The specific name used to describe ice falling from the sky depends entirely on how the ice formed and its characteristics upon reaching the ground. The simple phrase “ice falling from the sky” actually encompasses several distinct meteorological events, each created by a unique set of atmospheric conditions. Understanding these differences provides a more accurate picture of precipitation in its frozen states, which are classified by the precise temperature layers and air movements that shaped them high above the ground.
Defining the Major Types of Frozen Precipitation
The four main types of precipitation that fall as ice are distinct in their structure and density. Hail is associated with intense thunderstorms, consisting of layered, hard, opaque or translucent ice. Hailstones must be at least 0.2 inches in size and can grow quite large, sometimes exhibiting a layered structure similar to an onion when cut open.
Sleet, officially known as ice pellets, consists of small, translucent balls of ice typically smaller than 0.2 inches. When sleet hits a surface, it often bounces and makes a distinct tapping sound. This form is essentially a frozen raindrop.
Graupel, sometimes called soft hail or snow pellets, looks like small white balls resembling tiny beads of polystyrene. Graupel is notably soft and opaque, often crushing easily between the fingers, unlike the hardness of sleet or hail. Graupel results from riming, a process where supercooled water droplets freeze onto the outside of a falling snowflake.
The Atmospheric Conditions That Create Each Type
Each type of frozen precipitation requires a specific temperature profile in the atmosphere for its formation. Hail is produced in powerful cumulonimbus clouds, characterized by strong vertical air currents called updrafts. These updrafts repeatedly carry ice embryos high into the cloud, where they collide with supercooled water droplets that freeze onto the surface, forming the stone’s characteristic layers. The hailstone falls when its weight overcomes the strength of the updraft.
Sleet, or ice pellets, forms when a layer of warm air sits above a thick layer of freezing air near the surface. Snowflakes fall and melt as they pass through the warm layer, turning into raindrops. These liquid drops then fall through the deep layer of sub-freezing air near the ground and refreeze into solid, translucent ice pellets before impact.
Graupel Formation and Freezing Rain Distinction
The formation of graupel involves snowflakes encountering a layer rich in supercooled liquid water droplets. These droplets instantly freeze onto the surface of the snowflake, coating it in rime ice until the original crystal shape is no longer recognizable. This riming process transforms the snowflake into the opaque, fragile pellets known as graupel.
The difference between sleet and freezing rain hinges on the thickness of the cold air layer near the surface. If the cold layer is deep enough for the melted precipitation to refreeze mid-air, it becomes sleet. If the cold layer is too shallow, the drops remain liquid until they hit the ground, resulting in freezing rain.
When Ice Falls But Isn’t Standard Weather
Not all ice falling from the sky is a product of the standard atmospheric water cycle process. Freezing rain occurs when supercooled liquid precipitation instantly freezes on contact with exposed objects, such as trees, roads, or power lines. This results in a glaze of ice that can be extremely hazardous.
Megacryometeors
Another rare occurrence involves megacryometeors, which are exceptionally large chunks of ice that fall from clear or mostly clear skies. These ice masses can weigh from a few pounds up to tens of pounds, with one instance in Brazil exceeding 110 pounds. The exact process of their formation is not fully understood, but scientific analysis suggests their composition matches normal atmospheric rainwater. They are not a product of standard thunderstorm activity like hail and are considered a distinct, anomalous weather event.