小米摄像头怎么安装:美丽的雪花和其差异性

Photos document snowflake diversity
Physicist says the many possibilities guarantee no two crystals are alike
By Sara GoudarziSpecial to LiveScience

updated 1/13/2006 2:08:33 PM ET

USPSThe Holiday Snowflakes stamps are photographs of two basic snowflake patterns by physicist Kenneth Libbrecht. They are stellar dendrites, which form branching treelike arms; and sectored plates, which form platelike arms.
Through rain andsleet and dead of night and all that, your letters next winter can be deliveredbearing snowflakes artfully photographed by a physicist who weathers those samestorms to study nature's crystal magic.
Starting inOctober, the U.S. Postal Service will issue a set of four stamps featuringpictures of snowflakes taken by Kenneth Libbrecht, a professor of physics atthe California Institute of Technology.
For years,Libbrecht has been studying the physics of snowflakes, looking at the differentpatterns of crystal growth and snowflake formation.
"I'm tryingto understand the dynamics of how crystals grow, all the way down to themolecular level," Libbrecht said. "This is a very complicated problem, and I've been looking at iceas a particularly interesting case study."
Snowflakes arenothing more than ice, but the forms a single flake can take are dizzyinglycomplex. A single crystal of ice is known as a snow crystal. And one or moresnow crystals stuck together make a snowflake.
There is, as you'veheard, endless possibilities for how they stick together.
Searching forbetter snowflakes
When Libbrechtstarted making snowflakes in the laboratory, he took microscopic photographs inorder to be able to study the basic physics of each flake. In 2001, he startedcapturing images of natural snowflakes.
Location isimportant.
"Fairbanks sometimesoffers some unusual crystal types, because it's so cold," Libbrecht said."Warmer climates, for example, in New York state and the vicinity, tend to produce lessspectacular crystals."
"I visit thefrozen North and wait for snow to fall," Libbrecht said in a recente-mailinterview. "I'm in northern Ontarioright now."
Since ice ismostly clear, the flakes have to be lit properly to reveal their beauty.
"I use differenttypes of colored lights shining through the crystals, so the ice structures actlike complex lenses to refract the light in different ways. The better thelighting, the more interesting is the final photograph." He has to workquickly, using a paintbrush to place a flake into his portable studio for theshoot. When flakes have fallen, they stop growing — and within minutes theytypically lose their sharp edges and become less interesting.
Snow science
The pictures havehelped Libbrecht describe a growth instability in snowflakes that otherresearchers had missed. By applying high voltage to a growing snow crystal inthe lab, Libbrecht is able to analyze unique growth mechanisms, especially onvery small scales.
"Theseinstabilities are new and important for understanding how crystals grow, butthey're hard to explain," Libbrecht told LiveScience.
Nonetheless, theknowledge can be applied in making semiconductors, solid materials whoseelectrical conductivity operates many electronic gadgets. Semiconductors aremade in part by condensing certain substances into solid forms.
Libbrecht'sHoliday Snowflakes stamps will feature multi-branched stellar dendrites, withsix symmetrical main branches and many randomly placed side branches andsectored plates. These represent but one of seven primary types of snowflakepatterns.
As you'd expect,no two of the stamps are alike. But what about out in the field? Are theyreally all different?
"The answeris basically yes, because there is such an incredibly large number of possibleways to make a complex snowflake," Libbrecht said. "In many cases,there are very clear differences between snow crystals, but of course there aremany similar crystals as well. In the lab we often produce very simple,hexagonal crystals, and these all look very similar."

