Storms involved in the Fujiwhara effect are rotating around one another as if they had locked arms and were square dancing. Rather than each storm spinning about the other, they are actually moving about a central point between them, as if both were tied to the same post and each swung around it separately of the other.

To complete the effect, the entire system - the two storms and the central point between them - must move off in a single direction while the storms continue spinning about each other.

Fujiwhara looked closely at many different types of vortices to see how they acted when they came close to each other. He noted that if two vortices were equal in size and strength and spun in the same direction (like the hands of a clock but backwards, or "counter-clockwise"), they would move about each other as described above. But he also noticed other movements.

If two vortices spinning counter-clockwise approached and one of the vortices was larger than the other, they would begin spinning around each other for a short time with the larger one dominating. Eventually the smaller of the two vortices would get caught in the circulation of the larger one and be gobbled up.

If the similar vortices spun in opposite directions, one clockwise, one counter-clockwise, they would push each other away if they got too close. Other scientists since 1921 studied many cases where this happened in the atmosphere. Tropical cyclones called hurricanes or typhoons are perfect examples. This is where the term "Fujiwhara effect" gets used most often.

And there you go.