The flood plain of the Rhine from Strasbourg to Mainz is flat and very wide. Level terrain stretches outwards on both banks for 20 miles or more, and provides a scene like that which Tennyson described:
On either side the river lie
Long fields of barley and of rye,
That clothe the wold and meet the sky.
You will find me nowadays on the edge of this elongated plain, on the great river's eastern flank, a little north of Mannheim, where the flat ground rises sharply to a range of wooded hills, the Odenwald. And from my window I see on these westward-facing slopes the vineyards from which, a month or two from now, the Riesling grapes will be harvested to produce delicious inexpensive wines.
It has been sultry, warm and humid here on the fringes of the Odenwald in recent days, and the Winzers, those who tend the Odenwaelder vines, are edgy. There is thunder in the air, and at this crucial time for grapes, a thunderstorm directly overhead could spell disaster. It is not the thunder, or the lightning, or the heavy rain that troubles them; instead it is the heavy hailstones that strip the vines and devastate their crop. A year's livelihood could be swept away in minutes.
Hailstorms in this part of the world can be different in scale to those in Ireland. A hailstone begins its life as a small particle of ice. In the tumultuous anarchy of a thundercloud it encounters "supercooled" raindrops - water that has remained liquid at temperatures well below what we normally refer to as "freezing point"; these droplets freeze on impact with a hailstone, and make it larger.
An individual lump of hail may make journeys up and down a thundercloud, accumulating extra layers of ice with each trip. The hailstone becomes so heavy that it must succumb to gravity, and fall to Earth.
Irish hailstones are typically "pea-sized". Here in the Rhine valley, the vigorous thunderstorms produces hail as big as grapes, and golf-ball size is not at all unknown. A heavy hailshower effects a footprint or a "streak" corresponding to the movement of the cloud that bears it; a typical "hail-streak" might be half a mile in width and five miles long, aligned with the prevailing wind. If a hail-streak crosses a vineyard at this time of year when grapes are vulnerable, unhappy economic times for the unfortunate Winzer lie ahead.