Nancy Head, age 12, of Springfield, Ore., for her question:
What causes the different types of snow?
Snow is formed from minute fragments of frozen vapor, and billions by these icy particles gather together to form a single snowflake. We all have heard that the design of each snowflake is unique. And it also seems that the quality of snow in every snowfall is unique.
The snow may fall like great pompom balls of fluffy white cotton. It may sift down in small lacy flakes, and sometimes the ground is crusted with granular snow made of solid white pellets. If you examine it closely, every snowfall appears to bring snowflakes of a different kind. This is because the weather conditions, the air above the Earth, are always changing and are perhaps never quite the same.
Hail is formed from frozen liquid, and many people suppose that snow also forms from rain, cloud droplets or other liquid moisture in the air. But this is not so. Snow forms from water vapor, a gas made of separate molecules. A Certain amount of vapor is present even in the driest desert air. But even in air saturated with vapor, the water molecules are widely separated from each other.
These small particles must reach the temperature of the freezing point of water before they can become snow. And billions of them must get together to form a single small flake of snow. They can do this only when there are microscopic specks of dust or sand, soot or smoke in the air. The frozen fragments of vapor need these solid nuclei around which to gather.
The size and the shape of the snowflake depends upon several weather factors. The air must contain plenty of vapor, the temperature must be below the freezing point of water and there must be microscopic nuclei in the air. Very cold air tends to hold less water vapor, so not much snow can form and, the flakes tend to be hard and granular. The air may be loaded with vapor when the temperature is just below freezing and countless trillions of water molecules become frozen. There is plenty of material to make lots of whopping snowflakes. Dozens of the flakes join together to make big fluffy pompoms, and the air seems full of drifting white feathers.
It is harder for snowflakes to form when the air is very cold, and it is sometimes said that a winter’s day is too cold to snow. But there is always a little vapor in the air, and fine granular snow can fall in the coldest weather. Snow has been recorded in Alaska when the temperature was minus 52 degrees. However, this snow must have been made of hard little granular pellets.