Not often in Ireland do we have the opportunity to see an icicle. Even less frequently do they survive long enough for us to compare how they look in youth with their appearance in maturity. But now may be your chance; 'tis the traditional Christmastide, When icicles hang by the wall And Dick the shepherd blows his nail And Tom bears logs into the hall, And milk comes frozen home in pail.
Irish icicles, when they occur at all, are generally puny needles compared to the substantial stalactites that are common in more continental climes. Nevertheless, whenever cold water flows from some protruding overhang, and when temperatures are as low as we have recently experienced, examples can be seen that display all the characteristics of their kind.
If you inspect it closely, you will see that a recently grown icicle usually exhibits horizontal "ribbing" on its outer surface, while a train of bubbles can be seen running down its axis deep inside. Older icicles, on the other hand, often display vertical ribs running down their length from root to tip.
The horizontal ribs encircle a young icicle to form what seem like rings at regular intervals along its length - the protrusions are fragile features, and have their origins in the crystalline structure of the ice itself. The tip of a lengthening icicle is typically in the shape of an inverted elongated cup - rather like a deep volcano crater upside-down.
The hollow is formed as the ice builds down first around the icicle's external circumference, and it is usually full of liquid water.
Every now and then irregularities in the rate of ice formation cause the tip to lose its drop of liquid water, and as the "cup" refills, a bubble of air is often trapped inside. Successive happenings like this result in a row of bubbles running down the centre of the icicle - a feature often clearly visible through the transparent structure of the ice.
Older icicles can often be identified by the vertical ribs that extend along their length. The ribs are most commonly seen on icicles that have undergone renewed growth after a period during which their supply of water has temporarily ceased.
New water, streaming down the side of such an icicle, lays down a track of ice; successive water drops follow the same path as long as the surface of the track does not freeze between the flows, and the raised track may build outwards for several millimetres before the water shifts elsewhere to lay another track - and then another rib.