Not synonymous with snow: how and why does hail form?
Hail is associated with storm clouds (thunder and lightning) and can occur at any time of the year, including summers. How does it differ from snow? What is the largest hailstone ever documented in the world?
Although both belong to the family of hydrometeors (water particles in a liquid or solid state), snow and hail are different phenomena. Why? For one thing, hailstones can occur in high temperatures in the summer; an impossible environment to conceive of snow.
Hail is a type of solid precipitation (ice), which can range in size from a few millimetres to several centimetres. Hail can occur in any season of the year, even at high surface temperatures such as in summers or in tropical areas at sea level.
The origin of hail
Hail is generated inside storm clouds (Cumulonimbus), an imposing cloud of vertical development that can exceed 10 kilometres in height and which is responsible for producing powerful showers (intense precipitation, of short duration and affecting a specific place) and thunderstorms wherever it passes through during its maturity stage.
"Moisture, heat and other dynamic factors contribute to the formation of vertically developed cumulus clouds. All this moisture rises rapidly, driven by strong upward and downward air currents. As it rises, all this moisture condenses, forming small water droplets, which, when they pass the 0 isotherm line at sub-zero temperatures, solidify to form ice," explains Chilean meteorologist Luis Salazar.
And why do they fall? "This ice grows as it continues to ascend by joining with other drops. So, at high altitude, ice blocks the size of tennis balls and even bigger are formed. Until it reaches the point where the upward flow ceases and the downward flow begins. As it descends, attracted by the force of gravity, this sphere of ice, with the friction of the air, evaporates and loses mass until it reaches the ground with a smaller size", the specialist illustrates.
The biggest hail of all
Incredible! Do you know what the largest hail ever recorded in the world was? It was recorded on 14 April 1986 in the Gopalganj District of Bangladesh and weighed 1.02 kilograms. Yes, more than 1 kilogram! It fell in the midst of a storm that cost 92 people their lives, according to the World Meteorological Organisation (WMO).
Why are hailstones becoming increasingly larger in some countries, such as Chile? According to Luis Salazar, "we were used to receiving hailstones of a volume smaller than a small ball (polka or marble) on the surface, but now with greater instability and humidity, hailstones exceed the usual dimensions".
What about snow?
To precipitate snow, we need a cold and wet environment in the portion of the atmosphere closest to the surface, i.e. negative temperatures, or at most, under very special conditions, 1 or 2 degrees at most.
In simple terms, these are the raindrops that meet up with condensation nuclei, tiny particles in the air, such as dust, for example, which, when they freeze, form flakes.
If the temperature rises, sleet can fall, i.e. the flakes begin to melt before they touch the ground, and if the temperature rises even more, liquid precipitation falls instead: rain.