Why Does It Snow?
The question will be best answered by understanding the basic causes of changes in the three states of water from one state to the other. The three forms of water are: liquid (water), gas (water vapour) and solid (ice).
Precipitation as Snow:
In meteorology there is a phenomenon called precipitation. Depending upon different weather conditions, the water vapour present in the atmosphere is condensed to various forms such as drizzle, rain, sleet, snow or hail which fall on the earth surface due to gravity.
Snow is formed when atmospheric temperature goes below freezing point of 0°C and there is minimum moisture present in the air. The heaviest snowfall is normally between temperatures 0 °C to 2 °C. If temperature is warmer than 2 °C, then flakes will melt and fall as sleet. If still warmer it will fall as rain.
Ice crystals and Snowflakes:
The principle of the formation of ice or snow is the same, except that ice is formed from water and the snow is formed from water vapour in the atmosphere. Conversion of water to ice is direct. In the atmosphere, water vapour is first converted to water and then into snow.
Formation of ice or snow from water follows the same principle. When the cooling of water takes place the movement of water molecules slows down. At certain stage the molecules in cooled water are converted to ice crystals. Then, the crystals join together and form snowflakes which fall as snow.
Ice crystals can be in different shapes: stars, hexagonal planes, prisms. Ice crystals are formed at temperature of 0°C and below and also at higher temperature if the air is lifted and cooled. Depending upon the weather conditions, ice crystals attract water vapour and become bigger as flakes. The size and makeup of flakes differ under different weather conditions.
At temperature warmer than 0 °C, flakes melt on surface and become sticker and join together to form bigger and heavier flakes. Some common types of snowflakes are dendrite, thin plate and sector plate. It is the property of water that once the ice or snow is formed, there is always present some water on the surface of ice crystal and there is equilibrium between the two states. The lower the temperature the slower is the process of chance from ice to water. As the temperature rises the conversion to water becomes faster.
Weather Conditions and Snowfall:
The formation of snow is dependent upon the falling atmospheric temperature and presence of moisture or water vapour in the atmosphere. As the temperature goes down the molecules of water vapours become slower in movement and come together to form water but since these are very light they remain airborne.
If the weather is dry and cool the ice crystals are very fine and powder like. Such snow can be drifted away by wind. If the temperature is above 0°C the snow melts on surface and becomes sticky with the result that more crystals join to form flakes and over time the flakes become bigger and heavier.
When there is further lowering of temperature, the water molecules can change to larger drops and fall as rain. But, before rain could be formed, the lowering of temperature changes water to next tiny crystals of ice. Deep in the clouds water vapour condenses to water droplets. If the temperature reaches freezing point, water droplets are converted to ice crystals.
If the air does not have moisture and is dry it forms very small crystals of ice. If there is much moisture, water crystals join each other and form flakes of snow which fall on earth as snow. In the presence of moisture and slow lowering down of temperature the snow crystals join each other and form very large flakes which cause heavy snow fall.