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| Storm clouds form over the Menai Straights, North Wales. |
On my walk to work recently, I passed a woman broadcasting
her thoughts on the weather to the world, and the person on the other end of
her phone: “We’ve had sleet, which is
too warm for snow; and we’ve had hail, which is cold enough for snow; so we
should have had snow!”.
I’d missed the sleet, but I wouldn’t forget the hail in a
hurry; pea sized missiles had drawn a deafening clatter from the roofs of cars,
and had driven a pavement-full of pedestrians to shelter in doorways from the
prickly assault. Snow had been the last
thing on my mind, but for Mobile Telephone Lady, it was clearly a concern.
So why the apparent contradiction of near-simultaneous sleet
and hail? The answer lies in the clouds
that spawn them:
In Britain we’re used to rain, whatever the season, but most
of that rain starts life as ice, which only melts when it reaches warmer air
closer to the ground. The first few
flecks of a snowflake start life in freezing clouds, growing until their weight
starts to drag them earth-ward; they often collide with neighbours on the way
down, growing more, and taking on the familiar chaotic shape. The downward journey continues, until the air
is above freezing, and the flake begins to melt. First comes sleet - a part-melted mixture of
icy rain, and then (given enough time) a fully-fledged raindrop.
Hail, on the other hand, has a much more violent beginning. It all starts with ‘Super-cooled Water’: liquid
water vapour whose temperature, counter-intuitively, is below 0oC. Super-cooled vapour forms because water needs
something to freeze onto - a microscopic particle will do, but in the
atmosphere they can sometimes be hard to find, so water remains in liquid form,
ready to grow hailstones.
To create your hailstone you need a storm cloud; one large
enough to sustain powerful gravity-defying up-draughts, and one full of
super-cooled water. A passing fragment
of ice, or a frozen raindrop, forms the nucleus of the hailstone, but the up-draughts
prevent it from falling. The stone is dragged
through our super-cooled vapour again and again, adding layer upon layer of ice,
like the rings of an onion, and creating a larger and larger hailstone. Eventually its weight defies the up-draughts,
and the journey towards the ground begins.
Back on the ground, Telephone Lady was oblivious to the formation
of the hailstones and sleet above her, even though it provided the answer to
her problem: The violent birth of a
hailstone creates a dense mass of ice that takes much longer to melt than the
less substantial snowflake - the latter may be larger, but there’s very little
ice in the tiny filaments that form a snowflake. As temperatures nudged above freezing, close
to the ground, the snowflakes were already melting into sleet, but the dense
hailstones would take much longer.
The final act was played out on the pavements: The sleet
returned to its watery beginnings as soon as it touched-down, but the
hailstones remained, as layer upon layer of ice slowly melted away. Not that Telephone Lady noticed; she was already
too engrossed in another conversation.
Image copyright: Christopher Lee, 2008.
Text copyright: Christopher Lee, 2012.
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