Weather radar technology revolutionized the forecasting of severe storms and has saved many lives. The revolution began in the evening of April 9, 1953, when Don Staggs and Glenn Stout, two meteorologists at the Illinois State Water Survey, decided to break the rules. The protocol for operating the research radar at the Illinois State Water Survey office was to shut down and lower the dish if severe weather was ever in the area.
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Don and Glenn were too amazed at what they saw on the radar scope to shut it down that night. An interesting hook shape slowly emerged from the southwest side of the storm as the radar collected echoes from the rainfall within this storm. They called it a hook echo, because it looked like a fishhook was protruding from the backside of the storm.
They immediately recorded the position of this strange feature so they could drive out in the morning to see if there was any evidence of what it was, once the storm had ended. The next, their questions were indeed answered. Just north of Champaign, a tornado had passed from west to east across the county. Don and Glenn had inadvertently—and disobediently—discovered the very first tornadic signature on radar.
They immediately shared their discovery with friends and family, including a new professor at the University of Chicago named Dr. Theodore Fujita. Interest in tornado forecasting increased rapidly. Just under three years after the hook echo was discovered, the first successful radar forecast of a tornado took place in Bryan, Texas, on April 5, 1956—predicted 30 minutes before the tornado actually hit. Meanwhile Fujita published an analysis of Don and Glenn’s data, which later helped him formulate his now‐widely‐used Fujita scale for ranking tornado damage. These radar images enhanced his research as he built a database of over 30,000 tornado damage reports.
Radar Technology Improves…
Radar technology has improved significantly over the years. Color radar displays began in 1974, making it easier to distinguish the different levels of precipitation intensity. A new Doppler radar network was installed in the early 1990s, making it possible to track the wind speed and direction within a storm, a key factor in determining if a storm is rotating.
An example of this technology in practice came on the evening of May 4, 2007 as a supercell storm was gathering strength southwest of Greensburg, Kansas. It was night, so only radar could see this monstrous tornado that had emerged beneath the storm’s rotating updraft.
Meteorologists all over the world watched live animations of this storm in amazement as a pronounced hook echo took shape. This tornado was going to hit the town at night, a notoriously bad time for tornadoes to arrive, as people are home, possibly sleeping or not paying attention to severe weather alerts on TV, radio, or the internet. We expected a disaster. I watched the hook echo pass right over town, and I remember telling my wife that I expected hundreds of fatalities— this in a town that had roughly a population of 1,400 residents.
This storm had a pronounced, large hook echo and textbook characteristics, with a large forward flank, here, with an embedded hail core right here and the pronounced hook echo here. Watching this animation makes it possible to identify each of these features in the storm that advance towards Greenburg.
Well, damage photos the next day showed that destruction was almost complete. 95% of the structures in the town were severely damaged, with most being leveled. The winds were measured at 205 miles per hour, high enough to be classified as an EF‐5 on the new Enhanced Fujita scale.
Incredibly, most of the townspeople survived that night. The credit for this miracle was given to the Doppler radar in Dodge City, Kansas, which provided advanced warning starting 39 minutes before impact. All but 11 lives were saved that night.
Early Uses Of Radar
Radar stands for RAdio Detection And Ranging. It is a device that sends a focused microwave radio signal into the atmosphere and collects the reflection of these waves off of objects the beam intersects. Its original design did not include, though, the detection of precipitation.
World War II marks the first widespread use of radar technology. Its primary purpose was to detect enemy aircraft. Long before they could be seen or heard, the first radar systems used low‐frequency waves of 200 megahertz to 400 megahertz. But the Allies secretly developed so‐called microwave radars with higher frequencies of 3,000 megahertz, called S‐band, and 10,000 megahertz, called X‐band.
Weather was a nuisance to these early microwave radar systems. Radar echoes from rain would cover the imagery, preventing radar operators from detecting enemy aircraft. This discovery led the Allies to conduct many of their missions under the cover of bad weather, effectively preventing the radars used by the opposition from seeing the attack.
A Network Of Radars
After the discovery of the hook echo in 1953, the first successful tornado prediction in 1956, the United States built and deployed a network of weather radars across the US in the late 1950s to detect precipitation. This radar network was upgraded in the 1980s with the next‐generation radar network, called NEXRAD.
