question: what causes hail to form instead of snow?
comment: Hale is rain that freezes on its way down. Snow is tiny frozen water molecules that clump together to form fluffy flakes.
passages: Section::::Extraterrestrial snow.
  
  Extraterrestrial "snow" includes water-based precipitation, but also precipitation of other compounds prevalent on other planets and moons in the Solar System. Examples are:
  
  BULLET::::- On Mars, observations of the "Phoenix" Mars lander reveal that water-based snow crystals occur at high latitudes. Additionally, carbon dioxide precipitates from clouds during the Martian winters at the poles and contributes to a seasonal deposit of that compound, which is the principal component of that planet's ice caps.
   Depths of up to have been reported. A landscape covered in accumulated hail generally resembles one covered in accumulated snow and any significant accumulation of hail has the same restrictive effects as snow accumulation, albeit over a smaller area, on transport and infrastructure. Accumulated hail can also cause flooding by blocking drains, and hail can be carried in the floodwater, turning into a snow-like slush which is deposited at lower elevations.
   Section::::Classification.
  
  The "International Classification for Seasonal Snow on the Ground" has a more extensive classification of deposited snow than those that pertain to airborne snow. A list of the main categories (quoted together with their codes) comprises:
  
  BULLET::::- "Precipitation particles (PP)" (See below)
  
  BULLET::::- "Machine-made snow (MM)" – May be round polycrystalline particles from freezing of very small water droplets from the surface inward or crushed ice particles from crushing and forced distribution
   Hail
  
  Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets fall generally in cold weather while hail growth is greatly inhibited during cold surface temperatures.
   BULLET::::- Névé — Névé is a young, granular type of snow which has been partially melted, refrozen and compacted, yet precedes the form of ice. This type of snow is associated with glacier formation through the process of nivation. Névé that survives a full season of ablation turns into firn, which is both older and slightly denser.
  
  BULLET::::- "Penitentes" — Penitentes are snow formations, found at high elevations, which form of elongated, thin blades of hardened snow or ice up to 5 meters in height, closely spaced and pointing towards the general direction of the sun. They are evolved suncups.
   Snow clouds usually occur in the context of larger weather systems, the most important of which is the low pressure area, which typically incorporate warm and cold fronts as part of their circulation. Two additional and locally productive sources of snow are lake-effect (also sea-effect) storms and elevation effects, especially in mountains.
  
  Section::::Precipitation.:Cloud formation.:Low pressure areas.
   BULLET::::- "Slush" — Slush is substantially melted snow with visible water in it.
  
  BULLET::::- "Snirt" — Snirt is an informal term for snow covered with dirt, especially where strong winds pick up topsoil from uncovered farm fields and blow it into nearby snowy areas. Also, dirty snow left over from plowing operations.
  
  BULLET::::- "Spring snow" — Spring snow describes a variety of temperature and moisture conditions with corn snow.
  
  BULLET::::- "Watermelon snow" — Watermelon snow is reddish pink, caused by a red-colored green algae called "Chlamydomonas nivalis".
  
  Section::::Other cultures.
   BULLET::::- "Extratropical cyclone" — A winter extratropical cyclone (also "nor-easter") may cause snow, especially in its northwest quadrant (in the Northern Hemisphere) where the wind comes from the northeast.
  
  BULLET::::- "Lake-effect snow —" Lake-effect or ocean-effect snow occurs when relatively cold air flows over warm lake (or ocean) water to cause localized, convective snow bands.
  
  BULLET::::- "Mountain snow" — Orographic lift causes moist air to rise upslope on mountains to where freezing temperatures cause orographic snow.
  
  BULLET::::- "Snow flurry —" Snow flurries are an intermittent, light snowfall events of short duration with only a trace level of accumulation.
   "All are formed in cloud, except for rime, which forms on objects exposed to supercooled moisture."
   Snow develops in clouds that themselves are part of a larger weather system. The physics of snow crystal development in clouds results from a complex set of variables that include moisture content and temperatures. The resulting shapes of the falling and fallen crystals can be classified into a number of basic shapes and combinations, thereof. Occasionally, some plate-like, dendritic and stellar-shaped snowflakes can form under clear sky with a very cold temperature inversion present.
  
  Section::::Precipitation.:Cloud formation.
   Section::::Formation.:Hail.
   Snow is produced at upper levels in such a winter storm system, but it eventually melts into rain as it falls through a warm air layer of above freezing temperature (of at least in depth) associated with the overrunning. The rain passes through colder air near the surface and is supercooled. When that rain touches the ground or any other cold surface in the cold air below, the droplets freeze on contact, creating accumulations of ice. If the cold air layer is too thick, the droplets refreeze before hitting the ground and form ice pellets, which are usually less hazardous.
   BULLET::::- "Ice pellet" — Ice pellets have two manifestations, "sleet" and "small hail", that result in irregular spherical particles, which typically bounce upon impact. Sleet comprises grains of ice that form from refreezing of largely melted snowflakes when falling through into a frozen layer of air near the surface. Small hail forms from snow pellets encased in a thin layer of ice caused either by accretion of droplets or by refreezing of each particle's surface.
  
