Weathering the Summer of 1993
From floods to droughts, cold snap to heat wave, the jet stream blew it all our way last summer. Here's how.
When storms drenched the Midwest for six weeks nonstop last summer, no one was singing in the rain. The overflowing Mississippi and Missouri Rivers, and their veinlike tributaries, submerged areas of nine states. The muddy waters caused $12 billion in damages. Fifty people died; 70,000 were left homeless.
Meanwhile, in the South, a searing drought shriveled corn and soybean crops. "We set up fans in the henhouses and portable shades in the barn to keep the animals from dying of heat," laments 17-year-old Derrick Myers, who works on his family's farm in Bowman, South Carolina.
And July was not much kinder to other parts of the country. While people in the Northeast withstood 11 straight days of near 38°C (100°F) temps, people in the West went skiing out of season. A foot of snow fell in the Uinta Mountains near Salt Lake City, Utah. In Wisdom, Montana, the mercury plunged to -6°C (23°F), down from its 20°C norm.
No matter where you live, you probably weathered some of last summer's wicked weather. But did you know that it was all caused by the same weather phenomena? It's true, says climatologist Vernon Kousky.
Jet Setter" For reasons we're not sure of," says Kousky, it all began with the jet stream, a band of high-speed air currents originating above the Pacific Ocean. Last June, the jet stream took an unusual turn over the western United States.
Check out a "normal" summer weather report; you can usually see the jet stream speeding west to east across the map. Like a river of air, it curves north and then south, causing and maintaining winds, cool and warm weather shifts, and storms in the atmosphere below it. The movement resembles a wave, with a ridge (hump) in the west and a trough (dip) in the east.
But last summer, Kousky says, due to stronger winds than normal (no one's sure why), the jet stream took an unusual path. "Instead of going north to British Columbia, [it] took a dive over the western U.S. and then went north though the Great Lakes."
By forming a trough in the west, the jet stream opened the way for a cold Canadian air mass to "stretch" down into the western states. This enormous body of cold air lowered temperatures over the region, chilling Utah and Montana. The cool air, blocked by the trough's high-speed winds, could not continue moving south. Instead it moved in on the northern Midwest, around the Dakotas and Minnesota.
At the same time, winds at the base of the trough began pumping warm, moist air from the Gulf of Mexico northward, into the Midwest, around Iowa and Missouri.
When this warm air mass clashed with the cold Canadian air mass, the two stopped short, creating a barrier of air called a stationary front. Neither side gave way, but the warm, less-dense air rose high above the cool air. The forecast: lasting rain, as the water vapor in the rising air cooled and condensed into droplets. Because the front didn't budge, it just kept raining. And raining. And raining.
"That created a classic flood situation," says Kousky. Because the ground in the Midwest was already saturated from the spring snowmelt, it couldn't absorb the constant summer rains. In addition, human-made levees (dikes), built along the riverbanks to prevent flooding, forced the rising rivers into narrower channels. That increased the waters' height, causing the rivers to overflow their banks.
Sewage treatment plants were flooded, adding pollutants to the water supply. "You couldn't drink the water or use the restroom," says 14-year-old Pam Wilson of Harden, Missouri. "The water got higher and higher. It was dirty and stinky."
Southern DroughtSo why the drought in the South? The unusual path of the jet stream again. Instead of pumping warm, moist air into the region, as usual, the jet stream channeled that air into the Midwest. That left much of the South parched.
A high pressure air mass situated east of the jet stream's ridge added to the heat, says meteorologist Louis Uccellini. This tropical air mass, called a Bermuda High, for its normal position over that island in the summer, formed a hot-air "dome" over the East. It covered 2.5 million square kilometers. Warm air beneath the dome kept sinking and being compressed layer upon layer. This compression — like the kind you find in a pressure cooker — drove the temperature to record highs.
The AftermathAfter six grueling weeks, the jet stream finally shifted to its usual pattern. Summer's weather returned to its regular beat. And weather-beaten Americans began the long process of replanting and rebuilding.
But scientists are still grappling with some big questions: What made the weather so wild? Was it only tied to the jet stream's winds? What made the winds so strong this year?
"It will take more research to know exactly what happened," says Uccellini. Since wind forms when warm air rises and cool air rushes in to replace it, some scientists theorize that a stronger contrast between the warm and cool air could have intensified the jet stream's winds.
Extra warm air could have come from the Pacific. There, an irregular flow of warm surface water, called El Niño, whips a lot of warm air into the atmosphere. Since this past El Niño episode has lasted a year longer than usual — a first in weather history — much more warm air than usual may be present over the Pacific.
Added cool air could be the result of recent volcanic eruptions, such as that of Mount Pinatubo in July, 1991. Volcanic ash spewed into the atmosphere can block part of the sun's radiation, cooling the air.
Whatever the ultimate cause, the summer's weather reminds us all of nature's awesome power. Though we may one day be better at predicting the weather, we will probably never be able to control it. As climatologist Kousky says, "We just have to live with it, and accept the risks."
October 22, 1993