On the evening of September 29 at 7 pm, after the launch of the Long March 6A Yao-6 carrier rocket, a brilliant “seven-coloured cloud” appeared over Shijiazhuang as well as in the skies above Beijing, Tianjin, and other parts of northern China, resembling a dragon flying across the horizon.

The cloud was actually a contrail. After the rocket’s launch, water vapour in the exhaust condensed into ice crystals in the stratosphere (about 50 kilometres high). These ice crystals diffracted and refracted the sun’s rays after sunset, with different wavelengths separating to form iridescence (similar to the principle of a rainbow). Combined with the orange-red glow of the setting sun, the effect appeared as a “seven-coloured gradient.”