  BULLET::::- "Hail" — Hail forms in cumulonimbus clouds as irregular spheres of ice ("hailstones") with a diameter of 5 mm or more.
   Snow hydrology
  
  Snow hydrology is a scientific study in the field of hydrology which focuses on the composition, dispersion, and movement of snow and ice. Studies of snow hydrology predate the Anno Domini era, although major breakthroughs were not made until the mid eighteenth century.
  
  Snowfall, accumulation and melt are important hydrological processes in watersheds at high altitudes or latitudes. In many western states in United States, snow melt accounts for a large percentage of the spring runoff that serves as water supply to reservoirs, urban populations and agricultural activities.
   Snow refers to forms of ice crystals that precipitate from the atmosphere (usually from clouds) and undergo changes on the Earth's surface. It pertains to frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away. Snowstorms organize and develop by feeding on sources of atmospheric moisture and cold air. Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime. As snow accumulates into a snowpack, it may blow into drifts. Over time, accumulated snow metamorphoses, by sintering, sublimation and freeze-thaw. Where the climate is cold enough for year-to-year accumulation, a glacier may form. Otherwise, snow typically melts seasonally, causing runoff into streams and rivers and recharging groundwater.
   Snow interacts with vegetation in two principal ways, vegetation can influence the deposition and retention of snow and, conversely, the presence of snow can affect the distribution and growth of vegetation. Tree branches, especially of conifers intercept falling snow and prevent accumulation on the ground. Snow suspended in trees ablates more rapidly than that on the ground, owing to its greater exposure to sun and air movement. Trees and other plants can also promote snow retention on the ground, which would otherwise be blown elsewhere or melted by the sun. Snow affects vegetation in several ways, the presence of stored water can promote growth, yet the annual onset of growth is dependent on the departure of the snowpack for those plants that are buried beneath it. Furthermore, avalanches and erosion from snowmelt can scour terrain of vegetation.
   Orographic or relief snowfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains. The lifting of air up the side of a mountain or range results in adiabatic cooling, and ultimately condensation and precipitation. Moisture is removed by orographic lift, leaving drier, warmer air on the descending, leeward side. The resulting enhanced productivity of snow fall and the decrease in temperature with elevation means that snow depth and seasonal persistence of snowpack increases with elevation in snow-prone areas.
  
  Section::::Precipitation.:Cloud physics.
   BULLET::::- "Winter storm" — Winter storms may constitute any combination of sleet, snow, ice, and wind that accumulates or more of snow in 12 hours or less; or or more in 24 hours or of ice.
  
  Section::::Classification of snow events.:Precipitation.
  
  Precipitation may be characterized by type and intensity.
  
  Section::::Classification of snow events.:Precipitation.:Type.
   Section::::Accumulation.:Snow events.
   Bioprecipitation
  
  Bioprecipitation is the concept of rain-making bacteria and was proposed by David Sands from Montana State University before 1983. The formation of ice in clouds is required for snow and most rainfall. Dust and soot particles can serve as ice nuclei, but biological ice nuclei are capable of catalyzing freezing at much warmer temperatures. The ice-nucleating bacteria currently known are mostly plant pathogens. Recent research suggests that bacteria may be present in clouds as part of an evolved process of dispersal.
  
  Ice-nucleating proteins derived from ice-nucleating bacteria are used for snowmaking.
  
  Section::::Plant pathogens.
   Section::::Formation.
  
  Hail forms in strong thunderstorm clouds, particularly those with intense updrafts, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing . These types of strong updrafts can also indicate the presence of a tornado. The growth rate of hailstones is impacted by factors such as higher elevation, lower freezing zones, and wind shear.
  
  Section::::Formation.:Layer nature of the hailstones.
   Section::::Natural formation.:In the air.:Hail.
   Like other precipitation, hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust or dirt. The storm's updraft blows the hailstones to the upper part of the cloud. The updraft dissipates and the hailstones fall down, back into the updraft, and are lifted up again. Hail has a diameter of or more. Within METAR code, GR is used to indicate larger hail, of a diameter of at least and GS for smaller. Stones just larger than golf ball-sized are one of the most frequently reported hail sizes. Hailstones can grow to and weigh more than . In large hailstones, latent heat released by further freezing may melt the outer shell of the hailstone. The hailstone then may undergo 'wet growth', where the liquid outer shell collects other smaller hailstones. The hailstone gains an ice layer and grows increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls from the cloud.
   Industrial snow
  
  Industrial snow occurs near sources of heat, moisture and atmospheric particulate matter. It usually occurs at nighttime, in a combination of freezing temperatures, stagnant low stratus cloud or fog, and a temperature inversion. It falls from a low altitude as fine spikes of ice, having not had time to grow to the more usual shaped snowflake.
  
  It has been observed in Switzerland, Germany, in Canada (where it was referred to as "refinery snow", or "plant snow"), and Texas.
   BULLET::::- "Snowflake" — A snowflake grows from a single ice crystal and may have agglomerated with other crystals as it falls.
  
  BULLET::::- "Snow grain" — Snow grains (also "granular snow") consist of flattened and elongated agglomerations of crystals, typically less than 1-mm diameter, that include a range of crystal sizes and complexities to include a rime core and glaze coating. They typically originate in stratus clouds or from fog and fall in small quantities, not in showers.
  