Meteorologists know which conditions may spawn a tornado, but the actual birth process within a thunderstorm is still up for debate. The most likely scenario: a warm, humid layer of air forms near the ground under a layer of colder air in the upper atmosphere. When you have warm air near cooler air, you get an unstable atmosphere, and when there’s wind shear between the layers, a rotation forms.
If a layer of hot, dry air becomes established between the warm, humid air below and the cooler air aloft, it forms a boundary called a “convective cap” that keeps the layer near the surface from rising. A convective cap acts like the radiator cap on your car, keeping the atmosphere from boiling over. As solar energy passes through the cap, it heats up the humid air at ground level, which pressurizes the cap like steam in a radiator.
Tornadoes can form in other ways too: they often spring up within hurricanes, which already contain all the heat, rotation, and moisture that a tornado needs for survival. Tornadoes can even form in the winter during intense storms. Winter tornadoes are most common near the Gulf of Mexico, when warmer air collides with advancing storm systems.
Now add a dryline, or cold front, into the scenario: the convective cap can weaken to the point where all that built-up warmth near the surface explodes through the layer of hot air, mixing with the colder air above it to form a supercell, with strong updrafts rotating rapidly upward through the cloud. As the thunderstorm builds higher, a rotating “wall cloud” descends beneath the storm—the direct precursor to a tornado. Some theorists say that a tube of air spinning horizontally near the surface (like a rolling pin) can get picked up by the updrafts at this point, and with the tube now spinning vertically, a funnel cloud forms. Within the funnel is a strong downdraft, which descends toward the ground, creating a tornado.
Sometimes small whirlpools of wind form on the leading edges of gust fronts and are called gustnadoes. They’re not really tornadoes since they’re not connected to the cloud base, but they can still cause damage. On June 9, 1994, a line of strong thunderstorms racing through central Tennessee spawned a gustnado that passed within 100 yards of the Memphis National Weather Service, causing F1-level damage to houses and apartments nearby. Some gustnadoes have been clocked at speeds of up to 110 miles per hour.
Another type of tornado that doesn’t form in a supercell is the landspout, a weak column of spinning winds. These usually occur in Colorado and Florida. Unlike their larger cousins, landspouts don’t generally show up on Doppler radar and their life cycles are much shorter than a tornado’s. Landspouts usually form beneath building cumulus clouds, and although relatively weak, a few become powerful enough to cause serious damage.
A landspout over water is called—wait for it—a waterspout, and is also one of the weaker types of atmospheric funnels. In this case the word weaker is relative: a few of these seagoing cyclones can spin at speeds of up to 190 miles per hour, but they never reach the 300-miles-per-hour-plus velocity of an F5 tornado. Waterspouts form when moist humid air is pulled into a rotating updraft over a body of water. Until the rotation reaches a speed of 40 miles per hour or so, the funnel may be invisible. But as moisture begins to condense, a column of spinning water vapor reaches down from the cloud toward the surface. Waterspouts look like they’re sucking huge amounts of water into the clouds, but it’s really just vapor. Sometimes waterspouts can move inland and become tornadoes.
In the last few years, millions of dollars and thousands of hours have been spent learning how to predict these storms using sophisticated Doppler radar and other electronic methods. But the very first tornado forecast was accomplished back in 1948 without the aid of today’s high-tech gadgetry.
On March 20, California native Robert C. Miller, an Air Force captain and meteorologist, was putting together the evening forecast for Tinker Air Force Base in Oklahoma where he was stationed. Miller and Ernest Fawbush, a fellow forecaster, analyzed the latest weather maps from Washington and concluded it would be a relatively quiet night, with moderately strong winds but no storms, and that’s what their 9 p.m. forecast predicted. The two men didn’t realize that some of their source data were erroneous until a strong twister tore through the base an hour later, narrowly missing the aircraft hangars and operations center, and blowing the windows out of nearly every building on the base.
Five days later, on March 25, Miller was producing the morning charts when he noticed that the day’s expected weather conditions would be almost identical to those on the day of the tornado. He alerted General Fred S. Borum, who was by now in charge of the operation. The general ordered Miller to issue a thunderstorm warning, and by 2 p.m. a squall line had formed, just as it had before the last tornado.
“Are you going to issue a tornado forecast?” the general asked. Miller and Fawbush hemmed and hawed, neither relishing the idea of having another blown forecast pinned on them. “We both made abortive efforts at crawling out of such a horrendous decision,” said Miller in his memoirs. “We pointed out the infinitesimal possibility of a second tornado striking the same area within twenty years or more, let alone in five days. ‘Besides,’ we said, ‘no one has ever issued an operational tornado forecast.’ ”
“You are about to set a precedent,” said the general.
The forecast was composed, typed, and sent to Base Operations. A weather alert was sounded and base personnel flew into action, securing planes in hangars and tying down loose objects. At 5 p.m. a squall line passed through a nearby airport, but with only light rain and some small hail. Dejected, Miller drove home to commiserate with his wife. Later that evening as the couple was listening to the radio, an announcer broke in with an urgent bulletin about a tornado at Tinker Field.
