How
ice crystals are formed
Air temperature decreases with
height, and the amount of water
vapour that air can hold is proportional
to the temperature. In other words,
the warmer the air, the more moisture
it can hold. As air rises —
by convection or to cross a mountain,
for example — its temperature
falls and if it falls to below
the “dewpoint” temperature
some of the water vapour will
condense into droplets and a cloud
will form. If the temperature
falls below freezing, the water
vapour changes directly to ice
without passing through the liquid
phase.
This
sounds straightforward, but it
is not quite so simple. Water
vapour will not condense unless
there are minute particles —
cloud condensation nuclei —
onto which the droplets can form.
These are usually abundant over
land, but even so it is common
for air to be slightly supersaturated.
Ice crystals will not form unless
there are freezing nuclei present,
such as fine clay particles, onto
which water vapour can freeze.
These are much less plentiful
than condensation nuclei and most
do not initiate freezing at temperatures
above about 14°F (–10°C).
Consequently, most clouds capable
of releasing precipitation contain
a mixture of ice crystals and
supercooled water droplets —
liquid droplets that are several
degrees below freezing temperature.
Once ice crystals begin to form,
they do so at the expense of the
supercooled droplets. Water evaporates
from the droplets and is deposited
onto the ice crystals.
Where
the temperature is between about
14°F (–10°C) and
–4°F (–20°C)
a cloud contains approximately
equal amounts of water droplets
and ice crystals. Ice crystals
predominate where the temperature
is below –4°F (–20°C)
and there is almost no ice in
a cloud that is warmer than 14°F
(–10°C). If the temperature
is 75°F (24°C) at the
surface, it will be 14°F (–10°C)
at about 20 000 feet (6 km). That
is about one-third of the way
from the top of an average cumulonimbus
cloud capable of delivering a
heavy shower.
From
ice crystals to snowflakes
Because the shape of its molecule
determines the way it crystallises,
water invariably freezes into
a hexagonal crystal. This grows
larger as more water freezes onto
it, but it preserves its hexagonal
shape. Ice crystals vary in shape,
but whatever their shape they
always have six sides. Crystals
are classified according to their
shape. The classification recognises
seven crystal types, with a standard
symbol to designate each of them,
as well as three more symbols
for graupel (soft hail), sleet
(minute ice crystals, not the
mixture of rain and snow that
is called sleet in Britain), and
hail.
Once
ice crystals have formed they
start to fall and as they drift
this way and that, they collide.
When large ice crystals collide
they often shatter. Such collisions
release tiny splinters of ice
that act as freezing nuclei for
the formation of more crystals.
When large crystals collide with
smaller ones they tend to stick
together. That is how ice crystals
grow into snowflakes. The process
works best if the temperature
is above about 23°F (–5°C),
so there is a plentiful supply
of supercooled water droplets.
Water droplets also collide with
the ice crystals, forming a thin
layer of water that freezes when
another ice crystal arrives, so
the water acts as an adhesive.
Really big snowflakes form at
this temperature, from smaller
snowflakes that join by interlocking
and thereby preserving the six-sided
symmetry. Colder clouds produce
smaller snowflakes. Powder snow
— the consistency that is
best for skiing — forms
in very cold cloud.
The
snow pack
If the snow falls through air
that is warmer than freezing it
will start to melt. If it falls
for more than about 820 feet (250
m) through air warmer than about
35°F (2°C) the snow will
melt and fall as rain. If it falls
as snow it will not settle unless
the temperature in the air below
the cloud base and on the ground
is below about 39°F (4°C).
Once
snow has fallen, it packs together
under its own weight and becomes
denser by squeezing out the air
between grains. The character
of the pack depends on the amount
of liquid water that is trapped
between crystals. Wet snow contains
3–6 percent of water. This
makes it dense and heavy. It is
excellent for making snowballs,
but not so good for skiing. Wet
snow of this type falls on the
windward side of coastal mountain
ranges where it is moist air approaching
from the sea that produces the
climate, for example in western
Norway and in the western Alps.
Powder
snow, which is much drier and
therefore lighter, falls farther
inland, for example in central
Scandinavia, the central Pyrenees,
and the central and eastern Alps.
Freshly fallen powder snow consists
of small flakes with tiny pockets
of air between them. It is very
light and loosely textured, but
the spikes on the snowflakes interlock,
so powder snow has considerable
cohesion. Skiing across it has
the effect of packing it very
hard in some places, but throwing
it into much softer heaps in others.
In this condition it is sometimes
called “crud”.
During
the day the snow surface may warm
to above freezing. The snow will
start to melt, so a thin layer
of water covers the surface, but
as the temperature falls again
this water will freeze to form
a crust of ice coating the layer
of soft powder snow. Freezing
rain — rain comprising supercooled
raindrops that freeze on contact
with a cold surface — falling
on fallen snow will produce a
similar crust. The crust may be
so thin that it will not support
the weight of a person or so thick
that skiers pass over the top
of it. Between these conditions
the crust may be thick in some
places but thin in others, giving
the snow an uneven consistency
and making skiing difficult. Towards
the end of a prolonged dry spell
when the surface has been thawed
and frozen several times, the
old snow is called “sugar
snow” because of its texture.
Repeated melting and freezing
of a snow crust compacts the snow,
producing ice.
More
prolonged melting increases the
proportion of liquid water. Interlocking
snow crystals then turn to larger,
shapeless, ice grains. “Slush”
is the resulting mixture of ice
grains and water. The appearance
of slush usually heralds the complete
melting of the snow, but if slush
freezes it forms a rough surface
of solid ice that is useless for
skiing.
Physical
changes can also take place near
the base of a snowpack. Even when
it is densely packed, a layer
of snow contains tiny pockets
of air between the individual
crystals. At the base of the pack
crystals may sublime — change
directly into water vapour —
into these air pockets. The water
vapour is then immediately deposited
once more as ice crystals. These
crystals are called “hoar”
— not to be confused with
hoarfrost. They are dense, but
they pack loosely and can flow
like a liquid. The process of
sublimation and deposition may
then spread upward until a substantial
part of the base of the pack has
been transformed. The snow surface
has not changed, but the snow
beneath the surface layer is now
in a potentially dangerous condition
because the pack is no longer
securely bonded to the ground
beneath it. It can move, triggering
an avalanche.
AirportMonitor
is a dynamic, interactive display
of air traffic and flight information
available on many airport Web
sites. AirportMonitor tracks flights
from approximately 100 miles from
the terminal right down to the
runway. View information such
as flight origin, destination,
aircraft type, altitude, and flight
ID. AirportMonitor has a ten minute
delay and filtered information
for security purposes.
