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The unidentified flying object (UFO) reported by some residents in Sha Tin last Satruday (January 2) was found to be the condensation trails of a high-flying aircraft, according to an investigation conducted by the Hong Kong Observatory (HKO).

"The exhaust gas of aircraft contains a lot of water vapour. When this is injected into air with high relative humidity, the water vapour will condense into water droplets and form cloud-like streamers trailing behind the aircraft.

"This phenomenon is known as condensation trails, sometimes abbreviated to contrails," an HKO spokesman said today (Wednesday).

"In fact, what the residents saw -- two parallel white bands of smoke with a bright spot of light leading in front, is in line with the appearance of condensation trails," he said.

He pointed out that in nearly saturated air, contrails could persist for more than 30 minutes.

"Contrails usually occur at high altitudes.

"In the past, most aircraft in Hong Kong were seen at low altitudes either taking off or landing. Therefore, contrails have not often been recorded in Hong Kong," the spokesman added.

END/Wednesday, January 6, 1999

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