Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones). Hailstones usually consist mostly of water ice and measure between 5 and 50 millimetres in diameter, with the larger stones coming from severe thunderstorms. Hail is only produced by cumulonimbus clouds (thunderstorm clouds), and is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 mm thick.

Ideal conditions for hail formation

Hail forms in strong thunderstorm clouds, particularly those with intense updrafts, high liquid water content, great vertical extent, large water droplets and where a good portion of the cloud layer is below freezing 0 °C.

Hail forms when supercooled water droplets freeze on contact with condensation nuclei. The storm's updraft blows the hailstones to the upper part of the cloud. The updraft dissipates and the hailstones fall down, back into the updraft, and are lifted up again. The hailstone gains an ice layer and grows increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls out of the cloud.

Accordingly, hail is actually less common in the tropics (Such as Singapore) despite a much higher frequency of thunderstorms than in the mid-latitudes because the atmosphere over the tropics tends to be warmer over a much greater depth. Hail is also much more common along mountain ranges because mountains force horizontal winds upwards (known as orographic lifting), thereby intensifying the updrafts within thunderstorms and making hail more likely.

Source: [http://app2.nea.gov.sg/training-knowledge-hub/weather-climate/hail#sthas...