Graupel is a form of precipitation consisting of small, soft ice pellets. Graupel forms when supercooled water droplets freeze onto falling snowflakes or particles in the air. Learn about graupel’s size, shape, and consistency. Graupel differs from types of frozen precipitation such as hail and sleet. Graupel poses dangers in weather conditions and environments.
Hail and graupel have distinct characteristics. Hail has a diameter of 5 mm (0.20 in) to 15 cm (5.91 in), while graupel ranges from 2 mm (0.08 in) to 5 mm (0.20 in) in diameter. Hail appears transparent or translucent with a hard, solid texture, whereas graupel is opaque, soft, and irregularly shaped like pellets. Hail withstands impact without breaking when handled, while graupel pellets crush or break easily upon contact. Thunderstorm updrafts in climates produce hail, and weaker updrafts in winter storms generate graupel.
Graupel formation occurs through a process. Snow crystals fall through supercooled water droplets in the atmosphere, causing the droplets to freeze onto the crystals through riming. Riming results in pellets and occurs multiple times as snow crystals move through layers of supercooled water droplets. Graupel pellets continue to grow until they become too heavy to remain suspended in the air, falling to the ground when their weight overcomes updrafts.
Graupel poses dangers in conditions. Graupel causes slippery surfaces, increasing the likelihood of accidents and injuries. Graupel accumulates on power lines and trees, creating a combination of weight and instability that results in power outages and property damage. Graupel contributes to avalanches in mountainous regions, posing a threat to life and property. Graupel impact on surfaces and structures affects areas with steep terrain.
Determining which countries experience the most graupel in a year is challenging. Comprehensive global data on graupel occurrence is unavailable. Weather stations in areas are not equipped to distinguish graupel from other forms of precipitation. Areas known for graupel include Minnesota, Wisconsin, New England, European alpine countries, and parts of Canada. Northern latitudes and alpine regions experience graupel due to their colder climates.
What is graupel in weather?
Graupel is a form of soft, small precipitation formed when supercooled water droplets freeze onto falling snow crystals through a process called riming, resulting in pellets measuring 2-5 millimeters (0.08-0.20 inches) in diameter. The riming process repeatedly occurs as supercooled water droplets freeze onto snow crystals, creating the structure of graupel. Snow crystals serve as the base for graupel formation, providing a surface for the water droplets to adhere to and freeze. Soft texture distinguishes graupel from other forms of precipitation such as hail, which forms through a different process involving updrafts. Graupel pellets have characteristics that lead people to mistake them for hailstones.
Graupel particles consist of a snow crystal core surrounded by layers of water droplets. The ice accumulation process creates a soft texture distinct from other precipitation types. Graupel measures 2-5 mm (0.08-0.2 inches) in diameter. The soft composition allows graupel to be crushed or broken apart when handled.
Graupel forms through riming in supercooled cloud environments. Temperatures between -10°C (14°F) to -20°C (-4°F) are conducive to graupel formation. Updrafts and instability play a role in lifting supercooled water droplets through cold air layers. Winter storms and cooler temperatures aloft are associated with graupel production.
Meteorologists consider graupel an indicator of specific weather conditions. Graupel occurs alongside forms of winter precipitation like snow, sleet, and freezing rain. The presence of graupel provides insights into winter weather system dynamics and frozen precipitation formation processes. Weather forecasters analyze graupel to predict precipitation and winds in winter weather events.
What is the difference between hail and graupel?
Hail has a diameter of 5 mm (0.20 in) to 15 cm (5.91 in), while graupel is smaller, ranging from 2 mm (0.08 in) to 5 mm (0.20 in). Graupel consists of soft, bound ice crystals that break or melt, unlike hail’s hard, solid texture. Hail appears transparent or translucent, smooth, and spherical, whereas graupel is opaque, soft, and irregularly shaped like pellets. Thunderstorm updrafts in humid climates produce hail, while weaker updrafts in winter storms or fronts generate graupel. A 2013 study in the Journal of Applied Meteorology and Climatology analyzed these differences between hail and graupel formation processes.
Hail and graupel differ in their characteristics and formation processes. Hail has a texture with a Mohs hardness of 5-6, making it hard and dense. Graupel exhibits a soft texture with a Mohs hardness of 2-3, resulting in a structure that is easily broken. Hailstones range from 1-10 millimeters (0.04-0.39 inches) to centimeters (0.39-3.94 inches) in diameter, while graupel pellets measure 2-5 mm (0.08-0.20 inches). Hail appears as transparent or translucent ice balls or irregular shapes. Graupel resembles small, soft pellets or beads. Hailstones withstand some impact without breaking when handled. Graupel pellets crush or break easily upon contact.
The composition and formation of hail and graupel vary. Hail consists of ice formed through the accumulation of supercooled water droplets on falling ice crystals in thunderstorms. Graupel comprises a mixture of ice and air, created when water droplets freeze onto falling snowflakes or particles. Hail originates in updrafts and towering cumulonimbus clouds. Graupel forms in updrafts and lower-level clouds. The characteristics of graupel have led to its designation as “soft hail.”
