Remember Tuesday’s cold front?
It stretched southward from the weather bomb that tracked across Labrador. All four Atlantic Canadian provinces experienced wind gusts in the 80- to 130-km/h range. It was quite a storm. As the cold air funnelled down behind the front, temperatures tumbled. Not long after the system passed, emails started to come in from people who had seen hail!
There were small balls of “frozen something or other” falling from the sky but they were not hail stones. A photo I received from Catherine Hamilton of Anagance, N.B., confirmed it was graupel, also known as snow pellets.
What’s the difference?
Hail has thick uniform layers of ice, while graupel is an oblong shaped snowball. Ice from a hail storm only falls during thunderstorms, while graupel falls in a wintry mix – in this case, during a rapid transition from mild to cold.
Snowflakes that formed behind the front encountered super-cooled water droplets in the milder air ahead of it. Super-cooled droplets are very tiny and exist in liquid form, well below freezing. When in contact, these droplets collect and freeze on the surface of snowflakes through a process known as accretion. As more water droplets freeze on the snowflake, the original shape of the snowflake changes, resulting in the formation of graupel.