Weather at Sea by Simon Rowell

Author:Simon Rowell
Language: eng
Format: epub
Publisher: Fernhurst Books Limited

Forecast chart with the 528dm thickness line showing the edge of the blue ‘snow’ zone, the 546dm line the edge of the red ‘rain’ zone (Met Office, contains public sector information licensed under the Open Government Licence v1.0.)

HAIL

Hail occurs all over the world, whatever temperature the air at surface is. Hail is made up of rain drops that have been kept up above the freezing height by strong updrafts.

Strong updrafts are generated in deep squall clouds and tropical depressions in the Tropics, and in tall cumulonimbus clouds along cold fronts in the extra-tropical regions. Because the cloud is tall, a significant proportion of it will be above the freezing level, and any tiny droplets will actually be ice crystals or supercooled droplets. These will be bounced around in the strong updrafts and will accrete more and more ice, becoming larger and therefore heavier.

After a while the weight of the larger ice crystal (proportional to the cube of its radius) will become stronger than the upwards force provided by the updrafts (depends on its surface area, so proportional to the square of the radius) and it will fall as a hailstone. If it is large enough then not all of it will melt by the time it gets to the surface, and the un-melted bit is what we see as a hailstone.

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