question: If hail is ice, what causes rain to become hail instead of snow?
comment: Hail is a summer event. In the summer, you can have giant thunderstorm clouds which have huge temperature differences within the cloud (ice cold up high), very warm down low. There are often large updrafts in the clouds. Drops of water will get blown up into the colder part of the cloud where it will freeze into ice. The ice will fall for a bit but then get caught in another draft of upward flowing air and go back up into the colder part again where it gets another layer of ice. The cycle continues until the ball of hail gets too heavy and falls out of the cloud... reaching the ground before it can melt in the summer air. The size of the ball of hail tells you how strong the storm is because it tells you how many times the ball was blown back up into the highest part of the cloud before it finally fell. Snow is a winter event. Tiny ice crystals form in a cold cloud and fall to the earth. No updrafts of win. Freezing rain is another wintry event. Rain falls from a warmer cloud but freezes in colder air before it reaches the ground.
passages: 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.
   Section::::Formation.:Hail.
   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.
   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.
   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.
   Section::::Natural formation.:In the air.:Hail.
   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
   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 again. Hail has a diameter of or more. Within METAR code, GR is used to indicate larger hail, of a diameter of at least . GR is derived from the French word grêle. Smaller-sized hail, as well as snow pellets, use the coding of GS, which is short for the French word grésil. 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.
   When observed at an airport, METAR code is used within a surface weather observation which relates to the size of the hailstone. Within METAR code, GR is used to indicate larger hail, of a diameter of at least . GR is derived from the French word grêle. Smaller-sized hail, as well as snow pellets, use the coding of GS, which is short for the French word grésil.
   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.
   Precipitation can fall in either liquid or solid phases, or transition between them at the freezing level. Liquid forms of precipitation include rain and drizzle. Rain or drizzle which freezes on contact within a subfreezing air mass gains the preceding adjective "freezing", becoming known as freezing rain or freezing drizzle. Frozen forms of precipitation include snow, ice crystals, ice pellets (sleet), hail, and graupel. Their respective intensities are classified either by rate of fall, or by visibility restriction.
  
  Section::::Phases.
  
  Precipitation falls in many forms, or phases. They can be subdivided into:
  
  BULLET::::- Liquid precipitation:
  
  BULLET::::- Drizzle (DZ)
  
  BULLET::::- Rain (RA)
   Unlike other forms of water ice such as graupel, which is made of rime, and ice pellets, which are smaller and translucent, hailstones usually measure between and in diameter. The METAR reporting code for hail or greater is GR, while smaller hailstones and graupel are coded GS.
   Weather radar is a very useful tool to detect the presence of hail-producing thunderstorms. However, radar data has to be complemented by a knowledge of current atmospheric conditions which can allow one to determine if the current atmosphere is conducive to hail development.
   Rain droplets that are carried well above the freezing level become supercooled at first then freeze into small hail. A frozen ice nucleus can pick up in size traveling through one of these updrafts and can cycle through several updrafts and downdrafts before finally becoming so heavy that it falls to the ground as large hail. Cutting a hailstone in half shows onion-like layers of ice, indicating distinct times when it passed through a layer of super-cooled water. Hailstones have been found with diameters of up to .
   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.
   Snowfall arises from a variety of events that vary in intensity and cause, subject to classification by weather bureaus. Some snowstorms are part of a larger weather pattern. Other snowfall occurs from lake effects or atmospheric instability near mountains. Falling snow takes many different forms, depending on atmospheric conditions, especially vapor content and temperature, as it falls to the ground. Once on the ground, snow crystals metamorphose into different shapes, influenced by wind, freeze-thaw and sublimation. Snow on the ground forms a variety of shapes, formed by wind and thermal processes, all subject to "formal" classifications both by scientists and by ski resorts. Those who work and play in snowy landscapes have "informal" classifications, as well.
   The process of ice crystals sticking together is called aggregation. This happens when ice crystals are slick or sticky at temperatures of and above, because of a coating of water surrounding the crystal. The different sizes and shapes of ice crystals fall at different terminal velocities and commonly collide and stick.
  
  Section::::Accretion.
  
  When an ice crystal collides with supercooled water it is called accretion (or riming). Droplets freeze upon impact and can form graupel. If the graupel formed is reintroduced into the cloud by wind, it may continue to grow larger and more dense, eventually forming hail.
  
  Section::::Precipitation.
   "All are formed in cloud, except for rime, which forms on objects exposed to supercooled moisture."
   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.
   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.
   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.
   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.
   Rain and snow mixed
  
  Rain and snow mixed is precipitation composed of rain and partially melted snow. Unlike ice pellets, which are hard, and freezing rain, which is fluid until striking an object, this precipitation is soft and translucent, but it contains some traces of ice crystals, from partially fused snowflakes. In any one location, it usually occurs briefly as a transition phase from rain to snow or vice versa. Its METAR code is RASN.
  
  Section::::Terminology.
   Section::::Accumulation.:Snow events.
  