Types of Snowflakes
雪花的形状和结构
Simple Prisms 简单棱柱晶体
A hexagonal prism is the most basic snow crystal geometry . Depending on how fast the different facets grow, snow crystal prisms can appear as thin hexagonal plates, slender hexagonal columns (shaped a lot like wooden pencils), or anything in between. Simple prisms are usually so small they can barely be seen with the naked eye.
The examples at right show two stubby prisms and one thin plate. Snow crystal facets are rarely perfectly flat, being more typically decorated with various indents, ridges, or other features.
Stellar Plates 星盘晶体
These common snowflakes are thin, plate-like crystals with six broad arms that form a star-like shape. Their faces are often decorated with amazingly elaborate and symmetrical markings.
Plate-like snowflakes form when the temperature is near -2 C (28 F) or near -15 C (5 F).
Sectored Plates 扇盘晶体
Stellar plates often show distinctive ridges that point to the corners between adjacent prism facets. When these ridges are especially prominent, the crystals are called sectored plates.
The simplest sectored plates are hexagonal crystals that are divided into six equal pieces, like the slices of a hexagonal pie. More complex specimens show prominent ridges on broad, flat branches.
Stellar Dendrites 树枝星晶体
Dendritic means "tree-like", so stellar dendrites are plate-like snow crystals that have branches and sidebranches. These are fairly large crystals, typically 2-4 mm in diameter, that are easily seen with the naked eye.
Stellar dendrites are clearly the most popular snow crystal type, seen in holiday decorations everywhere. You can see these crystals for yourself quite well with just a simple magnifier.
Fernlike Stellar Dendrites 松枝星晶体
Sometimes the branches of stellar crystals have so many side-branches they look a bit like ferns, so we call them fern-like stellar dendrites. These are the largest snow crystals, often falling to earth with diameters of 5 mm or more. In spite of their large size, these are single crystals of ice -- the water molecules are lined up from one end to the other.
Some snowfalls contain almost nothing but stellar dendrites and fern-like stellar dendrites. It can make quite a sight when they collect in vast numbers, covering everything in sight.The best powder snow, where you sink to your knees while skiing, is made of stellar dendrites. These crystals can be extremely thin and light, so they make a low density snow-pack.
Hollow Columns 空心柱晶体
Hexagonal columns often form with conical hollow regions in their ends, and such forms are called hollow columns. These crystals are small, so you need a good magnifier to see the hollow regions.
Note how the two hollow regions are symmetrical in each column. Sometimes the ends grow over and enclose a pair of bubbles in the ice, as seen in the last picture on the right.
Needles 针形晶体
Needles are slender, columnar ice crystals that grow when the temperature is around -5 C (23 F). On your sleeve these snowflakes look like small bits of white hair.
One of the amazing things about snow crystals is that their growth changes from thin, flat plates to long, slender needles when the temperature changes by just a few degrees. Why this happens remains something of a scientific mystery.
Capped Columns 戴帽柱晶体
These crystals first grow into stubby columns, and then they blow into a region of the clouds where the growth becomes plate-like. The result is two thin, plate-like crystals growing on the ends of an ice column. Capped columns don't appear in every snowfall, but you can find them if you look for them.
The first example at right shows three views of a capped column. The first view is from the side, showing the central column and the two plates edge-on. The other two views show the same crystal from one end, with the microscope focused separately on the two plates.
Double Plates 双盘晶体
A double plate is basically a capped column with an especially short central column. The plates are so close together that inevitably one grows out faster and shields the other from its source of water vapor. The result is one large plate connected to a much smaller one. These crystals are common -- many snowflakes that look like ordinary stellar plates are actually double plates if you look closely.
The first picture at right shows a double plate from the side. The second picture shows a double plate with the microscope focused on the smaller plate. In the third picture, note the slightly out-of-focus hexagon that is about one-sixth as large as the main crystal. This hexagon is the second side of a double plate, connected to the main plate by a small axle.
Split Plates and Stars 裂盘和裂星晶体
These are forms of double plates, except that part of one plate grows large along with part of the other plate. The picture at right shows all eight ways to make a split star. Split plates and stars, like double plates, are common but often unnoticed.
You may have to stare at these pictures a bit to see how the two distinct pieces fit together. Note how in each case the crystals are connected in the center with short axles.
Triangular Crystals 三角晶体
Plates sometimes grow as truncated triangles when the temperature is near -2 C (28 F). If the corners of the plates sprout arms, the result is an odd version of a stellar plate crystal. These crystals are relatively rare.
Surprisingly, no one knows why snow crystals grow into these three-fold symmetrical shapes. (Note however that the molecular structure of triangular crystals is no different from ordinary six-sided crystals. The facet angles are all the same.)
12-Sided Snowflakes 12边雪花 （2个6枝叠和体）
Sometimes capped columns form with a twist, a 30-degree twist to be specific. The two end-plates are both six-branched crystals, but one is rotated 30 degrees relative to the other. This is a form of crystal twinning, in which two crystals grow joined in a specific orientation.
These crystals are quite rare, but sometimes a snowfall will bring quite a few. The picture at the far right shows a 12-sider where the two halves are widely separated.
Bullet Rosettes 子弹形莲座晶体
The nucleation of an ice grain sometimes yields multiple crystals all growing together at random orientations. When the different pieces grow into columns, the result is called a bullet rosette. These polycrystals often break up to leave isolated bullet-shaped crystals.
Sometimes a bullet rosette can become a capped rosette, as shown in the example at the far right.
Radiating Dendrites 辐射松枝晶体
When the pieces of a polycrystal grow out into dendrites, the result is called a radiating dendrite (also called a spatial dendrite).
The first example on the right shows radiating plates. The second example shows a fernlike stellar dendrite with two errant branches growing up out of the main plane of the crystal.
Rimed Crystals 霜面晶体
Clouds are made of countless water droplets, and sometimes these droplets collide with and stick to snow crystals. The frozen droplets are called rime. All the different types of snow crystals can be found decorated with rime. When the coverage is especially heavy, so that the assembly looks like a tiny snowball, the result is called graupel （软雹）.
The first two pictures at right have relatively light rime coverage. The final example is completely covered with rime, but you can still see the six-fold symmetry of the underlying stellar crystal.
Irregular Crystals
不规则晶体
The most common snow crystals by far are the irregular crystals. These are small, usually clumped together, and show little of the symmetry seen in stellar or columnar crystals.
Artificial Snow
人造（冰滴）雪
Snow machines shoot a mixture of water and compressed air out of nozzles. The water comes out as fine droplets, and the air cools as it decompresses, causing the droplets to freeze. A fan blows the ice particles onto the slopes.   You can see from the picture at right that artificial snow is made of frozen water droplets, with none of the elaborate structure found in real snow crystals.

