In order for snowflakes to form, we need cold clouds. Cold clouds are in air that is at freezing point (zero degrees centigrade or thirty two degrees Fahrenheit) or below freezing. The first stage of the life cycle of a snowflake is when water vapour condenses into a small droplet. As more and more water vapour condenses it will stick to the droplet it will grow, a process that is known as coalescence. Alternatively, the snowflake can grow through a process known as deposition, which is where water droplets freeze on to the snowflake.
As more and more water vapour condenses the rate of coalescence and/or deposition will increase and the snowflake will get bigger and bigger and heavier and heavier, until it falls from the precipitation cloud where it was formed. Each snowflake has a unique shape that will depend on the water vapour content and the air’s temperature as the snowflake is forming. In the right conditions, where the temperature is below freezing and there is an abundance of water vapour, the snowflake will grow six evenly spaced branches. These branches attract even more vapour hence making the snowflake even bigger.
As the snowflake falls in to somewhat warmer air it will melt, however this process acts like a glue and the snowflake will get larger. In addition, the snowflake will pick up even more water vapour as it falls, making it even bigger. If the snowflake melts too much before refreezing upon reaching the earth’s surface, it will fall as sleet as opposed to snow.
Provided there is some moisture and some way to lift or cool the air it cannot be too cold to snow. Snow will even form in extremely low and subfreezing temperatures. Heavy snowfall generally happens when there are relatively warm temperatures near the ground as warmer air holds more water vapour.
If the ground temperature is above freezing the snow can still reach the ground, but in order for this the conditions need to be just right. If the ground temperature is five degrees Celsius (forty one degrees Fahrenheit) or more it is highly unlikely the snow fall will reach the ground. In order for the snow to settle the ground temperature needs to be below freezing. If the ground temperature is above this the snowflakes will simply melt away as they hit the ground.
Now to finish, here’s some trivia about snow;
i) Snow is white because it reflects beams of light that are scattered by billions of ice crystals and come bouncing straight out again. Any light absorbed is done so over the wavelengths of visible light hence giving it its white colour.
ii) A single snowflake must be observes at four specific sites (including the London weather centre) between midnight on the 24 December and midnight on the 25 December for it to be declared a white Christmas. Based on the current rules England’s last white Christmas was 1999.
iii) In February 2007 orange snow fell in Omsk, Tomsk and Tyumen in Siberia. No-one really knows but it is believed the orange snow was caused when it mixed with sand particles during a sandstorm.
iv) In England there is “the wrong type of snow”. In 1991 the snowfall was too soft and powdery for British Rail to clear off the railway lines with their snowplough, and British Rail called this the wrong type of snow, which the papers soon picked up on.
v) The world’s largest snow castle is in Kemi, Finland. Every winter it is rebuilt and it contains a hotel, a restaurant and a chapel.
vi) When taking pictures of snow, the camera’s meter is often confused and will under expose making it grey. In order to make the snow look white you should take a meter from the snow and, adjust the exposure to dial in 2 stops of positive exposure compensation.