The Chinese have been showing off plans for changing the weather to ensure that the Olympics passes off without a hitch.

The Chinese Olympic stadium lacks a roof and Beijing's Weather Modification Office has been explaining how it intends to stop spectators getting wet.

An IBM p575 supercomputer is being used to provide hourly weather forecasts for 17,000 square miles around the city.

If rain clouds are expected the Weather Modification Office can call on 1,500 staff with 30 aircraft to drop chemicals into the clouds to force them to drop their water early.

A further 37,000 part-time staff can fire additional materials into the clouds using 7,113 anti-aircraft guns and 4,991 rocket launchers.

Zhang Qian, head of Beijing's Weather Modification Office, told MIT's Technical Review: "We use a coolant made from liquid nitrogen to increase the number of droplets while decreasing their average size.

"As a result, the smaller droplets are less likely to fall, and precipitation can be reduced."

Cloud seeding is not new, but it is thought that China has taken the technology further than any other country.

The state-run Xinhua news agency claims that the Weather Modification Office 'created' more than 250 billion tons of rain between 1999 and 2007.

The technology has been around since the 1960s, and was allegedly used by the US Army in Vietnam in an attempt to make travel difficult for the North Vietnamese.

The Soviet Union also apparently used it, notably in 1986 to keep radioactive rain clouds from reaching Moscow, according to G P Beryulev of the Central Aerological Observatory at Rosgidromet.