Snowflakes can be formed by the collision of ice crystals within clouds. This is known as the process of aggregation and usually accounts for the larger snowflakes that are seen to fall. Smaller snowflakes are formed by the Bergeron-Findeisen process. Super cooled water droplets (i.e. those with a temperature below freezing) are 'picked up' by the falling ice crystals. The ice crystals grow at the expense of the water droplets. For snow to reach the ground the air temperature must be no more than 2°C. One would expect the falling snow to melt as soon as the temperature rises above freezing, but this is not so. As the melting process begins, the air around the snowflake is cooled. At temperatures above 2°C the snowflake will melt to become 'sleet' or rain. In the UK, the heaviest falls of snow tend to occur when the air temperature is between zero and 2°C.