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How does ice react to different weather conditions? Intuitively we might think that very low temperatures are favorable for creating good waterfall ice conditions. Is it this simple? How do we make the connection between climbers' observations and a scientific experiment? A study led by the Laboratoire de Glaciologie et Géophysique de l’Environnement de Grenoble, supported by the Petzl Foundation, is providing part of the answer.
Warnings
Ice falls that form gradually can suddenly collapse, especially when vertical. Variations in temperature cause significant mechanical stresses in the ice that may in certain cases be big enough to cause a sudden break in the ice. In fact, the ice expands when heated and contracts when cooled. If cooling is slow, the ice can adapt to the stresses and deform "plastically"; this is referred to as ductile behavior. Conversely, if cooling is quick, the stresses can create cracks or expand those created by an ice axe. The ice therefore has a fragile character. Measurements by pressure sensors installed in the waterfall ice were used to find the link between drastic variations in temperature and a strong increase in mechanical stresses. By considering this last phenomenon, the ice climber holds the key to interpreting ice quality depending on temperature readings during the days preceding a waterfall ice ascent.
Ice has ductile behavior. This seems to be a favorable situation. We call this ice "sorbet".
Running water at the ice/rock interface provokes separation of the ice from the rock. This situation therefore seems to be unfavorable.
This leads to strong thermal contractions in the ice; the ice becomes fragile. The ice axe strikes contribute to the spread of cracks, with the possibility of a collapse created by the climber himself. Further, during contraction, a free-standing column will shorten, creating strong vertical mechanical stresses in the structure. There is therefore a high risk of sudden collapse of the structure.
Conditions are less critical, but the ice remains fragile and breaks under the strikes of the climber's axe.
These pieces of information are only partial clues and will never replace the experience acquired through practice. Content developed in collaboration with François Damilano, a member of the "Ice Crystal" research project. For more information, consult the Foundation at www.fondation-Petzl.org for the article: Ice crystals, a scientific journey into the heart of icefalls.