A storm was threatening.

The electric saturation of the atmosphere had become so great that about half-past two o'clock Robur witnessed a phenomenon that was new to him.
In the north, whence the storm was traveling, were spirals of half-luminous vapor due to the difference in the electric charges of the various beds of cloud.

The reflections of these bands came running along the waves in myriads of lights, growing in intensity as the sky darkened.
The "Albatross" and the storm were sure to meet, for they were exactly in front of each other.
And Frycollin?
Well! Frycollin was being towed--and towed is exactly the word, for the rope made such an angle, with the aeronef, now going at over sixty knots an hour, that the tub was a long way behind her.
The crew were busy in preparing for the storm, for the "Albatross" would either have to rise above it or drive through its lowest layers.

She was about three thousand feet above the sea when a clap of thunder was heard.

Suddenly the squall struck her.