Snow is formed when temperatures are low and there is moisture - in the form of tiny ice crystals - in the atmosphere.
When these tiny ice crystals collide they stick together in clouds to become snowflakes. If enough ice crystals stick together, they'll become heavy enough to fall to the ground.
Precipitation falls as snow when the air temperature is below 2 °C. It is a myth that it needs to be below zero to snow. In fact, in this country, the heaviest snow falls tend to occur when the air temperature is between zero and 2 °C. The falling snow does begin to melt as soon as the temperature rises above freezing, but as the melting process begins, the air around the snowflake is cooled.
If the temperature is warmer than 2 °C then the snowflake will melt and fall as sleet rather than snow, and if it's warmer still, it will be rain.
The size and make up of a snowflake depends on how many ice crystals group together and this will be determined by air temperatures. Snowflakes that fall through dry, cool air will be small, powdery snowflakes that don't stick together. This 'dry' snow is ideal for snow sports but is more likely to drift in windy weather.
When the temperature is slightly warmer than 0 °C, the snowflakes will melt around the edges and stick together to become big, heavy flakes. This creates 'wet' snow which sticks together easily and is good for making snow men.
Snowflakes are collections of ice crystals that can occur in an infinite variety of shapes and forms - including prisms, hexagonal plates or stars. Every snowflake is unique, but because they join together in a hexagonal structure they always have six sides.
At very low temperatures snowflakes are small and their structure is simple. At higher temperatures the individual flakes may be composed of a very large number of ice crystals - making a complex star shape - and can have a diameter of several inches.