Rockets and rainmaking

Rockets were probably first used for warfare in China about 900AD. The technology spread out from China over the following centuries and is still used at the end of the hot season in Thailand during bun bang fai or rocket festivals. In Yasothon today, the only difference is that bamboo has been replaced by plastic tubing and the chemical propellant is a lot more powerful than the gunpowder of the past.

How rockets work

There is certainly no better place to enjoy the Bun Bang Fai rocket festival than Yasothon province.

All rockets work on the same principle, the one described by Isaac Newton in his Third Law of Motion, namely: "For every action there is an equal and opposite reaction." This is also true for walking; we push backwards on the ground and the ground pushes us forwards. Or swimming; we push the water backwards and it pushes us forwards. Helicopter propellers push air downwards and the air pushes the propeller (and thus the helicopter) upwards.

So what does a rocket push on? At first the answer may not seem obvious, especially when you consider that rockets can even work in space where there isn't even any air to push on.

In a rocket engine the highly combustible fuel is mixed with oxygen and burns explosively. The hot waste gases are pushed backwards out of the rocket at high speed. This is the ``action''. The "reaction" is that the hot gases that are rushing backwards thrust the rocket forwards. This can lead to very rapid acceleration.

However, due to the huge amount of fuel needed to create such great thrust, rocket engines can't operate at high power for long. For example, the first stage of the Saturn V rocket, which launched the Apollo missions to the Moon, only burned for two and a half minutes, but during that time was able to reach a height of almost 61 km above the Earth.

Rainmaking — old and new ways

Going up in smoke. A rocket climbs rapidly into the clear blue Isaan sky.

According to tradition, rockets are fired to remind Phaya Taen, the king of the sky, of a deal he made with Phaya Kang Kok, the king of the toads, by which Phaya Taen is obliged to send enough rain for the rice harvest. After a successful harvest, large kites called wow tanu are launched. The kites have a bowstring which vibrates in the wind to produce a ``wow-wow-wow'' sound that tells Phaya Taen he has sent enough rain.

The importance of sufficient rainfall, as well as good water management, especially in the northeast of Thailand cannot be overstressed. According to Fred Pearce in his recent book When the Rivers Run Dry, it takes about 5,000 liters of water to produce one kilogram of rice!

However, modern times call for modern solutions, and due to the efforts of His Majesty the King Bhumibol Adulyadej, Thailand is now at the forefront of rainmaking technology. Anyone who has visited a cinema in Thailand will have seen tributes to HM the King, which are shown before the main feature. Footage often includes HM the King visiting drought-stricken regions. Indeed, the Royal Rainmaking Projects have made such advances that Thailand's advice is often sought by other countries in Asia and Africa, when they too face a drought.

"Super Sandwich"

HM the King's "super-sandwich" technique involves releasing crystalline particles (of silver iodide and other chemicals) into clouds. This is called seeding the clouds. The particles act as points around which tiny droplets of super-cooled water collect. These droplets join together to form snow which melts as it falls. Using aircraft to seed hot and cold clouds at different heights increases the chances of successfully producing rain and enables the rain to be targeted over particular areas.

While cloud seeding cannot stop a drought occurring, it has been used successfully to dras tically reduce the impact of drought over large areas of Thailand. One aspect of the operation is timing — it has to be carried out when the conditions are just right. If there are no clouds at all or the humidity is too low, then cloud seeding will not work. This was what frustrated attempts to produce rain over Chiang Mai to clear the smoke from forest fires in March of this year.

His Majesty was the first monarch ever to be awarded a patent for a waste water aerator, known as chai pattana. Since then, His Majesty has been awarded three further patents for rainmaking technology, the most recent of which was for the "super-sandwich", presented at Hua Hin on June 2, 2003.

Corrin Funnell is a laser physicist with a specialty in laser spectroscopy. He has taught in the UK, Egypt, and at Thailand's own Harrow International School, where he was head of physics. Currently, he is head of physics at Island School, Hong Kong.