Fujiwhara effect describes a stormy waltz

The Fujiwhara effect describes the rotation of two storms around each other. It's most common with tropical cyclones such as typhoons or hurricanes, but also occurs in other cases.

The effect is thought to occur when storms get about 900 miles apart. When Tropical Storm Iris was approaching the Windward Islands on Aug. 23, 1995 and Hurricane Humberto was close behind, they drew close enough together to begin a Fujiwhara dance.

As Humberto chased down Iris, Humberto began to lift northward over Iris while Iris slowed down and turned a bit to the south. Iris became a hurricane just as this "dance" began but both storms were weakened by their passion for each other. As they weakened, other forces in the atmosphere broke them apart and sent them on their separate ways.

About 8 days later, Iris, now a hurricane with 110 mph winds, was moving northward east of Bermuda. This time Tropical Storm Karen scooted in behind Iris. But Karen was a much weaker storm with winds of only 45 mph. As the storms moved closer the Fujiwhara effect began. But Iris' strength dominated and poor Karen was flung around Iris to the north and Iris absorbed Karen right into its circulation while barely flinching. (National Hurricane Center reports on Hurricane Humberto and Hurricane Iris)

Another example of the Fujiwhara effect occurred in 1974, when Twin Hurricanes Ione and Kirsten spun about each other in the eastern Pacific. This satellite image shows Ione on the left and Kirsten on the right.

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.

A good way to picture this is to think of two ice skaters who skate quickly towards each other, nearly on a collision course, grab hands as they are about to pass and spin vigorously around in one big circle with their joined hands at the center.

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.

The effect is named after Dr. Sakuhei Fujiwhara who was the Chief of the Central Meteorological Bureau in Tokyo, Japan, shortly after the First World War. In 1921 he wrote a paper describing the motions of "vortices" in water. Water vortices are little water whirls that spin around. Whirlpools are water vortices.

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.

One final example shows how the storms don't necessarily have to be tropical. Just before Christmas, 1994, a large coastal storm formed off Florida's east coast. The storm seemed to become tropical, developing an eye, almost like a late season hurricane. As the storm moved northward along the Eastern Seaboard a very large upper-level storm formed in its wake over Florida. This upper level storm was huge with a wide circulation spreading northward. It seemed to reach out and grab the first storm only to slingshot it on a northwestward path into New England.

When this first storm came ashore nearly over New York City early on Christmas Eve it blew down trees all over Southern New England with winds clocked as high as 99 mph in Westport Harbor, Mass. The storm cut power to 130,000 in Connecticut as it dumped nearly 5 inches of rain on the area.

The Fujiwhara effect is possible almost anywhere atmospheric or water vortices come close together. Scientists since Fujiwhara have studied the effect and think many other processes can hinder the effect so it doesn't happen all the time. Still many examples like those above seem to prove Dr. Fujiwhara's findings.

If you'd like to learn more about Dr. Fujiwhara's work, his limited number of papers can be found by looking up Fujiwhara in some of the larger libraries. His research can get quite technical with many atmospheric equations listed. So be prepared for some in-depth reading. In some references, his name is spelled, "Fujiwara," without the "h."

One reference that is much easier to understand is in the October, 1964 issue of Weatherwise magazine. Page 232 describes the "dance" two typhoons did near Japan in August of that year.