You're riding in a hot air balloon just south of the equator. since the balloon is moving west, you know that the are blowing the balloon?

You know that the tropical easterlies are blowing the ballon.

Explanation: A hot air balloon is mostly moved by the currents of wind that impacts the surface of the ballon and pushes it in a given direction.

If the ballon is moving west, this means that the wind is going from east to west, particularly, knowing that the ballon is right at the south of the equator, we can conclude that the winds are the Tropical Easterlies (that casually blows from east to west)

So the complete sentence can be:

... You know that the tropical easterlies are blowing the ballon.

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