Meteorologists have a challenging job. If they forecast snow, they need to determine when a moisture-laden air mass will pass over an area and if the temperature high at the snowflake-forming elevation will be below freezing. They also need to determine if the temperatures at lower elevations will allow the snowflake to fall to the…

If the snowflakes pass through a layer of warm air that is thick enough to melt them completely, then land on a cold Earth surface, the result could be freezing rain.

If the snowflakes pass through a thin warm layer of air, they could experience partial melting. When they exit the warm air, they will refreeze on the way down in the form of a tiny ice pellet. This is how sleet forms.

The formation of snowflakes high in Earth’s atmosphere does not guarantee snowfall on Earth’s surface. That will only happen if air temperatures are below freezing all the way to the ground, as shown in the accompanying illustration.

Although all snowflakes have a hexagonal shape, other details of their geometry can vary. These variations are produced by different temperature and humidity conditions through which the snowflake falls. Some temperature/humidity combinations produce flakes with long needle-like arms. Other conditions produce flakes with wide flat arms. Other conditions produce thin, branching arms. These different shapes…

The newly-formed ice crystal (snowflake) is heavier than the surrounding air and it begins falling. As it falls towards Earth through humid air, more water vapor freezes onto the surface of the tiny crystal. This freezing process is very systematic. The water molecules of the vapor arrange themselves so that the hexagonal crystal structure of…

A snowflake begins when a tiny dust or pollen particle comes into contact with water vapor high in Earth’s atmosphere. The water vapor coats the tiny particle and freezes into a tiny crystal of ice. This tiny crystal will be the “seed” from which a snowflake will grow. Hexagonal “Mineral” Crystals The molecules of water…