All those ice bucket challenges going on right now have got me thinking about ice and how it creates some beautiful weather phenomena.
The most obvious form that falling ice takes is hail. But it's the magical sounding diamond dust that I'll start with this week.
In simple terms, diamond dust is a cloud composed of tiny ice crystals that forms near the ground. In a way, it's similar to fog, but instead of being made up of liquid water, diamond dust forms directly as ice. Also, fog is dense enough to dramatically reduce visibility whereas diamond dust often doesn't affect it.
So how does diamond dust form in an otherwise clear blue sky? It usually happens when a temperature inversion is present above the ground - so instead of temperature dropping with height the air aloft is actually warmer than the air at the ground. Warmer air can hold more water vapour so when it mixes with the colder air at the surface it transfers this extra moisture which freezes into ice crystals if the temperature is low enough. If it's not cold enough you would get plain old fog.
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The magic number for the freezing to occur depends on how clean the air is. In theory, the temperature only has to be below freezing for ice to form but -13C seems to be the magic number. The reason is that while ice always melts if the temperature rises above freezing, water doesn't always freeze when the temperature drops below. Instead, water can exist in a supercooled state until something, such as a tiny particle of dirt in the air, encourages it to freeze.
When the air is very clean it takes much lower temperatures before the ice crystals form, in places like Antarctica diamond dust needs temperatures of about -30C! As cold as that sounds, it's a typical temperature for the Antarctica Plateau, where diamond dust is observed 316 days a year.
Diamonds in the sky
In case you were wondering why it's not called an ice fog - well, that is something different. Ice fogs start out life as normal fogs with drops of liquid water that then freeze into ice. Diamond dust, on the other hand, forms directly into crystalline pieces of ice, similar to tiny, thin six-sided pencils. On a sunny day these catch the sunlight and shine like a sprinkling of tiny diamonds in the sky, hence the name diamond dust.
What's more, if the crystals are orientated in the right way they can give rise to brilliant halos and other optical phenomena. These displays are even more spectacular than those you see under a cirrus cloud because the tiny ice crystals are all around the observer as layers of diamond dust can range in depth from 20 metres to 300 metres.
While diamond dust is pretty rare on these shores, the next icy weather phenomenon occurs so frequently that you've definitely seen it, almost on a daily basis. Called virga or fallstreak, it's a trail of precipitation that falls from underneath a cloud that evaporates before it reaches the ground.
OK, so sometimes virga will have started life as falling rain but high up in the colder reaches of the sky it's falling as ice. Named from the Latin for rod, twig or branch, virga can leave striking patterns in the sky, especially during a sunset, from light wisps to more defined shapes, depending on how the upper winds blow the ice crystals. The lines of virga can then look like commas or even resemble the trailing tentacles of a jellyfish cloud.
Let's finish up with something much rarer than virga or diamond dust, indeed it's the mysterious sounding megacryometeors. These are giant ice balls that fall from the skies, but other than that we don't know much about them.
No ordinary hailstone
We do know they are not just giant hailstones. They may have some similar characteristics but their formation is completely different, for a start they don't fall from cumulonimbus clouds. In fact, they often fall from clear blue skies... as happened in January 2000, when ice chunks weighing up to 3kg rained on Spain out of cloudless skies for ten days.
Since then more than 50 megacryometeors have been recorded, varying in weight from 0.5kg to one in Brazil that weighed in at 50kg. Apparently chunks of ice about two metres in size fell in Scotland back in August 1849. Being so large, megacryometeors are obviously dangerous and can leave small impact craters on the ground, leading to some confusion with meteorites.
So what's the story? Some people reckon these giant ice blocks must have fallen from aeroplanes. It's possible but it can't be from the toilets or they'd be blue in colour due to the disinfectants. The strange thing is that megacryometeors have been reported prior to the invention of aircraft. If not from planes then how did these ice blocks form, could strong high level winds really have supported them for days as they grew? For now, the answer remains in the sky.