Cold air, moving water and ice sheets together provide the setting for growing ice crystals. The arrow points to where Individual ice crystal balls are developing on a sheet of ice that is close to the moving water. Photo by Robert Dryja
Short or long ice crystals grow on the plant stems depending on how high they are above the ice sheet. Small lines of ice grow across the ice sheet after water molecules change directly from a vapor to solid. Photo by Robert Dryja
By ROBERT DRYJA
Somewhat unexpectedly, the Valle Grande with its broad grasslands can be a good place to see ice crystals. Further, the time to for a visit to see the ice crystals is in the late autumn, not when winter has arrived. Several conditions come together to make this happen.
The Valle Grande is part of the floor of large volcanic crater. It can be thought of as the bottom of a large bowl. This bottom is at 8,500 feet above sea level while the surrounding rim is at 9,500 feet with peaks approaching 10,000 feet. Cold air that develops at night accumulates in the bowl rather than flowing away. This cold air does not contain much moisture to make snow or ice. The grasses of the Valle Grande only turn brown in the autumn with no snow present.
The nightly temperature becomes quite cold in October. The temperature was in the -12 to 0 degrees range for seven nights this October. It was in the 1 to 10 degree for another ten nights. There was only one night when the temperature remained above freezing at 38 degrees. Unless a weather front passes through, this bowl of cold air tends to move slowly, settling toward the bottom where the East Fork creek is located.
The East Fork creek flows through the center of the Valle Grande and conditions change here. Whereas the nearby grasses are brown, the grasses alongside the creek are white with frost. The creek has areas along its banks that have a slow movement of water. Water in these areas has enough time give up its heat to the cold air, turning to sheets of surface ice. Faster moving water remains toward the center of the creek.
It is this combination of ice sheets, moving water and cold air that provides the conditions for the growth of large ice crystals. Vapor can evaporate from the moving water. This vapor immediately freezes in the cold air as it moves slowly across the adjacent sheets of ice. The creek provides this vapor continuously and so ice crystals can grow continuously.
The grasses emerging from the water have two types of ice crystals growing on them. Grass stems standing up over an inch high are covered with a coating of short ice crystals. Stems laying down and close to an ice sheet in contrast have much longer crystals covering them. There is a greater level of vapor just above an ice sheet that is adjacent to water. This provides more vapor for larger crystals to form.
A third type of ice crystal develops on the ice sheet itself. Small bits of plants just protruding through the ice sheet provide a “seeding” point where water vapor can condense into ice. The result is an irregular shaped ball of ice crystals growing in all directions.
A fourth type of ice crystal occurs when water molecules in the vapor stage turn into ice on their own rather than attaching to something. These initially grow as small linear crystals laying on the ice sheet. Some may then develop side branches.
These ice crystals are temporary from day to day. They develop at night when the air is at its coldest but melt or sublimate away during the day in warmer air or sunlight.
Vapor is no longer created when the creek freezes over completely in the winter. Crystals no longer can develop since no open water is available to provide vapor. However crystals may appear again in the early spring when the creek begins to thaw and open water again is present.
Balls of ice crystals develop on bits of plants protruding from the ice sheet. Photo by Robert Dryja
An ice crystal ball is composed of a batch of stems growing in all directions. Photo by Robert Dryja