Hail is solid precipitate that forms within thunderstorms and depending upon its size can do considerable damage.

Hail is primarily found during thunderstorm events where strong updrafts are present that repeatedly carry growing chunks of ice upward into the clouds. Hailstones generally begin forming as seeds of small frozen raindrops or soft ice particles known as graupel. These harden into conglomerates of snow flakes measuring around 1/4 inch or so. Graupel and/or frozen droplets are not the only embryo for hail formation. Hailstones are sometimes found containing sand, pebbles, leaves, nuts, and insects. There are even reports of hail containing turtles, baby crocodiles and frogs! All of which have been sucked into the storm by funnels or tornadoes and subsequently encased in ice through repeated updraft currents.

If graupel (or some other substance embryos) falls through the cloud and gets into an updraft region of a thunderstorm, it mixes with water droplets and ice particles that will freeze around it as it rises, like layers of an onion. These ice-stones are carried into the tops of the cloud gaining mass as they ascend. Somewhere up there, they become too heavy for the updraft to carry and so fall toward the ground. As they descend they warm adiabatically and the outside layer melts to a clear layer. These falling ice chunks invariably pass through other rising updrafts, and the same process is repeated - perhaps many times. Each time the hailstone grows and melts in its upward and downward pathways. Large hailstones are characterized by these onion-like layerings. Small hailstones would not have many of these up and down cycles before falling out of a cloud in a downdraft (or weakened) areas of the cloud. Clearly, once the hail stones become too heavy to be buoyant by the updrafts, they will fall to the ground as a hailshower.

The rain free region is produced by strong updrafts that effectively suspends rain and hail aloft as seen in the image to the right.
1. The graupel embryo is carried aloft by the updraft and begins to grow in size as it collides with supercooled raindrops and other ice particles.
2. Sometimes the hailstone is blown out of the main updraft and falls to Earth from 'Out of the Blue'.
3. When the updraft(s) are strong (and tilted) the ice-stone will fall back back through the cloud into another updraft for a repeat journey, growing on the way up, and melting a little on the way down. This process may be repeated several times, giving rise to ever larger hailstones.
4. In all cases, when the mass of the hailstones can no longer be supported by the updraft they fall to Earth as a hail shower. The stronger the updraft, the larger the potential of hailstones that can be produced by the thunderstorm.

Multicell thunderstorms are capable of producing several hailstorm showers. They do not produce the largest hailstones as their mature stage (in the life cycle) is relatively short giving little time for growth. In contrast, the sustained updraft of a supercell thunderstorm can give rise to large hail by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud.
While Tornado Alley is notorious, there is also a Hail Alley. As seen on the adjacent map, Hail Alley is situated just east of the Rockies covering areas of Colorado, Nebraska, and Wyoming. There are also hail-peak areas through Montana, South Dakota, Kansas, and across the High Plains curving southward through Oklahoma and north Texas that have recurrent frequencies of hail. Some locations in these areas have 10 or more hailstorms each year! Most of these hail storms occur between May and June, however, earlier and late-season storms are not out of the question from April through to October.

While Florida has the most thunderstorms in the US, most are not hail bearing. Those occurring through Nebraska, Colorado, and Wyoming reach freezing levels more readily where cloud glaciation can take place. Hail formation is a balance between vertical cloud temperatures for ice formation and ground height where melting can take place. Thus in the High Plains, the freezing levels are closer to the ground and hail is able to hit the ground before melting. In Florida, melting is likely to happen through a much greater depth of the cloud.

Hailstones can vary in size, shape, and hardness. Hailstones will therefore cause a variety of impact marks on buildings and vehicles depending on their form. Hail is sized by simply comparing it to a known sized object. Hail producing thunderstorms will likely produce a wide variety of hail sizes - sometimes falling out at different times or places relative to the storm. When reporting hail size to authorities, the largest hailstones should be the ones measured and recorded. There is a connection between the size of hail and the approximate speed and strength of the updraft within the thunderstorm.

Damage Impact

Pea to Penny Size Hail can cause light damage to trees and vegetation and possibly cause minor injuries to people and animals. It may also cause slight damage to automobiles and roofs if hard.

