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</td> <td valign="top"> <input type="checkbox" name="def"> Search Definitions<br> <input type="checkbox" name="case">case sensitive </td> </tr> </table> </form> <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/preface1">First Edition Preface</a>  <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/preface2">Second Edition Preface</a>  <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/acknowledge">Acknowledgments</a> </td> </tr> </table> <table width="550" cellpadding="5"> <tr> <td> <br><br><hr> <table> <tr> <td valign="top"><li></td> <td><b><font color="#007177">ice crystal</font></b>Any one of a number of macroscopic, crystalline forms in which <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=ice1">ice</a> appears, including <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=hexagonal-column1">hexagonal columns</a>, <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=hexagonal-platelet1">hexagonal platelets</a>, <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=dendritic-crystal1">dendritic crystals</a>, <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=ice-needle1">ice needles</a>, and combinations of these forms. <dd>The <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=crystal-lattice1">crystal lattice</a> of ice is hexagonal in its symmetry under most atmospheric conditions. Varying conditions of <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=temperature1">temperature</a> and <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=vapor-pressure1">vapor pressure</a> can lead to growth of crystalline forms in which the simple hexagonal pattern is present in widely different habits (a thin hexagonal plate or a long thin hexagonal column). In many ice crystals, trigonal symmetry can be observed, suggesting an influence of a cubic symmetry. The principal axis (c axis) of a single <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=crystal1">crystal</a> of ice is perpendicular to the axis of hexagonal symmetry. Planes perpendicular to this axis are called basal planes (a axes related to the prism facets) and present a hexagonal cross section. Ice is <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=anisotropic1">anisotropic</a> in both its optical and electrical properties and has a high dielectric constant (even higher than water) resulting from its water dipole structure. The electrical <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=relaxation-time1">relaxation time</a> for water is much shorter than for ice (10<sup>9</sup> Hz compared with 10<sup>4</sup> Hz), resulting from a <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=chain-reaction1">chain reaction</a> requirement for molecules to relax through defects in the ice lattice. In the <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=free-air1">free air</a>, ice crystals compose cirrus-type clouds, and near the ground they form the <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=hydrometeor1">hydrometeor</a> called, remarkably enough, “ice crystals” (or <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=ice-prisms1">ice prisms</a>). They are one constituent of <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=ice-fog1">ice fog</a>, the other constituent being <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=droxtal1">droxtals</a>. On terrestrial objects the ice crystal is the elemental unit of <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=hoarfrost1">hoarfrost</a> in all of its various forms. Ice crystals that form in slightly <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=supercooled-water1">supercooled water</a> are termed <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=frazil1">frazil</a>. Ice originating as frozen water (e.g., <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=hail1">hail</a>, <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=graupel1">graupel</a>, and <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=lake-ice1">lake ice</a>) still has hexagonal symmetry but lacks any external hexagonal form. Analysis of their sections (0.5 mm) in polarized <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=light1">light</a> reveals different crystal shapes and orientations, depending on the <a href="/web/20090217075936/http://amsglossary.allenpress.com/glossary/search?id=freezing1">freezing</a> and any annealing and subsequent recrystallization process. </td></tr></table> </td></tr></table></body></html> </td></tr></table></body></html>