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Icicles are one of our strongest symbols of winter and often used to depict the cold depths of prolonged winter. In its basic form, an icicle is a tapered, hanging spike or cone of ice formed by the freezing of dripping or falling water. We mostly think of them as forming off building roofs and edges from melting snow, but icicles can also form on trees, utility poles and fences, and on rocks near waterfalls or ground water seepage points, from rain, mist, spray and water seepage.
To simplify the discussion, let's think about the formation of icicles on roof edges.
We'll start with a slanted roof having a snow cover. To initiate the icicle formation, the temperature of the snow must rise above its melting temperature. Strong solar heating of absorbing roof materials and internal heat loss from an improperly insulated attic or under-roof are prime agents for raising rooftop temperatures and causing snowmelt.
When the snowmelt flows down the roof, the competition between gravity — pulling the water downward — and surface tension — trying to keep the water flow flat — leads to the formation of evenly-spaced ripples along the edge of the roof. These ripples freeze when and where the surface temperature dips below 0oC (32oF), and become icicle roots. The icicles then grow as water dripping over the roots freezes in progressive layers.
Icicles grow downward and outward simultaneously but at differing rates.
Horizontal ribbing encircles each new icicle to form a series of progressively smaller rings toward the tip. During active icicle growth, these rings are composed of fragile, thin ice plates which grow randomly outward. Eventually the spaces between the rings are filled with downward flowing melt water.
Vertical ridges form when melt water streams down the icicle exterior, laying a thin track of ice. Continued melt water flow adds additional ice along the same track for as long as the track surface stays liquid.
At the tip of a growing icicle is a pendent drop of water. Typically the water extends several centimeters (about an inch) up into the interior of the icicle from the tip. This tip is composed of randomly growing ice crystals which, once frozen, form an inverted cup, usually filled with water. Occasionally an air bubble enters the cup and drains it of its liquid. Water flowing down the icicle quickly restores the end drop, trapping some of the air and incorporating it into the solid icicle. Such bubbles are usually seen along the axis of the icicle, giving it a milky appearance.
Even after active growth has stopped, icicles can continue to change shape and appearance. At subfreezing temperatures some icicles change from solid water directly to water vapor, slowly altering and smoothing the icicle's surface.
Although icicles can grow to several meters (yards) in length, eventually, melting or gravity brings an end to the icicle, causing it to fall from its perch and crash to the surface 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 and forms an extensive, intertwining network of ice and hanging spikes giving the appearance of an ice curtain.
Nature Net News is brought to you by the Aldo Leopold Nature Center's Nature Net: The Environmental Learning Network with special thanks to American Girl's Fund for Children