Your Winter Service Berlin
Ice kernels that have a diameter of approx. 5 mm are referred to as hail; if the diameter is shorter it is referred to as sleet or grains. Hailstones can grow to the size of a tennis ball, but this does not happen very often. Hail is a kind of precipitation that only occurs in connection with a thunderstorm.
The upwinds and downwinds within a thundercloud make it impossible for snowflakes to fall to the ground. If they get locked into an upwind they are transported back up into higher regions where cold water can collect and can freeze and become ice. The kernel becomes heavier and falls back downwards. If it is swept up in an upwind again it drifts upward again and can get bigger. This process can be repeated several times until the hailstone actually becomes too heavy for the upwind and is caught in a downwind and falls to the ground.
Aside from the strength of the upwinds, a further precondition for large hail stones is a sufficient amount of moisture within the cloud.
According to how big they are hailstones hit the ground with great kinetic energy; huge hailstones with a weight of up to 0.5 kg can have rates of falls of more than 100 km/hour. That is why they are so dangerous for farmlands. They can hit crops on fields or orchards with such force that the harvest of a whole year is destroyed. Large hailstones can even dent cars and can damage other objects. Contrarily, icy and slippery road conditions only last for a short period of time, as the hailstones generally melt quickly when temperatures are above freezing.
Agriculture has developed several ways to reduce hail damages. One area that has been repeatedly devastated by hailstones is Lake Constance; due to the size of the lake and the great amounts of water that evaporate, the thunderclouds form large amounts of humidity resulting in very large hailstones. The most important counter measure is the insertion of silver iodide into the thunderclouds. Silver iodide generates ice kernels even at low temperatures so that the hailstones accumulate quicker and in larger numbers. As a result they are not as big and do less damage and frequently already melt when they are in the air and hit the ground as raindrops.
The silver iodide does not damage the environment and can be shot into the clouds with hail rockets and distributed under the clouds better, from where it is then transported up into the clouds with upwinds.
The mechanical protection of fields and farmlands by protective nets is restricted to certain local areas.