
Snowflakes: Pure and crystal clear
Snowflakes are transparent ice crystals formed around dust or other
small particles in the atmosphere when
water vapour condenses at
temperatures below the freezing point. Partly melted crystals usually
cling together to form snowflakes, which may, in rare cases, grow in
size up to seven to ten centimetres in diameter.
Structurally, elemental crystals of snow occur in any of various
hexagonal forms, depending upon exact atmospheric temperatures during
formation.
Among these six-sided, basically symmetrical both sides similar
shapes are needle, columnar or stud, plate-like, and star-shaped
crystalline types. Because of the infinite variability of weather
conditions, every snow crystal is unique in its precise configuration
shape and it is the large number of reflecting surfaces of the crystal
that make snow appear white.
The longer rays that constitute the arms of the six-rayed star are
generally hollow tubes. They are evidently built up by additions to the
edge of an original crystal.
Ice crystal process
Ice crystals grow larger through a process called the ice crystal
process, or Bergeron process, after the Swedish meteorologist Tor
Bergeron, who proposed that raindrops begin as ice crystals. If the
temperatures inside a cloud are below freezing, then liquid cloud
droplets and ice crystals may co-exist.
Liquid water droplets existing at below freezing temperatures are
called super cooled droplets. If super cooled droplets and ice crystals
come close to each other, then water vapour may leave the liquid
droplets and freeze on to ice crystals.
In this manner, the ice crystals grow larger at the expense of the
surrounding super cooled droplets. As ice grows larger by the Bergeron
process, many become heavy enough to fall. Falling ice crystals may
collide and stick to other ice crystals, forming a snowflake.
Ice crystals may also collide with super cooled cloud droplets,
changing the liquid droplets into ice on contact. These ice particles
may even stick together, producing a chunk of icy matter called graupel.
Role of temperature
Snowflakes are either single ice crystals or clusters of ice
crystals. Large snowflakes generally form when the temperature is near
zero degrees Celsius. Because of this temperature, the flakes are partly
melted and stick together when they collide.
Rain is falling drops of liquid water with diameters of 0.5 mm or
greater. Drizzle is falling drops of water smaller than rain. Some
raindrops are cloud droplets that grew by coalescence (combining) and
fell down as rain.
However, the majority of raindrops that fall over the middle and
higher latitudes begin as snowflakes or graupel.
As they fall, they enter warmer layers of air and melt, forming rain
drops. If the falling rain evaporates
(vaporise) before reaching the
ground, it forms streaks in the sky called virga. In the cold air of
winter, falling snowflakes and graupel may reach the ground without
melting and accumulate as snow. Graupel that reaches the ground is
called snow pellets.
If rain falls into a deep, sub-freezing layer of air near the ground,
some of the rain may freeze into tiny ice pellets called sleet. When
rain falls in to a shallow, sub-freezing layer of air near the ground,
it may remain as a super cooled liquid and freeze upon striking a cold
surface, forming freezing rain.
Freezing rain can coat everything with glistening ice, the weight of
which can break tree branches and snap power lines.
The measurement of snow fall is usually stated as depth in
centimetres, or other units, of newly fallen snow. It is also measured
in terms of the depth of the layer of water that would result if the
snow was to melt. 2.5-3cm of snow melts to 2.5 cm of water.
Compiled by Janani Amarasekara |