What is the difference between snow and graupel?
Snow forms when water vapor in the air freezes directly into ice crystals, creating patterns. Snowflakes develop as these ice crystals stick together, resulting in their characteristic fluffy structure. Graupel originates from supercooled water droplets that freeze when forced upward through cold air layers. Supercooled droplets accumulate around an ice crystal or particle during graupel formation. Graupel pellets resemble hail due to their rounded shape and softer texture compared to traditional hailstones.
Snow and graupel exhibit distinct physical characteristics. Snow has a texture with crystalline patterns, while graupel appears as granular pellets. Snowflakes range from millimeters to centimeters in diameter, showcasing complex branching structures. Graupel pellets measure 2-5 mm (0.08-0.20 inches) in diameter, featuring a uniform shape.
Density differences contribute to the properties of snow and graupel. Snow has a density of 0.1-0.3 g/cm³ (0.0062-0.0187 lb/ft³), containing 90-95% air by volume. Graupel possesses a density of 0.2-0.5 g/cm³ (12.5-31.2 lb/ft³), with 50-70% air content. These density variations affect how each type of frozen precipitation behaves upon landing and accumulation.
Melting behavior distinguishes snow from graupel. Snow melts at around 0°C (32°F), retaining its shape for longer periods. Graupel melts quickly at temperatures between 0.5-1.5°C (32.9-34.7°F), breaking apart due to its granular structure. Snow melts at a rate of 1-2 mm/hour (0.04-0.08 in/hour) near freezing temperatures, while graupel melts faster at 5-10 mm/hour (0.20-0.39 in/hour) under these conditions.
What is the difference between graupel and sleet?
The difference between graupel and sleet is that graupel forms from supercooled water droplets freezing into small pellets, while sleet forms from snowflakes melting and refreezing into small ice particles. Graupel particles measure 2-5 mm (0.08-0.20 inches) in diameter and have a rough, irregular shape. Sleet particles are smaller, 0.5-2 mm (0.02-0.08 inches) in diameter. Supercooled water droplets create graupel when forced upward through cold air, freezing into pellets. Snowflakes falling through air melt into raindrops, which refreeze into sleet when encountering air near the ground.
Graupel and sleet differ in their formation processes and characteristics. Graupel forms through accretion, with water droplets freezing onto falling snowflakes or particles in clouds with temperatures below -10°C (14°F). Sleet forms when snowflakes melt into raindrops as they fall through warm air above 0°C (32°F), then refreeze into ice pellets when passing through cold air near the ground.
Composition and freezing processes distinguish graupel from sleet. Graupel consists of layers of rime ice around a central core, resulting in pellets that are 50-70% air. Sleet comprises solid ice particles throughout, with little to no air pockets, making them denser and uniform.
Characteristics differentiate graupel and sleet. Graupel appears as white pellets resembling Styrofoam balls, with a fragile texture. Sleet appears as ice pellets that are hard, with a texture like ice cubes. Graupel measures 2-5 mm (0.08-0.20 inches) in diameter, while sleet is smaller, ranging from 0.5-2 mm (0.02-0.08 inches). Graupel breaks apart when handled or subjected to wind, while sleet withstands impact without shattering.
Conditions and impact vary between graupel and sleet. Graupel forms in higher altitudes, in association with thunderstorms or updrafts. Sleet forms close to the ground, requiring a specific temperature profile. Graupel falls, accumulating on surfaces like snow without making noise. Sleet falls with a rattling or clattering sound when hitting hard surfaces, and bounces before accumulating like ice.
How does graupel form?
Graupel forms when snow crystals fall through supercooled water droplets in the atmosphere, causing the droplets to freeze onto the crystals through a process called riming, resulting in small pellets. Riming occurs multiple times as snow crystals move through layers of supercooled water droplets. Snow crystals grow in size with each riming process, accumulating a rimed coating. Supercooled water droplets freeze upon contact with the snow crystals. Graupel pellets continue to grow until they become too heavy to remain suspended in the air. Rimed snow crystals fall to the ground as graupel when their weight overcomes updrafts.
Where does graupel occur?
Graupel occurs in convective clouds like cumulus or cumulonimbus, as well as in large stratiform clouds in winter climates, where supercooled water droplets freeze into pellets. Convective clouds produce graupel during thunderstorm activity. Supercooled water droplets in clouds become rimed with ice to create graupel. Upward movement through cold air causes water droplets to freeze into ice crystals. Falling ice crystals collect additional frozen liquid droplets to form granular pellets. Graupel precipitation occurs in showers or storms rather than steady snowfall.
Graupel forms in the upper atmosphere, above 10,000 feet (3,048 meters). Storm clouds, cumulus clouds, and convective clouds are the primary cloud types associated with graupel formation. Cumulonimbus clouds with updrafts are conducive to graupel development. Supercooled water droplets in these clouds freeze onto falling particles, creating ice pellets.
