IF YOU were in Dublin at around 5.15 pm on Tuesday last you may have seen, as Thomas Gray described it,
Iron sleet of arrowy shower,
Hurtle in the darkened air.
It was, of course, a hailshower but the hail stones were the largest that I, for one, have seen for quite some time. Many of them were well over a centimetre in diameter, conical rather than the usual spherical in shape, and in colour the purest brilliant white.
Large hailstones fall from the tall, towering cumulonimbus clouds that are formed when the thermal structure of the atmosphere is unstable, causing it here and there to bubble upwards like boiling water in a kitchen saucepan.
These giant cauliflower shaped clouds, in which also thunderstorms occur, are a brilliant white when viewed from a distance, but are black and menacing from underneath. They extend to great heights, to levels in the atmosphere where, even on a summer day, temperatures are well below zero. They contain powerful swirling currents of air, massive updraughts and down draughts that carry with them anything that lies within the cloud.
A hailstone begins its life as a small particle of ice. On its travels up and down in the tumultuous anarchy of a cumulonimbus, it encounters tiny "supercooled" raindrops drop of water that have remained liquid at temperatures well below what we normally refer to as the "freezing point": these droplets freeze on impact with a hailstone, enlarging it. An individual lump of hail may make several journeys up and down a cloud, accumulating an extra layer of ice with each successive trip. Eventually, of course, the hailstone becomes so heavy that it must succumb to gravity, and fall to earth.
The layers of ice on a hail stone are usually arranged concentrically, giving a structure rather like an onion, and are often alternating layers of clear and "milky" ice. The latter has its origins in the very rapid freezing of very small super cooled drops of water, a process which traps tiny bubbles of air within the ice to make it cloudy, or even nearly white. When the encountered water drops are large, however, they tend to solidify more slowly; the air escapes, and the ice is almost crystal clear.
Tuesday's Dublin hailstones, to judge by their pristine whiteness, were formed in a cloud where droplets very small in size predominated. Moreover, although it was not possible to check, conical shaped hailstones are usually found to consist of a cone of white or cloudy ice, standing, as it were, on a clear transparent base.