Icicles

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, I will consider the formation of icicles on roof edges, but their formation on other surfaces usually follows a similar process.

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 flow front. These ripples will freeze when and where the surface temperature dips below 0^oC (32^oF), and the frozen ripples become the icicle roots. Icicles then grow as water dripping over the roots freeze in progressive layers rather than being frozen all at once. This initiates icicle formation.

New icicles are typically cone-shaped, and often small bubbles can be seen within them. As they grow, vertical ridges and horizontal ribs, or rings, form on their outer surface. Icicles grow downward and outward simultaneously but at differing rates.

After a period of continuous growth, icicles display prominent horizontal ribbing which encircles the icicle to form a series of progressively smaller rings toward the tip. Each ring is separated from those surrounding it by shallow constrictions in the ice. Typically, rings extend outward less than a centimetre (half an inch). During active icicle growth, the rings are initially composed of fragile, thin ice plates growing randomly outward, but the spaces between them are soon filled with downward flowing meltwater.

Vertical ridges form during renewed growth after a period of dormancy. Melt water streams down the icicle exterior, initially laying a thin track of ice. Continued flow will add additional ice along the same track for as long as the track surface does not completely freeze. Tracks have widths approximately that of the meltwater stream percolating down the icicle. They can build ridges outward half a centimetre (a quarter inch) or more before the water shifts to another track.

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