Each radar in the NEXRAD system is housed within a radome. A radome is designed to protect the radar from adverse weather like strong winds, lightning, and ice accumulation, as well as from critters like birds and bats. Inside the radar is a large 30‐foot‐diameter dish which both transmits and receives the radar signal. The dish rotates on a large pedestal and broadcasts its signal in a radial pattern. This is why the radar data we see on TV and the internet is revealed by a sweeping line that fills out a circle.
Now, in the United States, the eastern US has the best radar coverage, with most locations scanned by several radars at once. Radar coverage in the western United States is challenging due to mountains blocking the radar beams. As a result, the radar network’s ability to monitor precipitation, intensity, and amount in the West is much more limited. The ideal scanning strategy to measure precipitation is to scan really low in the sky as possible. Mountains make this very, very difficult.
Some locations, though, are purposefully not scanned by NEXRAD radars. Notice, in the graphic below, the large void in south‐central Nevada, Home of one of the largest military bases in the United States. This is the region that in the fiction and conspiracy theories is sometimes called Area 51. In the movie Independence Day, this was where a fictional US government was conducting alien research.
OK, in reality, there is a US military base in this region, and they prevent inadvertent spying by broadcasting a microwave signal to block or blind the local weather radars. You can think of it like this. If someone was watching you at night with a bright flashlight, an easy way to prevent them from seeing you is to shine an equally bright flashlight back at them. By blinding the other person with a powerful flashlight, they cannot see what you’re doing. Similarly, when the United States government is testing equipment, like new aircraft technology, they don’t want the local NEXRAD radars observing what they’re doing.
An Elegant And Simple Solution To Monitor Rainfall
The radar broadcasts a powerful and focused microwave signal from its antenna as it scans in a circular pattern. These microwaves are scattered and reflected by raindrops, and some of that energy is directed back to the radar dish, where it is collected. The amount of power returned to the radar is proportional to the size and the number of drops that the radar is able to scan.
Unlike visible light, the majority of the radar beam passes through the clouds, through the rain and snow, to see storms behind storms.
Microwaves can pass through the atmosphere without being absorbed by the gases in the atmosphere, much like the way that visible light can shine through our atmosphere without being absorbed by the gases in the atmosphere. Well, unlike visible light, the majority of the radar beam passes through the clouds, through the rain and snow, to see storms behind storms. This allows the signal to pass through the atmosphere unattenuated, which in turn allows the radar to scan over greater distances—more than 200 miles, in some cases.
But weather radar is different from your microwave oven in your kitchen. On one hand, weather radar s operate at a power level equivalent to 1,000 times that of a standard microwave oven. And yet they’re not cooking raindrops. Why not?
Instead of continually blasting the rain with microwaves, which would cook it, weather radar sends out a focused, single, short‐lived pulse that last roughly one‐millionth of a second. This is called a pulsed Doppler radar. The radar dish then watches and waits for 1/1,000 of a second for some of that microwave pulse to be reflected or scattered back to the dish, where it’s collected.
Instead of continually blasting the rain with microwaves, which would cook it, weather radar sends out a focused, single, short‐lived pulse that last roughly one‐millionth of a second.
Again, the amount of energy it collects is directly proportional to the number of raindrops and their size inside of a contributing volume. The contributing volume is essentially a slice through the atmosphere. You can see these slices by looking at each pixel on a radar display.
So the amount of reflected energy lets us derive the intensity of the rainfall. And we can tell where the rain is, and its coverage, using the antennas azimuthal angle, the angle compared to true north. We can also use the elevation angle and the time it takes for the radar echoes to return to the dish. Together, these properties tell us exactly where the rain is.
Radar’s One Major Defecit
Well, radar has transformed how we track and predict severe weather, especially from tornadic thunderstorms. The United States National Weather Service has tracked tornadoes and fatalities since the 1940s. Their data shows that the number of powerful tornadoes has remained largely unchanged. And yet even as population has grown, putting more people in the path of these dangerous tornadoes, the number of deaths due to tornadoes has fallen sharply ever since the introduction of radar in the 1950s.
Weather radar has transformed the science and practice of extreme weather meteorology, allowing us to see under and through each storm. However, radar systems are ultimately limited by one factor, and that is their short range. 70% of Earth is covered in ocean, and our land‐based radar networks scan just a tiny fraction of coastal regions, with no coverage at all in the middle of the ocean.
This is where satellite technology proves its great worth. In a later discussion, I will discuss how heading to space lets us study the way that Earth’s largest extreme weather systems can evolve into mega‐ events, such as hurricanes.