|Lake ice cover is seasonal and occurs where average daily temperature is below the freezing point. Once formed the lake ice thickens over the course of the winter as the temperature gets colder. The lakes thermal structure prepares for ice with 0 degrees celsius water at the surface and denser 4 degrees celsius at the bottom. Once meteorologic conditions provide colder air, relative to warmer water, the water does not get colder - instead ice forms.
In our History of Winter (HOW) program we were able to study two very different kinds of lake ice.
CONGEALATION ICE is frozen lake ice and forms as two major types. (1)Spontaneous Nucleation takes place on cold calm nights when the surface of the lake supercools (i.e. - the temperature drops below freezing) and the ice nucleates -- spreading rapidly across the lake ice surface. The 'c' axis of the ice crystals are oriented vertically. Individual crystals can reach meter wide dimensions. (2)Heterogeneous Nucleation takes place when lake conditions are cold enough and strong winds blowing dust, snow or frozen rain hit the lake surface and the cause nucleation. The ice grows with the 'c' axis of the ice crystals in the horizontal direction. In this case columnar crystals are formed. These ice crystals take on a colorful tint when viewed in polarized light.
SNOW ICE occurs where snow falls on congealation ice and when thick enough the ice cracks and water rushes to wet the snow which will then freeze. The frozen snow ice is finely crystalline and easily recognized.
To the naked eye it is difficult to discern any details among various samples of ice except for the presence of trapped air bubbles. However, when a thin section of ice is sandwiched between 2 polarizer sheets on a light table the secret world of ice is revealed. The size, shape and orientation of the individual crystals are exposed in a kaliedoscope of color and shape. As a teacher, the use of ice in hands on experiments provides limitless educational possibilities.
|THE ICE 'C' AXIS All solids have a crystal structure which is like a fingerprint. The crystal structure determines what shape the crystals will form when they grow. Ice has a hexagonal crystal structure with a longer 'c' axis and three identical 'a' axes (called 'a1', 'a2' and 'a3'). The simple ice form is a hexagonal prism with the vertical direction being the 'c' axis direction.
The speed of light is different along the 'c' axis and the 'a' axes. This fact together with the orientation of the axes in a sample and the thickness of the sample will determine how the crystals look in polarized light