Hail

Unique identifier / Notation

MH0036

Synonyms

Hailstone,

Hailstorm.

Hail is precipitation in the form of particles of ice (hailstones). These can be either transparent, or partly or completely opaque. They are usually spheroidal, conical or irregular in form, and generally 5−50 mm in diameter. The particles may fall from a cloud either separately or agglomerated in irregular lumps (WMO, 2017).

Primary reference(s)

WMO, 2017. International Cloud Atlas: Hail. World Meteorological Organization (WMO). Accessed 22 November 2019.

Additional scientific description

Falls of hail always occur as showers. They are generally observed during heavy thunderstorms (WMO, 2017).

Hailstones usually form around a nucleus, that may not be at their geometric centre. The nucleus may be anywhere between a few millimetres and a centimetre in diameter. The nucleus is spheroidal or conical and is composed of ice that is usually opaque, but sometimes transparent (WMO, 2017).

Hailstones can occur with a great variety of forms and dimensions, even within a single fall. An ’onion skin’ formation, for example, consists of a nucleus surrounded by alternating layers of opaque and transparent ice. There are usually not more than five layers, except in very large hailstones, which have been found to have 20 or more layers. Some other hailstones do not have any layers and consist of transparent or opaque ice only. They typically have a density of between 0.85 g/cm3 and 0.92 g/cm3 but may have a lower density if they have large cavities filled with air. Some hailstones are partly composed of spongy ice, which is a mixture of ice, water and air. In exceptional circumstances, large hailstones can stick together to form irregular lumps of giant hail (WMO, 2017).

Hailstones form when a nucleus collects cloud droplets or drops of rain. There is no general agreement on the nature of this nucleus; the tendency is, however, to admit that it is usually a particle of small hail that has formed around a snow pellet (WMO, 2017).

Metrics and numeric limits

Hail size is often estimated by comparing it to a known object. Details on scales are available from NOAA (2019).

Key relevant UN convention / multilateral treaty

Not available.

Examples of drivers, outcomes and risk management

Most hailstorms are made up of a mix of different sizes, and only the very largest hail stones pose serious risk to people caught in the open.

The destructive effects of hailstorms upon plant and animal life, buildings and property, and aircraft in flight render them a prime object of weather modification studies (AMS, 2012).

Examples of National Alerting Parameters for hail include those issued in China (China Meteorological Administration, 2012) and the United States, where a hail warning is issued when hail is forecast to be one inch or more in diameter (NOAA, 2020).