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NEA: Hail not related to cloud seeding; rain is not toxic

Ice pellets the size of marbles were spotted mostly around western parts of Singapore on Tuesday. The National Environment Agency cannot confirm if the hail storm is related to the haze, but says it is not related to cloud seeding. -RazorTV

Tue, Jun 25, 2013
The Straits Times and RazorTV

NEA has confirmed that the hail that some residents, particularly those living in the western parts of Singapore experienced, is not caused by the cloud seeding in Indonesia.

SINGAPORE - The National Environment Agency (NEA) said in a briefing on Tuesday evening, that clouds do not travel that far and the clouds would be going in the wrong direction if it was related as the wind is currently blowing the haze away from Singapore.


Get the full story from The Straits Times.

SINGAPORE - After weeks of haze, the rain finally came - but along with it were hailstones.

Ice pellets the size of marbles were spotted mostly around western parts of Singapore on Tuesday. These areas include Jurong, Choa Chu Kang and Bukit Batok.

The National Environment Agency (NEA) said that while hailstorms are uncommon in tropical countries, this is not the first time Singapore has seen hail. The last hailstorm here was in March, 2008.

Hail usually occurs because of thunderstorms. It is a form of rain and it is not harmful.

NEA said that it cannot confirm if the hail storm is related to the haze, but said it is not related to cloud seeding. The agency added that the heavy downpour this afternoon was not toxic or acid rain.

This is because it does not contain sulphur dioxide.

NEA said it is unlikely that the rain is the result of cloud seeding being conducted in Indonesia, as clouds do not travel that far and the winds are currently blowing the haze away from Singapore.

While the rain was a short respite from the haze, it cannot wash away all the PM2.5 particles in the air.

Slight hazy conditions and thunderstorms are expected over the next one to two days.

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What is hail?

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 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.

Taken from the National Environmental Agency website at nea.gov.sg.

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'Hail' and fierce storms reported in western parts of Singapore
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