Solving the Puzzle of Triangular Snowflakes
Snowflakes ought to be hexagonal. So why are triangular ones so often observed?
KFC 11/27/2009

The beautiful six-fold symmetry of snowflakes is the result of the hydrogen bonds that water molecules form when they freeze.
But snowflakes can form other shapes too when the growth of the crystal is perturbed on one side. In theory, diamonds, trapezoids and other irregular shapes can all occur. And yet the one most commonly observed (after hexagons) is the triangle. The puzzle for is why? What process causes deformed snowflakes to become triangles rather than say squares or rectangles?
Today, Kenneth Libbrecht at the California Institute of Technology in Pasadena provides an answer. Libbrecht, you may remember, has built an amazing snowflake machine to study the formation of these remarkable tiny crystals.
Their growth and shape, he says, is governed by two processes: the diffusion of water molecules through the air and the molecular dynamics on the surface of the crystal and it is the former that explains triangular crystal formation.
Libbrecht says various phenomena influence the way water molecules get to the surface of a snow crystal but perhaps the most important is aerodynamics. This orients the crystals and ventillates it, determin the rate at which it can grow.
Libbrecht has calculated how small perturbations in the growth rate on one side of a crystal change its shape. He says that whatever the cause of the perturbation, a hexagonal crystal will always continues to change shape in one way or another.
But the curious property of triangular crystals is that they are stable against this kind of change. So when a crystal has become triangular, other perturbations cannot change its shape further.
Triangles are a kind of valley in the energy landscape of snowflake morphology. And this explains why they are so common. Cool!
Keep an eye out for one next time you're out in the snow.
Ref: arxiv.org/abs/0911.4733: Aerodynamical Effects in Snow Crystal Growth