Tennis Ball to Baseball size hail is obviously damaging. It will cause severe damage to trees and shrubs and produce life-threatening injuries to people and animals. Hail to this scale would cause extensive damage to automobiles and roofs.

Implications for Storm Spotters
Hail is a solid precipitate that falls from severe thunderstorms.
The size of hail varies greatly- largely depending upon the strength, longevity and structure of the parent thunderstorm.
Hail Storms
Observation of an approaching thunderstorms can sometimes offer clues of hail formation within the storm. While hail is found throughout the downdraft areas of thunderstorms, the most significant (and damaging) hail is usually found skirting the updraft area of a thunderstorm. This is sometimes obscured behind a heavy rain core. This Hail Shaft or Hail Core is often a precursor of a more dangerous weather phenomena – tornadoes.
Spotters can identify Hail Shafts within a storm by looking for an intensification of whitening among the shafts of rain. These whitish streaks can indicate an intense area of hail skirting the updraft area where it can help hide tornadic development from those in the path of the storm.
When viewed from an areal perspective, hail falls along paths known as Hail Swaths or Hail Streaks. These range in size from a few tens-of-yards to large 10 miles by 100 miles long swaths. These swaths can pile hail so deep that the hail needs to be removed by snow plow.
When a hail storm produces an accumlation of hail at the surface, a phenomena known as Hail Fog can result. Hail fog occurs when hail that has accumulated on the ground cools the air above it (by as much as 20˚F). When cooling the air it easily reaches the dewpoint and condensation occurs. Since this occurs at ground level, the end result is fog (cloud on the ground).
Slow moving hailstorms can create hazardous winter-like driving conditions.
Significant hail accumulations in low lying or confined areas often enhance or become a cause for flash flooding. Since Hail floats (like an ice cube) it can be responsible for clogging up drainage pathways, culverts and storm sewers.
Lastly, it is worth pointing out a relationship found between +Positive lightning (Killer strikes) and hail. Approximately 5% of lightning is positive. It is common for thunderstorms to start with mainly negative flashes with the positive strikes occurring towards the end of the life cycle. In supercells and tornadic thunderstorms, there appears to be larger number of positive flashes occurring throughout the storm's development. Research has shown that the occurrence of positive strikes appears to be correlated with the production of large hail. Something to watch for when in the field.
Implications for Storm Spotters
White curtains under dark thunderstorm clouds are likely to be hail shafts- areas of falling hail.
Hail shafts are also important visual indicators of potential tornadic activity.
Hail accumulation in low lying or confined areas often enhance the potential for flash floods.
Human Impact
Deaths caused by hail are rare in the US with the last known fatality occurring in March of 2000 in Lake Worth, TX. Here, a young man died from head injuries after being stuck by a softball size hailstone while trying to save his new car. Deaths and injuries are more common in other parts of the world where people live in poorly constructed buildings with little protection. Hailstorms do have a tremendous economic impact with yearly costs estimated to be around $1 Billion in damages to crops and properties in the US.
During a severe hailstorm, it is recommend to seek shelter in a sturdy structure or vehicle. Larger hail can fall at speeds of over 100 mph, thus causing damage and injuries to anything in its path. If caught in a hailstorm while driving, attempt to stop immediately and/or pull into a nearby safe place to avoid damage to windshields and windows. By continuing to drive, the impact of the hail will be maximized. Once stopped, try to orient the vehicle into (toward) the hail so it is hitting the front of the vehicle. Windshields are generally reinforced and can withstand larger impacts than side and back windows. This will lessen any damage from flying glass to both occupants and equipment in the vehicle.
Driving on hail covered roadways can offer another set of challenges for storm spotters, in addition to vehicle damage and breaking glass. Small or soft hail covered roads can become quite slick - comparable to that of snow and ice. Larger hard hail can be like trying to drive on ball bearings!

Implications for Storm Spotters
While rarely lethal, hailstones can cause considerable damage and injuries.
Wear protective head and eye gear to avoid injuries from large hail and glass fragments.
Hail covered roads can be unexpectedly slippery.

Author's Bio: 

Athena is not just a High Priestess of Wicca and a spiritual adviser, she is also an avid weather geek and is a storm spotter and storm chaser.