Six o'clock in the morning.

In the sky above Zurich, a sharp sound of breaking wind rang out, as if there was an extremely powerful gale that swept across from high in the sky.

In the city, many people who woke up early for their morning walks looked up to check, thinking that it was a change in the weather that was about to rain, but when they raised their heads, what they saw was an azure, cloudless sky.

This situation has left local residents puzzled.

On the ...