Hail

Hail is created when small water droplets are caught in the updraft of a thunderstorm. These water droplets are lifted higher and higher into the thunderstorm until they freeze into ice. Once they become heavy enough, they start to fall. If the smaller hailstones get caught in the updraft again, they will collect more water on the way up.
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As the stone gets higher and higher the water will freeze on the outside of the ice pellet, causing the hailstone to get bigger. Once the hail stone becomes heavy enough it will begin to fall. Depending on how many times this processes happens before the hailstone finally falls to the ground, will determine how big the hailstone will ultimately be.
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In terms of diameter, the largest hailstone on record fell in Vivian, South Dakota, in the United States on 23 July 2010 (see image below). The stone measured 8 inches (20.32 cm) across, which is similar to the size of a small bowling ball. It likely was even larger when it fell, however, because it is believed to have melted somewhat before it was measured.

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Hailstones can fall from a height of 9000 m (30,000 feet) and approach the earth at speeds of as much as 193 km/h (120 miles per hour).

A hail stone shape is circular at smaller sizes and becomes more irregular at larger sizes.  Hail is generally compared to common, everyday objects when reporting size. The following chart will give you a rough idea of the size of hail and the estimated updraft speed required to carry it high into the thunderstorm. When the updraft is no longer strong enough to support the weight of the hail, it will fall to the earth.

Object         Size        Updraft Speed
pea              6mm         39 km/h
marble         13mm       56 km/h
20c              27mm       79 km/h
50c              32 mm      87 km/h
golf ball        44mm       103 km/h
egg              50mm       111 km/h
tennis ball     64mm       124 km/h
baseball        70mm       130 km/h
grapefruit      100mm     158 km/h
softball          114mm     166 km/h

Giant hail stones are usually more irregular in shape. This is because irregularities of a smaller size hail stone are exacerbated as the hail stone gets bigger. Also, smaller hailstones can merge onto a bigger hailstone. When this happens the hail stone will have bulges and will have a larger diameter in certain directions.

Below are some of the recent images we have received from PWL chasers and followers.

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Image© Daniel Pardini/PWL – Various sizes from 18th October 2014, Perth

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Image© Adam Delves/PWL – Single pea size hail from 18th October 2014, Perth

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Image© Carmen Mallard/PWL – Boyup Brook, 22nd October 2014.

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Image© Danica Justine/PWL – Large hailstone from Boyup Brook, 22nd October 2014

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Image© Blake Moore/PWL – Egg size hailstones from Boyup Brook, 22nd October 2014

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