Hailstorm is a severe weather phenomenon, which causes extensive damage to crops, property and livestock. Hail is solid precipitation made of balls or irregular lumps of ice, each of which is called a hailstone. Unlike graupel or snow-ice pellets that are smaller and translucent, hailstones consist mostly of water ice and measure between 5 mm and 15 cm in diameter. Any thunderstorm, which produces hail that reaches the ground, is termed a hailstorm.
Hail can cause serious damage, notably to aircraft, automobiles, glass-roofed structures, skylights, besides crops, people and livestock. Hailstorms occurring primarily in the months of March and April cause maximum damage to ‘rabi’ crops when it is ripening for harvest and when the mango orchards are flowering. One of the most damaging hailstorms hit the north Indian city of Moradabad on April 30, 1988, and resulted in the death of 246 people, in addition to 1,600 livestock fatalities. In 2015 India suffered an estimated loss of INR 20,453 crore due to unseasonal rains and hailstorms in March.
The United States gets a large share of hail each year, resulting in 1 billion USD in damages. Australia has a history of both small and large hailstorms, although the 1999 hailstorm remains one of the worst on record when around half a million tonnes of hail fell from the sky during a single hailstorm that hit Sydney on April 14, 1999. It ripped apart roofs of 20,000 houses, 40,000 vehicles and 25 aircrafts, resulting in whopping losses of over 1.7 billion USD.
Favourable conditions for a hailstorm
Hail forms in the strong updraft region of a thunderstorm. Atmospheric conditions favourable for the formation hail bearing thunderstorm are:
- High degree of instability,
- High moisture content,
- Low freezing level,
- High vertical wind shear.
Hailstorms occur mostly over mid-latitude continental regions and decrease in frequency towards the pole and equator and over oceans and seas with the exception of Kericho, which is located close to equator at an elevation of 7,200 ft in Kenya and gets an annual average of 50 days of hail. The hail at Kericho is typically small but frequent. Other regions of significant hailstorm occurrence in the world include the Great Plains of the United States, Australia, China, India, Central Europe, Central Asia and adjoining parts of Russia.
In India the favourable hail conditions are met by active western disturbances during winter and the pre-monsoon season. Low level circulation associated with western disturbance, middle level trough in westerly winds and presence of a jet stream at higher level provide favourable conditions for the formation of hail-bearing thunderstorms along the line of discontinuity over north and northeast India. During the passage of an induced circulation, a deep westerly trough belt of hailstorm extends to central India and, at times, up to Telangana, Andhra Pradesh and Karnataka. Hailstorm occurrence however is maximum over the Northeast and Western Himalayas followed by Vidarbha, eastern Maharashtra, parts of Madhya Pradesh and Jharkhand. Southern states generally do not experience hailstorms.
In general, most hailstorms over North, Northeast and Central India occur from January to May with the months of March and April recording the highest number. Latitudinal and longitudinal distribution with the monthly frequency and diurnal variation of hailstorms over Northeast India is shown in Fig. 1a and b. Hailstorms, occur mostly in the afternoon and evening between 1500 hrs and 1900 hrs.
Hail Formation process
Hail forms as super-cooled droplets, ice crystals and water freeze upon an embryo hailstone. Once hail forms, gravity will bring the hail to the earth’s surface. If the updraft is stronger than the pull of gravity, the hailstone will continue to grow. Once the hail reaches a size that the updraft can no longer keep aloft, the hail stone will make its way down (Fig. 2).
Structure of a hailstone
On cutting through a hailstone, one can see rings of ice. Some rings are milky white; others are clear, which suggests that a hailstone can grow by two different processes, wet and dry. The wet growth occurs when the hailstone is in the storm where the air temperature is below freezing, but not super cold. When the hailstone collides with a drop of water, the water does not immediately freeze on the ice. Instead, the liquid water spreads over the hailstone and slowly freezes. During this slow freezing, air bubbles can escape, forming a layer of clear ice.
Dry growth of hailstones occurs when the air temperature is well below freezing (Fig. 1). In these conditions a water droplet freezes immediately upon colliding with the hailstone. Air bubbles quickly freeze in place, leaving cloudy ice. Counting the layers of clear and milky white ice gives an indication of how many times the hailstone travelled to the top of the storm.
Hailstones can be as large as oranges and grapefruits. If hailstones collide with each other and freeze together, they can form hailstones with a large variety of aerodynamical shapes. Hailstones are commonly spherical, conical, pear-shaped, ellipsoidal or discoidal in shape. The shapes are circular at smaller sizes and irregular at larger sizes. The shape is used as a guide for reporting hail size by comparing it to objects of the same diameter for simplicity in reporting (Table 1). It is considered to be severe hail if the diameter is 0.75 inches/1.84 cm or greater.
Giant hailstones are usually irregular in shape, exacerbated with the merging of smaller hailstone on to a bigger hailstone ending up with bulges and irregularities. As per official US records, Vivian in South Dakota saw a hailstone of 20 cm diameter fall on 23 July 2010 which was the largest hailstone recovered in the US. There are unconfirmed reports of Gopalgunj district in Bangladesh having been hit by a one kg hailstone, the heaviest hailstone known, on April 14, 1986.
Hailstorm reports are based on observations of hail falling on ground. Since hailstorm is a mesoscale phenomenon, it occurs in a limited area—thus goes unreported in remote areas. However, radar is an extremely useful tool in the detection of any hydro meteor in the atmosphere. The reflectivity of any cloud is dependent on the number, type and size of hydro meteors, which includes rain, snow and hail. Today, dual polarmetic radars are able to detect hail from differences in radar reflectivity of rain and hail at horizontal and vertical linear polarisation. This technique holds great promise in improving the detection of hailstorms.
The Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) allows a unique, consistent comparison between regions that cannot be otherwise compared using ground-based records owing to varying standards of data collection. Daniel J. Cecil and Clay B. Blankenship of Earth System Science Centre, University of Alabama, have developed an eight-year climatology of storms producing large hail estimates from satellite measurements. The Study shows that severe hailstorms are indicated in a broad region of northern Argentina and southern Paraguay and a smaller region in Bangladesh and eastern India. Numerous hailstorms are also estimated in the central and south-eastern United States, northern Pakistan and north-western India, central, western, and south-eastern Africa. Fewer hailstorms are estimated for other regions over land and scattered across subtropical oceans. (Fig. 3)
The need to protect crops from the damage caused by hailstorm has been a matter of interest amongst scientists since 1896, when Alber Stinger tried using a hail cannon to protect orchids in Styria, Austria. However, since Vincent J Schaefer’s discovered that dry ice acts as a good nucleating agent for ice crystals in atmosphere in 1946, hail suppression programmes have been undertaken by 15 nations.
Cloud seeding is undertaken so as to reduce accumulation of large amounts of super cooled water by supplying the cloud with numerous freezing nuclei capable of converting water to ice. This prevents the growth of large hailstones. Smaller hailstones are believed to inflict lesser damage.
Of the early ambitious projects were those by Russians in the Caucuses region. Russian scientists claimed a 50 per cent reduction in losses through cloud seeding with silver iodide nuclei. Since statistical evaluation of the Russian projects has not been possible, their claims remain open to interpretation. The United States initiated the National Hail Research Experiment (NHRE) in 1972, monitoring 30 hailstorms near the Colorado-Nebraska border. An analysis of the NHRE in 1975 failed to support statistically significant increases in hailstorms with small hail or an increase in rain.
Although there has been much interest in hail suppression, lack of an adequate standard to determine the success or otherwise of experiments undertaken leaves its efficacy open to doubt. However, the research has certainly yielded a lot more information on hail formation.