Icicles are a classic symbol of cold winter weather. In order to form, however, they need a mix of both warm and cold conditions.

These hanging pieces of tapered ice develop when the air temperature is below freezing, but there is enough heat from the sun – or in the case of a building, a  poorly  insulated roof – to thaw some snow. As the melt water runs off the edge of a surface, it re-freezes in the cold air. Starting with only a few water droplets, an icicle can begin to form. Over time, as melt water continues to drip and re-freeze, an icicle gains both layers of thickness and length.

Often seen forming along the edge of roofs, icicles can also be found on tree branches, power lines, and rocks where water seeps out of the ground. The size and shape of an icicle, according to experts at the National Snow and Ice Data Center, depends on a few different factors. These include the shape of the surface on which it forms and the number of different directions from which melt water approaches the growing icicle.

Icicles can range in size from a few inches to tens of feet.

Icicles in the sunshine. Credit: MLE