Friday, January 23, 2015

The Science of Icicles

One would think that in this modern age of instant information that we would know all there is to know about the simple things in life, such as icicles.  It is only within the last few years that scientists have begun to understand how they are formed at all, and still there are mysteries. 

Ironically, icicles form the way they do because of heat.  Of course water freezes at 32 degrees Fahrenheit, so generally the outdoor air temperature needs to be below that mark for icicles to form.  However, a ray of sunshine is just enough to slightly melt a bit of snow, which turns into water and drips off an edge.  It in turn loses its heat to the surrounding cold air and freezes, turning from water droplet into ice droplet.  After a few freezing droplets begin to coalesce, water droplets begin to flow down the sides of the formation, freezing as a thin film onto the ice already there. What doesn’t freeze to the sides of the ice already there, drips off the tip, which isn’t actually pointy, but concave. 

Heat rises.  So as the water runs down the side and the colder air is pulling heat out of it via the process of conduction, the air then becomes slightly warmer as a result and begins to rise along the sides of the forming icicle.  Thus, the blanket of air that surrounds the icicle being warmed by the water, becomes thicker nearer the top.  So it becomes slightly warmer at the top and therefore ice forms most rapidly at the tip where the surrounding air is cooler, which is why icicles are generally long and pointy. 

Variations in the final shape of icicles are influenced by air flow.  Intuition tells us that the “perfect” shaped icicle would be most likely to form in a still environment where there is no airflow to disrupt the process.  Our intuition fails us in this instance.  Icicles formed in still air tend to have odd anomalies such as little projections and branches, and a generally “non-perfect” icicle shape.  Perhaps like baking a cake, without a bit of stirring the cake doesn’t come out quite right!        

The final shape of an icicle is also heavily influenced by the water itself.  The sides of icicles are rarely smooth.  Instead there tends to be rows of undulating ripples.  It is impurities in the water itself, such as salt and dirt which are the culprits in causing these surface features.  Icicles formed from pure distilled water for instance, have sides that are quite smooth and without much surface texture.  The reason for this exists only in the realm of theory, currently unknown to science. 

Also only in the realm of theory, is this interesting fact about these ripples on icicles: regardless of air temperature, airflow, water flow, or the size of the icicle, the distance between ripple peaks is universal.  It’s always about a centimeter or so.

Ash Cave, Hocking Hills Ohio
As it turns out, icicles are the way they are due to heat, breeze, and dirt!  Go figure.

Hey, a great place to see them if conditions are right, is The Hocking Hills! 

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