Almost 200 aircraft flew over Beijing's Tian'anmen Square Thursday as part of the parade to mark the 70th anniversary of the end of the Second World War.

They included airborne warning and control system aircraft, bombers, fighters and maritime patrol aircraft.

Many left smoke trails that brightly colored the blue skies.

A China Central Television commentator said the colored smoke represented Chinese people's advance toward the future.

People were curious how the smoke trails were created. More than 140 accounts mentioned them in one hour on Sina's Weibo microblog service.

The Bayi (August 1) Team aerobatics squadron brought their new liquid smoke generators to the parade.

"The liquid smoke generators are installed under the belly," Cao Zhenzhong, commander of the Bayi team, told Xinhua.

They work when high-boiling-point reagents pour into the nacelle, or engine casing. With nitrogen, the smoke sprays into the exhaust coming from the engine, and transforms into steam.

The steam then turns into a form of fog as it cools.

"So the smoke lines should be colorful fog," Cao said.

During the parade, the aircraft flew at lower than normal speed, which helped the atomization of the reagent.

The success of the display was achieved after dozens of trials of varying formulas and nozzle adjustments.

White is the most simple reagent, being a made of diesel, lubricating oil, kerosene, or even soybean oil, according to cannews.com.cn.

Other colors require ingredients with higher boiling points and dyes.

During the trials, soybean oil was used to produce a reagent as it was the most environmentally friendly ingredient, but it was also easily carbonized in high temperatures and choked nozzles.

Other environmentally non-toxic raw materials were used, but the final details were not released.

Different jets have different ways of spraying the smoke, depending on their engine types, such as turbojet and fanjet. But they have similar basic principles. The high-boiling point reagents are heated by the flue from the engines, producing abundant smoke dyed different colors.

What looks like white smoke could have three possible causes: a release of jet fuel as the aircraft prepares to land; an eddying current around the wing tips in humid air; or a vapor trail made of condensed water vapor coming from the engine.

The aircraft use hydrocarbon fuel. After burning, the carbon dioxide and water vapor merge. At high altitude, above 8,000 meters, and temperatures below minus 40 degree centigrade, the vapor trails occur.