This image shows the surface of a 6 inch diameter icicle when the air temperature was about 40 degrees F.   The surface of the icicle was melting away from the warm air.  The result is two different patterns:  the larger polygons which look like the cracks in mud on a dry lake bed, and inside the polygons the   little rivulet patterns which run vertically on the ice.

What causes the polygon pattern?

By understanding what causes the patterns, then ice patterns could be used as a thermometer.