Global Weather Factors Set Hurricane Factory Spinning
EYE ON THE STORM
BOSTON — THERE'S good news about the North Atlantic hurricane season that runs from June through November. The outlook is for tropical cyclone activity generally to be a bit below normal.
Veteran hurricane forecaster William Gray of Colorado State University at Fort Collins says he expects nine tropical storms strong enough to be given names. Five of them should be hurricanes, including one intense hurricane. The long-term average is nine named storms, including six hurricanes, two of which are intense. Also, Dr. Gray expects 35 days when named tropical storms are active - including 15 hurricane days. The long-term average is 46 storm days, including 23 hurricane days.
There are caveats. Gray's forecasting record is good, with only two outright failures in the past 10 years. But he warns that his scheme could fail in any given year. More important, it doesn't tell when or where a hurricane may form or hit land. Also, the scheme doesn't work for the Pacific or Indian Oceans.
Gray and his colleagues have found several global factors that influence North Atlantic tropical-storm formation. These include El Nino - the presence of warm surface water in the equatorial Pacific and associated wind patterns that discourage North Atlantic hurricane activity. Other factors are atmospheric patterns that influence West African rainfall - drought, for example, indicates low tropical-storm activity - and the direction of stratospheric wind patterns.
Hurricanes are an intense form of tropical cyclone that occur in the North Atlantic, eastern and western North Pacific (where they are called typhoons), and in the Western South Pacific and Indian Ocean (where they are simply called cyclones). They are unknown in the eastern South Pacific and South Atlantic Oceans.
Tropical cyclones don't form when the sea surface is cooler than about 80 degrees F. They never form within 4 or 5 degrees latitude of the equator, where the influence of Earth's rotation is too weak to spin up their winds. Only about 13 percent of the storms form above 22 degrees latitude, about the latitude of Cuba.
Some 50 to 75 percent of the tropical cyclones that arise annually achieve the 74 m.p.h.-minimum wind speed to be classed as typhoons or hurricanes. Also, most Atlantic hurricanes occur during the so-called ``intense'' part of the hurricane season - August to October. Gray will update his forecast in early August.
WITHIN about 250 miles of a developing hurricane, inflowing air begins to converge strongly. Towering convective cumulus clouds, which form the main body of the storm, appear. Wind speeds pick up as the inward spiraling air nears the storm center. While still about 10 to 60 miles out, the air flow turns sharply upward in the ring of massive cumulus clouds, called the eye wall. Here is where the heaviest rains fall and the strongest winds blow. Yet inside their fury lies the calm of the hurricane eye. There, sea-level air pressure is 5 to 10 percent, or more, lower than it is outside the storm. The sea surface domes up, forming a mound of water that moves with the storm. This adds to water heaped up by winds to create the storm surge that sweeps in like a monstrous tide when a hurricane comes ashore.
Outside the eye wall, hurricane clouds tend to organize into rain bands that spiral around the eye. This is why areas where hurricanes come over land receive varying amounts of rainfall. Tornadoes may also spin out of the storm when it comes ashore.
As a rough analogy, you can think of the central storm ``machinery'' as an ``engine,'' Kerry Emanuel of the Massachusetts Institute of Technology suggests. It has a fuel supply - the warm sea surface supplying heat and moisture. It has ``cylinders'' - the convective cloud systems that transform that fuel into energy for the storm's circulation. And there is an exhaust system - the outflow at the top that carries air away and spreads cirrus clouds hundreds of miles from the storm center.
This vigorous circulation - in at the bottom, up through the convective clouds, out at the top - moves about two million metric tons of air a second to heights of 40,000 to 60,000 feet. The power required to do this is several hundred times the electrical generating capacity in the United States. The so-called latent heat energy, released as 16 to 20 billion metric tons of water vapor a day, condenses into droplets and is more than adequate to supply that power.
While Gray's statistical scheme can foretell North Atlantic seasonal hurricane activity, the prize daily forecasters seek is sharper projections of storm development and movement. Aided by aircraft, radar, and satellites, they can maintain vigilance. Computer projections also have added to their forecasting skill. But they still cannot foretell storm characteristics well enough to avoid overwarning areas at risk of hurricane attack. As the American Meteorological Society has noted, detailed forecasting ``remains rather subjective.'' So, for now, extensive warnings with attendant false alarms are the only safe course of action.