图书馆面积:关于雪的知识

什么是雪？

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(2004年10月28日)

水是地球上各种生灵存在的根本，水的变化和运动造就了我们今天的世界。在地球上，水是不断循环运动的，海洋和地面上的水受热蒸发到天空中，这些水汽又随着风运动到别的地方，当它们遇到冷空气，形成降水又重新回到地球表面。这种降水分为两种：一种是液态降水，这就是下雨；另一种是固态降水，这就是下雪或下冰雹等。

大气里以固态形式落到地球表面上的降水，叫做大气固态降水。雪是大气固态降水中的一种最广泛、最普遍、最主要的形式。大气固态降水是多种多样的，除了美丽的雪花以外，还包括能造成很大危害的冰雹，还有我们不经常见到的雪霰和冰粒。

由于天空中气象条件和生长环境的差异，造成了形形色色的大气固态降水。这些大气固态降水的叫法因地而异，因人而异，名目繁多，极不统一。为了方便起见，国际水文协会所属的国际雪冰委员会，在征求各国专家意见的基础上，于1949年召开了一个专门性的国际会议，会上通过了关于大气固态降水简明分类的提案。这个简明分类，把大气固态降水分为十种：雪片、星形雪花、柱状雪晶、针状雪晶、多枝状雪晶、轴状雪晶、不规则雪晶、霰、冰粒和雹。前面的七种统称为雪。

为什么后面三种不能叫做雪呢？原来由气态的水汽变成固态的水有两个过程，一个是水汽先变成水，然后水再凝结成冰晶；还有一种是水汽不经过水，直接变成冰晶，这种过程叫做水的凝华。所以说雪是天空中的水汽经凝华而来的固态降水。（十种大气固态降水形态：雪片、星形雪花、柱状雪晶、针状雪晶、多枝状雪晶、轴状雪晶、不规则雪晶、霰、冰粒、雹）。

雪
维基百科，自由的百科全书
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美国科罗拉多州森林中的雪
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美国科罗拉多州森林中的雪

雪是水或冰在空中凝结再落下的自然现象，或指落下的雪花。雪是水在固态的一种形式。

雪只会在很冷的温度下才会出现，因此亚热带地区和热带地区下雪的机会较微。以香港为例，现有的纪录只有1893年1月才下过。但非洲的高山有“赤道雪”的奇景。所以某些主题公园会提供人工雪给人玩。

雪在中国文化内象征纯洁。
雪花的不同形状
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雪花的不同形状

雪花是指雪落下的普遍形状——六重对称的形状。它之所以有这样的形状，是因为它要在平面上以最有效率的方式布置，它是结晶学的研究对象之一。

威尔逊·班特利是第一位的雪花拍摄者。

开普勒曾写过一本研究雪花结构的书，叫《六角的雪花》(De Nive Sexangula)。

科赫曲线的形状很像雪花。
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娱乐

* 进行冬季运动如滑雪的场地
* 堆砌雪雕或雪人

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雪害
火车被雪阻着了
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火车被雪阻着了

* 雪崩
* 造成冻伤
* 雪融后造成水灾
* 阻塞交通

取自"http://zh.wikipedia.org/wiki/%E9%9B%AA"

页面分类: 天气 | 水体

Snow
From Wikipedia, the community-written free encyclopedia.
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For other uses, see Snow (disambiguation).

A fresh snowfall in Colorado's (USA) high forests.
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A fresh snowfall in Colorado's (USA) high forests.

Snow is precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes. Since it is composed of small rough particles it is a granular material. It has an open and therefore soft structure, unless packed by external pressure.

Snow is commonly formed when water vapor undergoes deposition high in the atmosphere at a temperature of less than 0°C (32°F), and then falls to the ground.
Contents
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* 1 Types
* 2 Occurrence
* 3 Recreation
* 4 Geometry
* 5 See also
* 6 External links

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Types

Flurries are similar to rainshowers and only last for short periods of time. Snow which has partially thawed while falling is called sleet; if this re-freezes on further descent, the resulting small icy pellets or granules of snow are called soft hail. A related phenomenon is freezing rain, where rain falls on ground sufficiently cold for it to freeze on contact, forming black ice on the ground.

A snow squall is a brief, very intense snowstorm while a blizzard is a long-lasting snow storm with intense snowfall and usually high winds. Particularly severe storms can create whiteout conditions where visibility is reduced to less than 1 m, while blizzards can also create large snowdrifts. A ground blizzard occurs when a strong wind drives already fallen snow to create drifts and whiteouts.

Snow can be also manufactured using snow cannons, which actually create tiny granules more like soft hail (this is sometimes called "grits" by those in the southern U.S. for its likeness to the texture of the food). In recent years, snow cannons have been produced that create more natural looking snow, but these machines are very expensive and are found only on the most prestigious places.
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Occurrence

Snowfall varies by time and location, including geographic latitude, elevation and other factors which affect weather in general. In latitudes closer to the equator, there is less chance of snow fall, 35° N and 40°S are often quoted as a rough delimiter. The western coasts of the major continents remain snowless to much higher latitudes.

