有的雪花都拥有相同的六边形。但是，没有两片雪花的分子排列是完完全全一模一样的。

This makes each one unique.

这样说来，每片雪花都是独一无二的。

No two snowflakes are the same

没有哪两片雪花是相同的

Like fingerprints, no two snowflakes are exactly alike. In fact they are so diverse, there is actually an international classification system for snowflakes. This system is broken down into mostly eight principal snow crystal types: stellar dendrites, sectored plates, rimed crystals, hollow columns, needles, spatial dendrites, capped columns and irregular forms.

和指纹一样，没有哪两片雪花是完完全全一模一样的。事实上，雪花非常多种多样。对于雪花，国际上实际还有一套分类体系。该体系将雪花分为8大类：恒星树枝状、扇形板状、边缘晶体、空心圆柱、针状、空间树枝状、带帽圆柱和不规则形状。

To understand how they get their unique shape, we must start at the beginning. A snowflake first forms when an extremely cold water droplet freezes onto a pollen or dust particle in the sky.

要想了解为什么雪花会有不同的形状，我们就要从雪花形成之初开始说起。当遇到极寒天气时，水滴会冰冻成花粉状或尘土状，这时雪花初次形成。

"This creates an ice crystal," according to the National Oceanic and Atmospheric Administration (NOAA). "As the ice crystal falls to the ground, water vapor freezes onto the primary crystal, building new crystals -- the six arms of the snowflake."

根据国家海洋和气象局（NOAA）介绍，“这就形成了冰晶。当冰晶落到地面时，水蒸气会冷冻成初生晶，并形成新的冰晶-一片六角形的雪花。”

Each individual snowflake encounters slightly different atmospheric conditions on its path from sky to ground. Therefore, they all end up unique, while still maintaining the basic six-sided structure.

每片雪花在从天空飘落到地面的过程中都会遇到稍许不同的气候条件。因此，所有的雪花形成后，尽管都具有基本的六边形，但是都是与众不同的。

"Ultimately, it is the temperature at which a crystal forms — and to a lesser extent the humidity of the air — that determines the basic shape of the ice crystal. Thus, we see long needle-like crystals at 23 degrees F and very flat plate-like crystals at 5 degrees F. "

“最终起作用的是冰晶形成时的温度-其次是空气的湿度。空气的湿度决定了冰晶的基本形状。因此，在23华氏度时，我们会看到长长的像针一样的冰晶。在5华氏度时，则会看到非常扁平的、像盘子一样的冰晶。”

Although the core six-sided shape is always maintained, each of the six arms may branch off in new directions, or change shape as it incurs slight changes in the surrounding temperature or humidity, but because each of those arms is experiencing the exact same atmospheric conditions, all the arms will look identical.

尽管最基本的六边形往往能够保留下来，但是每个边都会朝着新的方向分叉，或当周围的温度或湿度有细微变化时，形状发生改变。但是，由于每个边所经历的气候条件都完全相同，因此所有的边看上去都是一模一样的。

Cold air is paramount冷空气至关重要

As the famous quote says "To appreciate the beauty of a snowflake it is necessary to stand out in the cold."

就像一句著名的引文所说：“唯有矗立在寒冷中方能欣赏到雪花的美。”

The temperatures at the surface are only part of the story though. You really have to look at the entire column of air to know what type of precipitation you will get.

地表温度仅仅是一部分。要想知道会产生哪种降水，则需要查看整个空气量。

Most precipitation that forms in wintertime clouds starts out as snow because the top layer of the storm is usually cold enough to create snowflakes. Precipitation continues to fall as snow when the temperature remains at or below 32 degrees F from the cloud base to the ground.

绝大多数在冬季的云里形成的降水会首先以雪花形式出现，这是因为暴风雪的最高处通常足够寒冷并可以形成雪花。当温度保持在或低于32华氏度时，降水会继续以雪花形式从云端降落到地面。

"As a general rule, though, snow will not form if the ground temperature is at least 41 degrees F (5 degrees C)," according to the National Snow and Ice Data Center (NSIDC) website.

