• 5 months ago
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  • Author: SW101

In this section we will take a look at how and why hail forms, what kind of environments favour hail formation. We will see different types of hailstones and how to read their history. We will also take a look at criteria for severe hailstorms and some hail records. Let us start!

Hail is precipitation of hard ice particles – hailstones. By definition, hail is 5 mm or larger in size. Hailstones are generally in the form of balls, but can be irregular lumps in shape, particularly in larger sizes.

Hail forms in the part of the cloud between the freezing level (height at which air temperature drops below 0 °C) and the upper part, where all the water is in the form of ice crystals. In the layer in between – mid-levels – the water is liquid, despite the temperature below zero. This is called supercooled water. Supercooled water droplets freeze partially or completely either by a process called nucleation in which they begin to freeze from the inside, or on contact with a small ice particle or any other solid matter (called a nucleus). Thus a hailstone begins to form. As more supercooled water droplets collide with the hailstone, they add to its mass and size. This is the main mechanism by which hailstones grow. The hailstone remains aloft as long as the updraft is strong enough to keep it airborne. Once the hailstone grows too large for the updraft to keep it in the air, it drops towards the ground.

Interesting fact: water in clouds is present in liquid form above freezing level. At 0 °C to -15 °C most of the water in the cloud is in the form of supercooled water droplets. At -15 °C to -40 °C the cloud is composed of a mixture of ice crystals and supercooled water. Only at temperatures below -40 °C is most of the water in the form of ice crystals.

Conditions for hail formation

Factors that influence the likelihood of hail include: instability, the depth (thickness) of the part of the troposphere in which hail can grow, the depth and temperature of the layer near the ground in which the temperature is above freezing, amount of moisture I n the updraft and wind shear. Let us take a closer look at each:

  • Adequate instability: the stronger the updraft, the larger the hailstones it is able to hold aloft. The most powerful updrafts in severe thunderstorms can support hailstones larger than 10 cm in diameter.

  • Sufficient amount of supercooled water droplets – but not too much in the air for the hailstone to be able to grow on while in the updraft. Too much water droplets in drags down the updraft and reduces its potential to hold hailstones aloft. This is called water loading.

  • Small depth of the layer of troposphere where the temperature is above freezing: as soon as the hailstone falls below the freezing level, into air that is warmer than 0 °C, it begins melting. Therefore the smaller the height the less time the falling hailstones will be melting.

  • Large depth of the layer of troposphere where the temperature is below freezing: two factors generally increase it:

    • A low freezing level. The lower, the more favourable for hail.
    • Dry mid-levels, at approximately 3-8 km height. In dry air rain droplets falling through evaporate, cooling the air further. The cooler air encourages hail formation. This effect also further lowers the freezing level.
  • Places at higher elevations are also more likely to get hail: hailstones begin to melt rapidly when the temperature of the surrounding air rises above freezing. The further a hailstone has to drop through a layer of warm air, the more it melts.

  • Thunderstorms forming in more sheared environments have the potential to produce larger hail. In particular, thunderstorms with rotating updrafts (supercells) can produce very large hail (>5 cm).

Recap: favourable conditions for formation of hail include strong updrafts, which can hold hailstones aloft, a shallow layer of air where the temperature is above freezing and a deep layer of air aloft where the temperature is below freezing. Dry air aloft is very favourable for hail, as precipitation falling through it evaporates, cooling the air further.

Types of hailstones

There are a number of different types of hailstones. They are all made of water ice, but their shape and structure is different, reflecting a different mechanism by which they formed:

  • “Onion skin/layer” hailstones: in an onion skin or onion layer hailstones you can see alternating concentric rings of clear and white ice.

  • Aggregate hailstones: form by aggregation (‘gluing together’) of smaller hailstones into large hailstones. Aggregate hailstones are typically large, 4-5 cm or larger.

  • Irregular hailstones: tend to be the largest in size. Often, very large hailstones, larger than 10 cm in diameter grow irregular shapes and protrusions / spikes.

Next time you see hail take a look at the structure of hailstones in find out which types are present.