Miller rushed back to the base to find a scene of devastation, with power poles down and debris strewn everywhere. Miller relates what a jubilant Major Fawbush told him he’d missed:
“As the line approached the southwest corner of the field, two thunderstorms seemed to join and quickly took on a greenish black hue. They could observe a slow counterclockwise cloud rotation around the point at which the storms merged. Suddenly a large cone shaped cloud bulged down rotating counterclockwise at great speed. At the same time they saw a wing from one of the moth-balled World War II B-29s float lazily upward toward the visible part of the funnel. A second or two later the wing disintegrated, the funnel shot to the ground and the second large tornado in five days began its devastating journey across the base very close to the track of its predecessor.”
Buoyed by their success, the Air Force set up the National Severe Storms Forecast Center, now the National Weather Service’s Storm Prediction Center, in 1951, and soon the public was clamoring for its own storm warnings. In 1952 the Weather Bureau finally set up its own storm prediction agency, the Weather Bureau Severe Weather Unit, which became the Storm Prediction Center in 1995.
In 1971, the same year Ted Fujita came up with his tornado damage scale, Doppler radar was first used to confirm that winds within a hook echo were rotating, giving scientists a picture of a storm’s internal mesocyclone—the “smoking gun” that pointed to the origins of tornadoes. In 1973, Doppler radar pinpointed an area in a thunderstorm near Union City, Oklahoma, where winds abruptly changed direction, which turned out to coincide with the occurrence of a violent tornado. For the first time meteorologists had direct evidence that Doppler radar could spot a twister in its formative stages.
Doppler is great if a tornado happens to pass by a radar installation, but few twisters are that accommodating. In 1994 and 1995, NOAA’s National Severe Storms Laboratory (NSSL) formed a plan to hunt tornadoes on their own turf, taking the war to the enemy for the first time. The main purpose of the VORTEX1 project (Verification of the Origins of Rotation in Tornadoes Experiment 1) was to find out exactly how and under what conditions tornadoes form.
After formulating twenty-two hypotheses for the project to either prove or disprove, Dr. Erik Rasmussen was assigned the role of project director and field coordinator, and several universities were brought on board to aid in the project and analyze the data. Unlike previous efforts to study tornadoes, all the equipment and manpower available would be brought to bear on only one storm at a time, analyzing each twister in exhaustive detail to obtain as much data as possible from different vantage points and with myriad instrument types.
Twelve cars and five vans were outfitted with the latest in sensors designed to measure temperature, humidity, wind speed, and air pressure. The data would be gathered every six seconds and stored for later analysis and comparison. Another van would serve as a traveling command post where the field commander could constantly monitor the position of the other vehicles. The vans were equipped with weather balloons designed to transmit upper-air information back to the convoy, which would form a “mobile mesonet” (a network of weather and environmental monitoring stations) that could cover the tornado from all angles.
The crown jewel of project VORTEX1 was a mobile Doppler radar unit mounted on a truck that would allow researchers to intercept and study supercells and tornadoes wherever they occurred. This “Doppler on wheels” would become the most important element during the hunt, peering deep inside a supercell’s mesocyclone to watch the actual birth of a twister.
Orbiting overhead as the convoy spread out around a storm were NOAA’s WP-3D Orion (the same type of aircraft used by the famed Hurricane Hunters of the 53rd Weather Reconnaissance Squadron of the Air Force Reserve) and a Lockheed Electra owned by NCAR. The two planes gave the project three-dimensional coverage of any target, using their belly radars to scan horizontally to determine a storm’s internal structure, and their tail radar to scan vertically for wind speed information.
VORTEX1 launched in 1994. That year turned out to be one of the slowest ever for tornadoes in the target area. Even though there were no twisters to study, scientists practiced deploying the mobile mesonet and were able to gather data on several supercell thunderstorms.
Things changed in 1995, when the team was able to intercept nine tornadoes and study them at close range. One tornado that formed near Dimmitt, Texas, became the most intensely examined twister in history. On June 2, after gathering data on a tornado that destroyed part of the town of Friona, Texas, radar showed another mesocyclone forming near Dimmitt. The teams quickly moved into their assigned positions around the developing storm, and just as quickly a tornado formed south of the town. The team was able to capture high-resolution Doppler data of the debris cloud caused by the tornado, as well as some video footage.
The tornado evolved into a powerful F4, literally sucking the pavement off a stretch of State Highway 86 and snapping telephone poles like matchsticks. Several vehicles were destroyed, and two trucks disappeared completely. Ten miles away, another tornado had formed, so the Electra was able to gather data from only one side of the Dimmitt storm. Even so, the planes were able to document how air flowed through the mesocyclone. Ground units meanwhile collected nearly 1,000 automated surface observations near the storm and around 1,000 additional measurements by balloon and other methods.
Some of the findings include:
• Most of the elements that lead to tornadoes are present in one small part of a supercell, allowing scientists to narrow their focus.
• Tornadoes can form rapidly at the beginning of a storm’s life, in less than half an hour from the formation of the supercell.
• Before a tornado forms, a large invisible rotating segment of the storm’s mesocylone already extends to ground level.
In 1997 a follow-up to the project, called Sub-VORTEX, was conducted, but with fewer vehicles and a tighter focus. Sub-VORTEX used two Doppler radar trucks to look into the same tornado from different angles, forming a two-dimensional image of the storm’s interior.
In 2009 another project was launched under the name VORTEX2 to study why some thunderstorms produce tornadoes, while others don’t. Although many questions remain, the VORTEX projects have substantially increased our knowledge of this dangerous weather phenomenon.