Such a question can even be surprising. What's so difficult? Because ice icicles appearing on the roofs, is ordinary frozen water. But not so all is simple. The fact that the icicles just fall from the roof during thaws, as they form and grow at low temperatures. According to all the laws of physics of water for the formation of icicles under such conditions can not be. So where did it come from?
Causes of water, "building material" for future icicles can be different. For example, a layer of snow on the roof can be heated from below by a warm attic. Snow gently melts, and thin streams of water flow down to the ledge. Here they are on the frosty air, freeze and become icicles.
Could be another reason. On the southern slope roof snow melts under direct sunlight, even a light frost. But as soon as the water jets out of the shade, they turn into ice. In both cases, because there is a constant "feeding", icicles are growing gradually. Sometimes they can reach impressive sizes.
But here comes the thaw, when everything begins to melt. At that point, icicles, where it is attached to the roof, ice needle melts particularly hard: the effect of warm air is added to the warmth of the sun-warmed roof. At some point under their own weight icicles break off and fall down.
For passers-by falling ice needles is considerable danger. So once again it is worth mentioning: in early spring should be careful not to come close to the homes and often to glance up.
We also recommend you read:
Chapter from the book "Entertaining physics" - "the Task of the ice icicles"