根据国家冰雪数据中心网站的介绍，“一般而言，如果地面温度至少在41华氏度（或5摄氏度）时，则不会形成雪。”

"While it can be too warm to snow, it cannot be too cold to snow. Snow can occur even at incredibly low temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur when there is relatively warm air near the ground—typically -9 degrees C (15 degrees F) or warmer—since warmer air can hold more water vapor."

“尽管太温暖就无法下雪，但是不会出现因为太冷而无法下雪的情况。即使是在非常低的温度下，雪花也可能会出现，只要空气中存在某些湿度源和某些可以将空气抬升或冷却的途径。事实上，当地面附近的空气相对温暖-通常为零下9摄氏度（15华氏度）或更暖时，往往会出现大雪天气--因为更加温暖的天气也可以保留住更多的水蒸气。”

The NSIDC explains that while most snowflakes are less than 0.5 inches across, some can reach sizes nearing 2 inches across. In order to get such large snowflakes, however, you need very specific weather conditions including near-freezing temperatures, light winds and unstable atmospheric conditions.

NSIDC解释说，尽管绝大多数雪花的直径都少于0.5英寸，但是有些雪花的直径也可以达到将近2英寸。要想天空中下起如此大的雪花，就需要达到非常特定的天气条件，包括接近零度的气温、微风和不稳定的气候条件。

Frozen precipitation and travel do not mix well冻雪和交通格格不入

What's interesting about snowflakes is that independently they are quite delicate, but in large enough quantities they can shut down a city.

关于雪花有趣的一点是，它们一片片单独看起来非常纤巧，但是如果是很多雪花放在一起，它们就会导致一座城市与外界隔绝。

Snow isn't the only form of frozen precipitation, but it is the most complex. In addition to snow, you can also have sleet, freezing rain, hail and graupel.

雪花并非是唯一一种冰冻降水，但是它却是最复杂的。除了雪花，还会有雨夹雪、冻雨、冰雹和霰。

Graupel occurs when supercooled water droplets collect on the outer surface of snowflakes. It happens when temperatures at the surface are above freezing and the air aloft is very cold.

当超级冷的水滴在雪花外表面聚集时，就会形成霰。当地表温度高于零度，但是上方空气却十分寒冷时，就会出现霰。

Sleet is actually just rain (liquid precipitation) that freezes on its way down to the ground as it hits cooler pockets of air near the surface.

雨夹雪实际上就是雨（液体状降水）。降水在从空中下降到地面的过程中遇到了地面附近更冷的空气团时，降水发生了冰冻。

Hail is actually falling raindrops that get caught up in a thunderstorm's strong updraft winds. They then get carried aloft into the coldest part of the cloud tops and freeze.

冰雹实际上是下降的雨滴被雷暴天气强烈的上升气流所困住并因此被向上带到最冷的云层中并结冻。

If you are driving you can actually hear graupel and sleet as they hit your windshield of your car because of their composition. It tends to make a "tap, tap, tap" sounds as it hits the glass and bounces slightly.

如果你正在开车，你会听到霰和雨夹雪打在挡风玻璃上的声音。因为它们的构成，它们在掉落在玻璃上时会发出“啪啪声”并会轻轻弹起。

Hail also makes noise, but more importantly, it can cause significant damage to a vehicle because they are usually much larger in size.

冰雹也会发出噪音，但是更为重要的一点是，冰雹会对车辆造成极大的损害，这是因为冰雹的体积通常要比前面二者大得多。

Freezing rain is when the entire column of air is warm and the surface is at or below the freezing mark, resulting in rain actually freezing on contact. This is one of the more dangerous kinds of precipitation because if you are out walking or driving, it appears as though it is just raining. However, what you can't see is that those rain drops are turning into ice as soon as they make contact with the road, sidewalk or even your vehicle.

冻雨是当整个空气都很温暖并且地表温度处于或低于零度时，雨水会在接触地面时冰冻。这是最为危险的一种降水现象，因为如果你步行外出或开车时，冻雨看上去好像仅仅是在下雨。但是，你无法看见的是，这些雨滴一接触地面、人行道或甚至你的车辆时，就会变成冰。

来源：CNN，编辑&翻译：译锐翻译Susan