Jewels from the Sky
You have just braved the hot May sun.. And you long for a refreshing glass of lemon juice with tinkling ice cubes in it. That’s a cool thought! The ice cubes of course, come from the refrigerator, but did you know that nature is a mega refrigerator that churns out a tremendous amount of ice? We are talking about snowflakes! In many parts of the world the winter months bring this white, powdery, innocuous stuff that blankets the earth in a mantle of pristine white. About one septillion (1, 000, 000, 000, 000, 000, 000, 000, 000 or a trillion trillion) snow crystals drop from the sky every winter!
Henry Thoreau – the American poet – once compared snow crystals to real stars. Why? Well, snow crystals are actually miniature works of art. And no two crystals have been found to be the same. A serious scientific study of snow crystals started only in the 1800's when a self-taught naturalist called Wilson'Snowflake' Bentley examined and photographed over 5000 of these fragile beauties. He discovered that snow crystals came in a variety of shapes – needles, plates, hollow columns, capped columns and many more.
The reason for this amazing diversity is to be found in the process of a crystal’s growth. Water vapour in a cloud condenses around a speck of dirt or even a bacterium. As many as 180 billion molecules of water join together to form a crystal. The shape and size of the crystals is determined by the temperature and humidity of the clouds. In the highest clouds where the temperature is lowest the snow crystals are shaped like hollow columns. At slightly warmer temperatures, in the middle cloud layers, they are shaped like columns or flat hexagons. (See Box)
But whatever the diversity of shapes every snow crystal starts out as a hexagon because they have a specific bond geometry. This mystery is hidden deep within the molecular structure of water. Water molecules bond only at 120 degrees so ice crystals have a hexagonal shape.
This image (X-ray vision image) shows the structure of a water molecule. The red dots are oxygen while the blue dots are hydrogen. But this is merely the beginning of a rather long journey to the ground. The crystals may join together after leaving the cloud or collide with other crystals. And if the air beneath is warm they might melt into raindrops.
Wilson Bentley examining a snow crystal under a microscope.“Every crystal was a masterpiece of design and no design was ever repeated,” — Bentley (1925
Differences in temperature have a huge role to play in the formation of snowflakes.
Thin plates: form when the temperature is between 0 to –4 degrees C.
Needles: columnar crystals grow so long and thin that they look like needles. They occur when temperatures are between –4 to –10 degrees C.
Sector plates: ice ridges divide the plate like arms into sectors. They are flat and thins plates of ice and form at temperatures varying between –10 to –12 degrees C.
Dendrites: dendrite means tree-like. It describes the multibranched shape of these crystals that have six symmetrical main branches and many other side branches. They from at –12 to –16 degrees C.
Hollow Columns: these are often the main constituents of many snowfalls. They are hexagonal columns shaped like a pencil. They form when the temperatures dip between –22 or –50 degrees C.
What happens once the snowflakes reach terra firma? Well in most places it melts during summer. But depending on climactic conditions snow can also accumulate on the ground. Many parts of the northern hemisphere remain snowbound for the entire year. The part of earth where water remains in a solid form is called the Cryosphere. It includes:
This frozen world is the repository of our planet's geological history. Scientists can decipher earth's history by collecting data from ice. They drill hollow tubes into thick ice on glaciers or ice sheets and derive long ice cylinders, called ice cores. Ice cores give a superbly detailed account of what was happening in the world eons ago. Each layer in the ice core corresponds to one year or season. Everything that fell with the snowflakes – dust, ash or atmospheric gases – remains preserved in the ice. These frozen archives give an unprecedented view of ancient global climate. Latest expeditions to the Peruvian mountains in South America may shed light on a global climate change that occurred over 5,000 years ago. More importantly, these records allow researchers to predict the impact of events – volcanic eruptions and global warming – that could strike us today.
Future – Etched in ice
Our future may be dependant on this icy world. Changes in snow and ice cover affect temperature, sea level, ocean currents and storm patterns. Snow and ice reflect 60-90 per cent of solar energy back into space, helping to keep the earth cool.
Reduction of snow cover and sea ice may lead to global warming, as more solar energy is absorbed. Studies reveal that global warming will affect Polar Regions most. Scientists have already noted changes in the Arctic-spring and summer sea ice have declined and warmest temperatures in 400 years were recorded recently.
Snow and ice store freshwater and act like earth's water bank. Scientists say that higher temperatures have led to a decline in the moisture content of snow packs. This finding has a crucial implication on future water resources. Countries in the northern hemisphere, where around 70 per cent of water comes from snowmelts are already facing a water crisis. The past, present and future of the earth is linked to snow. Can we then afford to brush it off casually?