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Ode to Icicles
Fri Feb 14, 2003
Listen in RealAudio 
Hi, I'm Bryan Yeaton for The Weather Notebook.
Few things signify winter to folks like a row of icicles reaching down from a roofline. They
can provide a prism for holiday lights, a treat for exhausted sledders, but can also become
deadly falling daggers. What creates them?
The process starts when above-freezing temperatures, solar heating, or interior heat loss melt
rooftop snow. If, while flowing off the roof edge, meltwater encounters sub-freezing surface
temperatures, it crystallizes, becoming the icicles'roots.
Water dripping across these roots freezes in progressive layers to produce long, typically
cone-shaped icicles. Growing both downward and outward, icicles build horizontal and vertical
ridges on their surfaces.
Extended growth produces a series of prominent, horizontal rings separated by a narrowing of
the icicle. The rings, initially fragile, thin ice plates, ultimately become rounded peaks
usually less than a half-inch high.
Vertical ridges build as melt-water trickles down the icicle laying a thin track of ice.
Continued flow along the same track adds more ice until the track surface completely freezes.
These ridges may grow out a quarter-inch before shifting the water flow onto another
track.
At the tip, growing icicles are primarily liquid water with a pendant drop on the end.
Icicles can grow to several yards in length, but even after growth has stopped, they continue
to change as melting and sublimating surface ice alter and smooth their surface
features.
The Weather Notebook is a program of the Mount Washington Observatory, and funded by Subaru,
and The National Science Foundation. Additional funding comes from Davis Instruments, maker
of the Vantage Pro weather station, on the web at www.davisnet.com.
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