An icicle is a spike of ice formed when water dripping or falling from another object freezes. Typically, icicles will form when ice or snow is melted by either sunlight or some other heat source (such as heat leaking from the interior of a heated building), and the resulting melted water runs off into an area where the ambient temperature is below the freezing point of water (0°C/32°F), causing the water to refreeze. Over time continued water runoff will cause the icicle to grow. If an icicle grows long enough to touch the ground (or its corresponding ice spike growing up from the ground) then it is called an ice column.

Icicles can pose both safety and structural dangers.

Ice hanging from buildings may break away and fall and pierce the flesh of pedestrians below or cause damage to other objects below. In addition, ice is relatively heavy and the tendency of icicles to form on overhangs, eaves, and gutters may cause more stress on those structures than they were designed to bear, causing them to break or tear away from the building.

Similarly, icicle buildup on trees may cause branches to break.

Everyday ice and snow has a hexagonal crystal structure. Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases.

Rime is a type of ice formed on cold objects when drops of water crystalize on them. This can be observed in foggy weather, when the temperature drops during night. Soft rime contains a high proportion of trapped air, making it appear white rather than transparent, and giving it a density about one quarter of that of pure ice. Hard rime is comparatively denser.

Aufeis is layered ice that forms in arctic and subarctic stream valleys. Ice frozen in the stream bed blocks normal groundwater discharge and causes the local water table to rise, resulting in water discharge on top of the frozen layer. This water then freezes, causing the water table to rise further and repeat the cycle. The result is a stratified ice deposit, often several meters thick.

Ice can also form icicles, similar to stalactites in appearance, as water drips and re-freezes.


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