Interesting fact: this alternating structure is produced by two different mechanisms of growth: wet growth and dry growth. Wet growth happens in lower parts of the cloud, where the temperature is already below freezing, but it is still not very cold. Therefore both the air and supercooled water droplets are not very cold. When a hailstone collides with a supercooled water droplet, the droplet does not freeze entirely immediately. This happens due to latent heat release during freezing. The liquid water spreads over the hailstone and freezes slowly. During this slow freezing any trapped air bubbles escape and clear ice remains. Dry growth happens higher up in the cloud, where the temperature is lower. Supercooled water droplets are cold enough to freeze fully on contact with the hailstone. Any air that was trapped on contact remains within the ice. This produces small air bubbles and more opaque, white, non-transparent ice. Therefore each pair of clear and white ice layers is a record of one trip of the hailstone through the cloud. Like tree rings by counting the layers you can see how many times the hailstone travelled up and down the cloud.

Hail shafts and swaths

Hail often forms very distinct hail shaft, hail-filled white downdrafts, which often display curtain-like structure. Typically, a hail shaft is more distinct white and opaque than a rain shaft. Also, the wispy, curtain-like structure is more distinct than in a rain shaft.

Occasionally, storm chasers and spotters close to intense hail shafts report audiable sounds, resembling a waterfall.

The zone where a hailstorm produces significant accumulation of hail is called a hail swath. Slow moving or nearly stationary hailstorms tend to produce thick hail swaths, such as the example below.

Further examples of hail shafts

Hail shaft on a severe hailstorm in central Slovenia on June 16, 2010. Hail up to ~4-5 cm was reported. Note the intense and sharply delineated hail shaft. Also note the hail on the ground and ‘hail fog’. The hailstorm is moving away from this location towards the left in the photo.


Severe hailstorms

Particularly intense hailstorms are damaging to property and vegetation. A hailstorm with high potential for damage is a severe hailstorm. There are generally two definitions of severe hailstorms, depending on which side of the Atlantic you are on.

In Europe a severe hailstorm is defined (by the European Severe Storms Laboratory) as a hailstorm that:

  • Produces hailstones 2.0 cm in diameter or larger.
  • Produces hail accumulations on flat ground, that are 2.0 cm thick or thicker (deposits by storm runoff do not count).

In the USA a severe hailstorm is defined (by the National Weather Service) as a hailstorm that:

  • Produces hailstones 1” (2.5 cm) in diameter.

Interesting hail facts and records

  • The hailstone with the largest recorded diameter fell during a severe hailstorm in Vivian, South Dakota (USA) on July 23, 2010. It was 20 cm in diameter and weighed 0.88 kg.

  • The largest circumference hailstone ever recorded was during the June 22, 2003 Aurora, Nebraska, USA hailstorm. The hailstone was 17.8 cm in diameter and had a circumference of 47.6 cm. It is noteworthy that the hailstone was damaged and parts had broken of on impact with a roof of the house; initially the hailstone was even larger. It is very noteworthy that the hail that hit Hamilton County (including Aurora) produced impact pits on the ground up to 36 cm across!

  • It is likely that parts India and Bangladesh have seen hail at least as large and possibly larger: the area is well-known for extremely severe hailstorms, however, the vast majority are not officially recorded.

  • The heaviest recorded hailstone was recorded in Gopalganj district, Bangladesh on April 14, 1986. It weighed 1.02 kg.

  • The severe thunderstorm that produced the May 31, 2013 El Reno EF-5 tornado may also have produced hail over 20 cm in diameter. This was suggested based on photogrammetric analysis of videos.

  • The thickest hail swath was recorded on July 29, 2010 Boulder, Colorado, USA hailstorm. A virtually stationary thunderstorm dropped hail in the same area for 1.5 hours, producing a hail layer up to 100-120 cm thick.

  • The previous record for the thickest hail swath was held by the June 3, 1959 Seldon, Kansas, USA hailstorm: it produced a blanket of hail up to 45 cm thick over an area of 140 square kilometers.

  • The hailstorms of July 27-28, 2013 in Reutlingen and Pfortzheim, Baden-Württemberg and in Wolfsburg and Hanover, Lower Saxony, Germany produced an estimated 3.6 billion € damage. The July 27 Reutlingen hailstorm produced the largest hailstone recorded in Germany: 14.1 cm in diameter, 360-380 g weight.

  • The May 8, 2017 severe hailstorm that hit Denver, Colorado, USA is one of the costliest hailstorm in the US history. The hailstorm hit the west Denver Metro area and caused extensive damage, totalling $2.3 bilion dollars. It was likely surpassed by the May 5, 1995 Dallas and Fort Worth, Texas hailstorm (Mayfest Storm), which likely produced over $3 billion dollars in damage.