Snowflake Science

Background Information: (info taken from http://www.snowcrystals.com) ● What is a snowflake? ○ When people say snowflake, they often mean snow crystal. The latter is a single crystal of ice, within which the water molecules are all lined up in a precise hexagonal array. Snow crystals display that characteristic six-fold symmetry we are all familiar with. The picture on the left shows a snow crystal. ● A snowflake, on the other hand, is a more general term. It can mean an individual snow crystal, but it can also mean just about anything that falls from the winter clouds. Often hundreds or even thousands of snow crystals collide and stick together in mid-air as they fall, forming flimsy puff-balls we call snowflakes. Calling a snow crystal a snowflake is fine, like calling a tulip a flower. ● Snow crystals are not frozen raindrops; that is called sleet. A snow crystal appears when water vapor in the air converts directly into ice without first becoming liquid water. As more water vapor condenses onto a nascent snow crystal, it grow and develops, and that is when its ornate patterns emerge ● Why such complex, symmetrical shapes? And why are no two alike? ○ A stellar snow crystal begins with the formation of a small hexagonal plate, and branches sprout from the six corners when the crystal grows larger. As it tumbles through the clouds, the crystal experiences ever changing temperatures and humidities, and each change makes the arms grow a bit differently. ● The exact shape of the final snow crystal is determined by the precise path it took through the clouds. But the six arms all took the same path, and so each experienced the same changes at the same times. Thus the six arms grow in synchrony, yielding a complex, yet symmetrical shape. And since no two snow crystals follow the exact same path through the clouds as they fall, no two look exactly alike. ● We have made "identical-twin" snowflakes by exposing a pair of tiny seed crystals to nearly identical varying conditions as a function of time. This shows essentially what would happen if two snow crystals traveled side-by-side as they fell from the clouds.

● What synchronizes the growth of the arms? ○ Nothing. The six arms of a snow crystal all grow independently, as described in the previous section. But since they grow under the same randomly changing conditions, all six end up with similar shapes. There are no mysterious forces -- quantum-mechanical, acoustical, or anything else you might have heard about -- that provide communication between the arms to ensure they all grow alike. ● Now, let me assure you that the vast majority of snow crystals are not very symmetrical. Don't be fooled by the pictures -- irregular crystals (see the Guide to Snowflakes) are by far the most common type. Just take a look for yourself next time it snows. Near-perfect, symmetrical snow crystals are fun to look at, and sought after by photographers, but they are not common. ● The six-fold symmetry you see in a snow crystal arises from the arrangement of water molecules in the ice crystal lattice.

The Snow Crystal Morphology Diagram

● The way snow crystals grow depends strongly on the temperature and humidity in the clouds. This is summarized in the Snow Crystal Morphology Diagram shown on the right. This is also called the Nakaya Diagram, after Japanese physicist Ukichiro Nakaya, who discovered this behavior by growing snow crystals in his lab in the 1930s.

● The diagram shows that the largest, most photogenic stellar snow crystals only grow in a narrow temperature range around -15 C (5 F). Needles and columns are best found around -6 C (21 F). Capped columns appear when the temperature changes as the crystals grow. Remember these are temperatures in the clouds; it is often substantially warmer on the ground.

● You can also see that more elaborate, branched crystals grow when the humidity is high. Simple prisms grow when the humidity is low (or when the crystals are tiny).

● Exactly why snow crystals grow this way is not yet fully understood, although much progress has been made. The growth behavior of ice depends on the molecular structure and dynamics at the crystal surface, and this is all so complicated that no one really understands everything about it.

● Although science has made great advances in understanding the secrets of the Universe, there remains a bit of mystery still in these remarkable ice structures.

Materials: ● White printer paper ● Pipe Cleaners ● Instructions for making borax crystals ● Printouts of snowflake types

Activity:  Help patrons explore different patterns of snowflake crystals and encourage them to think of a time that they have seen a singular snow crystal. What did it look like?  Encourage them to fold and design and cut out their own paper snow crystal (make sure it only has 6 sides!) Check out this link for instructions: https://www.instructables.com/id/How-to-Make-6-Pointed-Paper-Snowflakes/