Have you ever heard of your local meteorologist referring to the precipitation falling as graupel? Just the other day it was snowing one minute, then something a little different started falling from the sky. It’s not snow, but it’s not exactly hail or sleet either, so what is this stuff?
Graupel, which are also known as “snow pellets” are defined to be white, opaque ice particles that form as a result of supercooled water droplets forming around snowflakes as they fall to the ground.
When referring to graupel, I personally like to describe them looking similar to the frozen dessert Dippin’ Dots due to their shape, size and texture. When I ask people if they know what the “Dippin’ dot” looking stuff is that’s on the grass or hood of their car, they almost always know what I’m referring to.
Instead of being made up of pure ice crystals like snow, or formed as pure, liquid water freezes completely before reaching the surface like sleet, they’re sort of a hodge-podge of both; all rolled into tiny snowballs. They’re also much softer than sleet would be, and if you gather some in your hand like in the picture above they usually will stick together in clumps relatively well.
How does it form?
The key to the formation of graupel like any precipitation is the depth of cold (or not cold) air, and how cold or warm these different layers are.
Like most frozen precipitation, graupel initially starts out as snow way up in the atmosphere. Typically, you need temperatures inside the cloud to be freezing with at least some portion being around 15 degrees Fahrenheit with temperatures near the surface around 45 degrees or colder.
Snowflakes will fall from the clouds and remain as snow as they pass through this shallow layer of cold air. The snow will then partially melt allowing for supercooled water droplets to form around it, which can happen in both freezing and slightly above freezing conditions.
Did you know? Supercooled water is water that exists as a liquid at temperatures below freezing. This is why the temperature of 32 degrees Fahrenheit is mainly referred to as the “melting point” and not the “freezing point” since water always melts at 32 degrees Fahrenheit, but can exist as a solid or liquid at or just below freezing.
As this mix of snow and supercooled water continues to pass through a layer of below freezing air, the water will refreeze over the snow into tiny balls of ice, which we call graupel! This process of supercooled water droplets freezing onto the surface of ice crystals is known as riming.
Depending on how intense this riming process is, these tiny balls of snow and ice can grow to fairly decent in size, but are typically about the size of about 0.2 inches or less, around the size of a pea if not a bit smaller.
Here are examples of other frozen precipitation including hail, graupel, sleet, and snow:
Sleet and graupel can be tricky to tell apart from each other. Although they are similar in appearance, they are different enough that you can tell them apart if you know just what to look for. Sleet is usually clear and transparent in appearance along with being more firm, while graupel is white, opaque and softer.
Graupel also has the neat propensity to bounce when hitting a surface rather than to melt or just “plop” onto the ground like snow. You can see just how bouncy graupel can be in the video clip above.
Just remember sleet as tiny balls of ice, and graupel as tiny but softer, miniature snowballs!
~Meteorologist Christine Gregory