People look at a hot air ballon during the balloon festival at Rheinaue leisure park in Bonn, Germany, on June 8, 2019. The four-day festival, which kicked off on June 7, attracted more than 30 ballon teams from Germany, Belgium and the Netherlands. (Xinhua/Lu Yang)

People visit the balloon festival at Rheinaue leisure park in Bonn, Germany, on June 8, 2019. The four-day festival, which kicked off on June 7, attracted more than 30 ballon teams from Germany, Belgium and the Netherlands. (Xinhua/Lu Yang)

Hot air ballons are seen during a hot air ballon night glow show of the 11th Ballonfestival Bonn at Rheinaue leisure park in Bonn, Germany, on June 8, 2019. The four-day festival which kicked off on Friday attracted more than 30 ballon teams from Germany, Belgium and the Netherlands. (Xinhua/Lu Yang)

People view a hot air ballon night glow show of the 11th Ballonfestival Bonn at Rheinaue leisure park in Bonn, Germany, on June 8, 2019. The four-day festival which kicked off on Friday attracted more than 30 ballon teams from Germany, Belgium and the Netherlands. (Xinhua/Lu Yang)