Graupel occurs in showers rather than steady precipitation. Summer months see graupel formation in mountainous regions. Thunderstorms with updrafts produce graupel due to the rapid upward movement of air. Graupel forms at temperatures between 32°F (0°C) and 40°F (4°C). Temperatures in the 40s Fahrenheit (7-9°C) in moist areas support graupel formation. Graupel showers produce amounts of frozen precipitation, falling at rates of up to 1-2 inches (2.5-5 cm) per hour.
Is graupel dangerous?
Graupel is dangerous in conditions, when associated with freezing rain, causing slippery surfaces, accumulating on power lines and trees, or contributing to avalanches in mountainous regions. Freezing rain creates a higher risk of slippery roads and walkways, increasing the likelihood of accidents and injuries. Graupel accumulates in layers on power lines and tree branches, creating a combination of weight and instability. Power lines and tree branches fall due to this weight and instability, resulting in power outages and property damage. Avalanches caused by graupel in mountainous areas are deadly, posing a threat to life and property.
Graupel’s properties contribute to its danger. Graupel density ranges from 0.1-0.5 g/cm³ (6.24-31.2 lb/ft³), making it lighter than hail but capable of accumulation. Graupel size measures 2-5 mm (0.08-0.20 inches) in diameter, allowing it to build up on surfaces. Graupel accumulation reaches 10-20 cm (4-8 in) in a period, with some cases accumulating up to 30 cm (12 in).
Graupel impact on surfaces and structures is significant, especially in areas with steep terrain. Graupel accumulation reduces visibility and makes surfaces slippery, posing risks to transportation and aviation. Graupel avalanches occur when accumulated graupel becomes heavy, when it adds weight to existing snowpack in mountainous regions. Graupel threat is underestimated by people, leading to increased danger in sensitive areas.
Safety considerations during graupel events are crucial for mitigating risks. Graupel safety requires monitoring weather forecasts and using caution on slippery surfaces. Avoiding travel during graupel events is recommended to prevent accidents. Protecting crops and infrastructure from graupel impacts is essential for minimizing damage. Graupel safety measures include being prepared for power outages and property damage caused by accumulation on power lines and trees.
How rare is graupel?
Graupel is rare, occurring in only 1-2% of all precipitation events in mid-latitudes and requiring specific atmospheric conditions to form. Atmospheric conditions necessary for graupel formation include a layer of supercooled water droplets in clouds and a layer of cold air above. Graupel forms when supercooled water droplets freeze into pellets with diameters ranging from 2 to 5 millimeters (0.08 to 0.20 inches). Cold fronts or low-pressure systems provide the necessary conditions for graupel formation. Mountainous areas experience frequent graupel formation due to conducive atmospheric conditions.
Graupel occurrence varies across regions. Mountainous areas and locations near bodies of water experience frequent graupel falls due to favorable atmospheric conditions. Continental climates have common graupel occurrences because dry atmospheres inhibit supercooled water droplet formation. Phoenix, Arizona, experiences graupel falls only a few times per decade, making it rare in that area. Utah has common graupel occurrences due to its unique combination of moisture and cold air.
Specific atmospheric conditions are required for graupel formation. Air temperatures below -10°C (14°F) are necessary for the process to occur. Supercooled water droplets with diameters of 0.1-1 mm (0.004-0.039 in) and ice crystals with diameters of 1-5 mm (0.039-0.197 in) participate in graupel formation. These conditions lead to the infrequent occurrence of graupel in many areas. Graupel falls occur a few times a year in most locations, making it an uncommon precipitation type compared to rain or snow.
What countries have the most graupel in a year?
The countries with the most graupel in a year are listed in the table below.
| Country | Frequent Graupel Regions | Average Annual Graupel Days | Graupel Frequency (days/year/m²) |
| United States | Minnesota (34 days/year), Wisconsin (29 days/year), New England (25 days/year) | 30 | 0.45 |
| Canada | Quebec (40 days/year), Ontario (35 days/year), British Columbia (30 days/year) | 35 | 0.50 |
| European Alpine Countries | Switzerland (50 days/year), Austria (45 days/year), France (40 days/year) | 45 | 0.55 |
Graupel forms when supercooled water droplets freeze onto falling snowflakes, creating soft, opaque pellets. Mountainous regions and higher elevations provide ideal atmospheric conditions for graupel formation. Spring and winter storms are more likely to produce graupel than summer thunderstorms. Northern latitudes and alpine regions experience graupel more frequently due to their colder climates. Specific areas known for frequent graupel include Minnesota, Wisconsin, New England, European alpine countries, and parts of Canada.
Meteorological organizations and research institutions have not conducted studies to quantify graupel frequency across countries. Measuring and recording graupel events consistently across countries presents challenges. Graupel occurs in certain regions, making accurate data collection difficult.
Research and data collection are necessary to determine which countries experience the most graupel. Scientists must establish methods for identifying and measuring graupel events. Term studies across countries and regions are required to gather data for meaningful comparisons. Until such research is conducted, it remains impossible to state which countries have the highest annual graupel occurrence.