As temperature decreases with altitude, high mountains, even at or near the Equator, have permanent snow cover on their top. Examples include Mount Kilimanjaro in Tanzania and the Tropical Andes in South America; the only snow actually on the Equator is at 4,690 m altitude on the south slope of Volcán Cayambe in Ecuador (Google Earth images). Conversely, many regions of the Arctic and Antarctic receive very little precipitation and therefore little snow despite the bitter cold (below a certain temperature, air essentially loses its ability to carry water vapor).

Although density of fresh snow varies widely, a guide is that the depth of snowfall is 10 times that of a rainfall containing the same mass of water.

Substantial snowfall sometimes disrupts infrastructure and services even in regions that are accustomed to them. Traffic may be snarled or even completely stop. Basic infrastructure such as electricity, phones and gas supply can be shut down. This can lead to a snow day, a day on which school or other services are cancelled owing to unusually heavy snowfall. In areas that normally have very little snow, this may occur even with light accumulation — something often made fun of by those people used to colder climates, where streets would remain passable given the same amount of snow.

The highest seasonally cumulative precipitation of snow ever measured in the world was on Mount Baker, Washington, U.S.A during 1998–1999 season when they received 28.96 meters (1,140 in); this surpassed the previous record holder, Mount Rainier, Washington, U.S.A which during 1971–1972 season received 28.5 meters (1,122 in) of snow; and the world record daily precipitation was recorded in Silver Lake, Colorado, U.S.A in 1921 1.93 meters (76 in).

See also: List of Countries receiving snowfall
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Recreation
Building a snowman.
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Building a snowman.

Forms of recreation dependent on snow:

* Many winter sports, such as skiing, snowmobiling, snowshoeing and snowboarding
* Playing with a sled or riding in a sleigh
* Building a snowman or snow fort
* Throwing snowballs mutually in a snowball fight or at others to tease them. (Humans seem to be the only animal that throw their snowballs. Pygmy chimpanzees have been seen carrying snowballs around, but never to throw them.)

Where snow is scarce but the temperature is low enough, snow cannons may be used to produce an adequate amount for such sports.

Tightly packed snow may be used as a construction material in, for example, Inuit snow houses.

The world´s biggest snowcastle is built in Kemi, Finland, every winter.
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Geometry
Snow flakes by Wilson Bentley, 1902
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Snow flakes by Wilson Bentley, 1902

An interesting question is why the arms of snowflakes are symmetrical, and why no two snowflakes appear to be identical. The answer is believed to be due to the fact that the distances between snowflakes are much greater than the distances across snowflakes.

The symmetry of snowflake arms is always six-fold, which arises from the hexagonal crystal structure of ordinary ice (known as ice Ih) along its 'basal' plane.

There are, broadly, two possible explanations for the symmetry of snowflakes. Firstly, there could be communication (information transfer) between the arms, such that growth in each arm affects the growth in each other arm. Surface tension or phonons are among the ways that such communication could occur. The other explanation, which appears to be the prevalent view, is that the arms of a snowflake grow independently in an environment that is believed to be rapidly varying in temperature, humidity and so on. This environment is believed to be relatively spatially homogenous on the scale of a single flake, leading to the arms growing to a high level of visual similarity by responding in identical ways to identical conditions, much in the same way that unrelated trees respond to environmental changes by growing near-identical sets of tree rings. The difference in the environment in scales larger than a snowflake leads to the observed lack of correlation between the shapes of different snowflakes.
The "Japanese Tsuzumi" is an unusual variation of snow crystal, getting its name from the hourglass-shaped tsuzumi drum.
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The "Japanese Tsuzumi" is an unusual variation of snow crystal, getting its name from the hourglass-shaped tsuzumi drum.

However, the concept that no two snowflakes are alike is incorrect; it is entirely possible, but unlikely, that a pair of snowflakes may be visually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Society has reported that matching snow crystals were discovered by Nancy Knight of the National Center for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

High-resolution gallery

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See also

* Avalanche
* Sleigh
* Skiing
* Snow sculpture
* Snowball
* Snowboard
* Snowman
* Snow cannon
* Frost
* Snowshoe
* Snow removal
* Igloo
* Grit bin
* Lake effect snow
* List of countries receiving snowfall
* Eskimo words for snow

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External links
Commons
Wikimedia Commons has media related to:
Snow

* National Science Digital Library - Snowflake
* Kenneth G. Libbrecht's Snowflake FAQ
* http://www.its.caltech.edu/~atomic/snowcrystals/photos/photos.htm

Retrieved from "http://en.wikipedia.org/wiki/Snow"

Category: Snow