The great drought of 1986, the worst in the Southeast in at least the 115 years that government meteorologists have been keeping records, can be blamed on the jet stream and the odd way it was behaving late last winter and early this spring, according to climatologists at the National Weather Service.
While many people in the Southeast were enjoying a mild winter and a balmy spring, the jet stream, which should have been steering rain and snowstorms into the region, had already retreated up toward the Canadian border, giving the precipitation to the Midwest and New England.
The jet stream, a permanent high-altitude wind encircling the Earth in a generally west-to-east direction, normally drifts northward during spring, leaving behind soil wet enough to turn the grass green and get the crops off to a strong start. Through most of the summer the Southeast lives off the wet soil, helped by the occasional summer evening storm. Most of those storms, however, have disappeared -- another casualty of the wayward jet stream.
"Droughts will occur for their own reasons, but when you get one like this -- one that's definitely in the record books already -- you start to look at other factors: long-range cycles that could mean we're in for droughts recurring year after year," said Murray Mitchell, a research climatologist who recently retired from the National Oceanic and Atmospheric Administration.
Mitchell said there is evidence that the current drought may be part of a 22-year climatic cycle linked to the behavior of the sun's magnetic field. The last major dry spell in the Southeast occurred during the mid-1960s.
The 22-year drought cycle has been well established for the western United States, going as far back as 1600. Mitchell was one of the scientists who discovered this cycle by studying the growth rings of very old trees. During droughts, trees grow slowly and have narrower rings.
Tree ring studies in the East have not been completed. However, Mitchell said, climatologists have noticed a general "seesawing pattern" in which the western droughts coincide with wet spells in the East and vice versa. Most of the United States west of the Mississippi is currently enjoying normal or unusually wet weather.
Mitchell emphasized that although droughts can occur for various noncyclic reasons, the 22-year solar magnetic cycle seems to increase the likelihood of droughts and worsen droughts that begin for other reasons.
No one knows how the solar cycle affects Earth's climate.
"That's one of the great mysteries," Mitchell said. "But when you look at the data going back over the centuries, it pops right out at you: The cycle is real."
The seesaw pattern is a consequence of the jet stream. Although the jet stream is often depicted as a tightly constricted current of air, it actually is a diffuse wind, the core of the prevailing westerlies. At its center, winds blow about 100 mph in summer and up to about 200 mph in winter, usually between five and seven miles above the ground.
The jet stream is a natural consequence of hotter air at the equator and colder air at the poles. Warm equatorial air rises, flows northward (in the southern hemisphere it flows southward), cools over the North Pole and sinks back to the ground to flow south, completing the circuit.
If the Earth did not spin, winds near ground level would tend always to blow from the north. The Earth's spinning, however, imparts a sideways motion that, at the core, is the jet stream.
During late winter the jet stream, or at least its subtropical branch, normally cuts across Mexico and picks up moisture over the Gulf of Mexico, said James Wagner, a climate analyst for the National Weather Service. It carries the clouds in a northeastward direction, across the Southeast and over the Atlantic around Cape Hatteras.
"As spring moves up from the South, it pushes the jet stream northward," Wagner said. "By May it's running from east Texas to up along the Ohio Valley. As it moves to the north, it's bringing the rain farther and farther north."
In a normal year the humid spring of Georgia becomes the humid summer of Washington. Most of the soil moisture on which farmers rely in the Southeast is an endowment deposited as the jet stream migrates northward over the region.
"This is the main source of the water we have to live on for the summer," Wagner said. "Once the jet stream is up north, the summer rains we get are mostly a result of evaporation that forms thunderheads in the late afternoon."
In other words, summer rains are largely a recycling of the same water brought in by the jet stream. The water evaporates from the soil and from plant leaves, from rivers and lakes, and forms clouds that drop the water down again not far away. Summer storms, however, usually return less water to the soil than was lost through evaporation.
As evaporation draws on the original endowment, only part of the water is reinvested in the soil. As summer wears on, the balance dwindles. Normally the effects don't show in the grass or on the crops until late August, when the crops are mature.
This year, however, the lack of a substantial endowment from the jet stream has meant that there is little water that can evaporate back into the air. As a result, summer rains have been unusually spare and spotty. July's water balance is down to where, in a normal year, it would be in late August or early September.
The lack of evaporation also has meant hotter air temperatures. Normally some of the sun's energy is absorbed by water molecules, turning them from liquid to vapor. With evaporation rates so low, more of the solar energy simply goes into heating the soil and air.
Still, there is enough evaporation to keep the air humid. Normally this warm, humid air would rise until a cooler upper atmosphere condensed the water vapor back into rain. This summer the upper atmosphere has usually been too warm to condense much water. The water vapor remains muggy water vapor.
Wagner said other factors also have contributed to the drought. The Bermuda High, a dome of high air pressure that normally sits over the Bermuda region, has moved west, closer to the Southeast. High pressure causes air in the upper atmosphere, normally dry, to descend. Storm formation is favored by low pressure regions, which allow humid air from lower levels to rise into the cooler upper atmosphere.
The Appalachian mountains also have contributed to the drought. The few potential storms that have come from the West have been pushed higher as the air flowed up the mountains' western slopes. That has often cooled the water vapor enough to cause rain, but only on the west side of the mountains -- leaving little water for the farmers on the east side of the mountains.
With summer's usual source of rain gone, Wagner said, the drought is not likely to break until the fall hurricane season begins. Most fall rains come from tropical storms that form in the Caribbean and move over the Southeast. Most don't become hurricanes, but they can bring heavy rains.
"We won't really know whether we're in a long-term drought until we see what happens next year or the year after," said Mitchell. "If you want to worry, though, there's some evidence to support you."
Officials yesterday estimated the agricultural losses from the drought at up to $1.9 billion. And Secretary of Agriculture Richard E. Lyng approved disaster relief for 69 counties in North Carolina.