The tip of a growing icicle is primarily liquid water with a pendent drop on the tip end. Typically liquid water extends several centimetres (about an inch) up into the interior from the tip. The outer region of the tip is composed of randomly growing ice crystals which meld to form an elongated, inverted cup which is usually filled with liquid water. However, occasionally an air bubble will enter the cup and drain it of liquid. Water flowing along the exterior quickly restores the end drop, trapping some of that air and incorporating it into the solid icicle as a bubble several millimetres (tenths of inches) in diameter. Such bubbles can usually be seen located along the axis of the icicle, giving it a milky appearance.
Even after active growth has stopped, the icicle will continue to change its shape and appearance, even at subfreezing temperatures. Some ice may sublime from solid water to the vapour state, thus slowly altering and smoothing the icicle's surface features.
Although icicles can grow to several metres (yards) in length, eventually, melting or gravity will bring an end to the icicle, causing it to fall from its perch and crash onto the surface below.
For home and building owners those tapering spikes of ice hanging down from roof edges are common winter problems. They can cause damage to the building since large icicles have the potential to pull down gutters. During thaws or wind storms, falling icicles can be extremely dangerous to people below.
Icicles forming on or near waterfalls and other areas, where liquid water seeps from the rock or soil, add further beauty to natural winter scenes. In some cases, the spray from the falling water drifts onto existing icicles can form an extensive, intertwining network of merged ice and hanging spikes giving the appearance of an ice curtain.
Photo courtesy of NOAA, US Department of Commerce.
Copyright 2003, Keith C. Heidorn, All Rights Reserved. (Illustrations, unless otherwise specified, ©2003, Spectrum Educational Enterprises, All Rights Reserved.)
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