You have seen snow, but have you ever studied a snowflake? Have you looked hard at falling snowflake and seen its design up close?
It' s not easy to look at something so tiny．The flake often melts or blows away as soon t it lands．If looking at one seems difficult, imagine trying to photograph one．
But that' s what Ken Libbrecht has spent much of the past four years doing：catching snowflakes, putting them under a microscope and taking pictures of them．
Libbrecht is an American scientist who has written four books about snowflakes．Four of his snow photos will be put on US postage stamps this holiday season．
It' s the structure
In his latest book "Ken Libbrecht' s Field Guide to Snowflakes", he answers lots of questions about snowflakes in a way that' s easy to understand．
The beautiful structure of snowflakes is mainly the result of two things：conditions in the sky and the structure of water molecules(分子)．
A snowflake starts as a very small spot of dust．Water molecules in the air get attached to the dust, then freeze．These molecules are triangular．When enough of them become attached, a six-sided shape is formed．
Some snowflakes become flat and stay that way．Sometimes more water molecules freeze at the six corners and make the snowflake look like flowers．Other snowflakes turn into six-sided columns．
But if the changes in temperature and the amount of water in the air are just right, thin branches may grow on a baby snowflake as the snowflake travels in the air．The six-pointed starbursts(星放射状)are usually the prettiest．They are most likely to form when the weather is either just below freezing or around 5 degrees．
Be a snowflake watcher
1．A gentle snowfall is best．
2．Take a magnifying glass(放大镜)and a piece of black paper outside and wait a few minutes for the paper to get cold．
3．Hold the paper out flat until a few flakes have landed on it．Quickly examine them with the magnifying glass．Look for the best ones, or see how many types you can find．
(1)
It is not easy to study snowflakes because ________．
[ ]
A．
they are tiny and triangular
B．
they are difficult to catch by hand
C．
they are white and cool
D．
they are easy to melt and blow away
(2)
We can learn from the passage that Libbrecht
[ ]
A．
has written a lot of books about postage
B．
has taken a lot of pictures of snowflakes
C．
has designed some snow stamps
D．
has published only four books
(3)
Which of the following is true about snowflakes?
[ ]
A．
Starbursts form when it is freezing cold．
B．
Snowflakes are flat and contain lots of water．
C．
Snowflakes are six-sided．
D．
Snowflakes won' t melt．
(4)
What is the passage mainly written for?
[ ]
A．
Introducing Libbrecht and his latest book．
B．
Encouraging readers to watch snowflakes．
C．
Telling people what fun it is to study snowflakes．
