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