Graupel

Graupel is a specific type of precipitation that falls from the sky.

It is often called soft hail, snow pellets, or popcorn snow. Some people also use the terms granular snow or hominy snow. Graupel consists of crisp, opaque balls of rime. It is quite fragile and will typically fall apart when pressed. This distinguishes it from true hail, which is made of hard, uniform layers of ice. While graupel can occur during thunderstorms, it is also common in winter storms and at high elevations.

The formation of graupel relies on a process called accretion. This process begins when a snow crystal falls through the air. The crystal encounters supercooled water droplets. These droplets remain in a liquid state even at temperatures far below the freezing point. They stay liquid as long as they are above the homogeneous nucleation point of water. When a snow crystal makes contact with these droplets, the liquid freezes onto the surface. This creates a rimed crystal, which means it has frozen droplets on its exterior. As accretion continues, the mass of frozen droplets eventually obscures the original snowflake shape. The crystal grows until it becomes a ball-like particle of graupel.

Scientists use specialized tools to study the microscopic structure of these particles. A low-temperature scanning electron microscope, or LT-SEM, allows researchers to see the rime clearly. Even though the topography is difficult to record with a light microscope, the LT-SEM reveals frozen cloud droplets. These droplets can measure up to 20 micrometers on the surface of the crystals. This riming process can happen to all four basic forms of snow crystals. These include plates, dendrites, columns, and needles. Regardless of the starting shape, the accumulation of droplets eventually transforms the crystal into graupel.

As graupel falls through the atmosphere, its physical shape affects its movement. Many particles deform into a conical shape during their descent.

The size of the particle's base diameter determines how it travels. Small particles with a diameter of less than 1mm generally fall with the conical base facing down. Particles between 1mm and 3mm exhibit persistent oscillations around the center of the base. If the particle is larger than 3mm, it will start to tumble. As the base diameter increases, the conical particle generally falls further horizontally from its original starting point.

Meteorologists have a specific way to identify this weather phenomenon. In METAR weather reports, the code for graupel is GS. Historically, meteorologists referred to graupel as "soft hail." It is distinct from ice pellets, which often fall in a wintry mix alongside graupel. While true hail is usually associated with cumulonimbus clouds in thunderstorms, graupel is frequently found in general winter weather. It acts as a replacement for typical snowflakes in many wintry conditions.

Graupel is particularly significant in high-altitude climates due to its physical properties. It is denser and more granular than ordinary snow because of its rimed exterior. Macroscopically, it looks like small beads of polystyrene. These properties can make mountain slopes very unstable. Layers of graupel that are 0.5 meters or thicker create a high risk for dangerous slab avalanches. Thinner layers falling at low temperatures can also act like ball bearings. This occurs when graupel sits below subsequent falls of more stable snow, making surfaces very slippery.

Understanding how graupel behaves over time is also important for safety. Fresh layers of graupel are unstable on slopes due to their density and low viscosity. However, graupel does not stay in this state forever. It tends to compact and stabilize, a process sometimes called "welding," after it has fallen. This stabilization typically takes approximately one to two days. The exact timing depends on the local temperature and the specific properties of the graupel itself.