Clouds may look white and fluffy, but they can get quite heavy with water vapour. When clouds get so full that they cannot hold onto any more water vapour, the water droplets fall back to earth in the form of rain, hail, sleet or snow. Rain occurs when water droplets in clouds join together to form drops between two and 6.5 millimetres across. Larger drops are heavier so they may fall as rain.
Hail forms when droplets of water freeze. As they are tossed about by rising and falling air, more layers of ice can form around the hailstone causing it to grow. Sleet, or frozen raindrops, forms when snow melts and refreezes or raindrops freeze as they fall through air colder than 0°C. Snow forms when water vapour in the air chills very quickly into ice. Snowflakes are usually six-sided ice crystals and can be found in many different patterns depending on the temperature and amount of dust in the air. The air temperature must be less than 0°C for snow to form. Colder temperatures produce snowflakes with sharper tips and more complex shapes. Dew is formed when water vapour condenses near the ground and forms on cold surfaces. Frost forms when the surface temperature falls below freezing (0°C) allowing ice crystals to form directly out of dew-laden air on cold surfaces. It's amazing!
Many people believe that all snowflakes are unique, in that no two snowflakes will be alike. (No one has ever compared every snowflake to know for sure, of course!)