Fixed weather radar is built to watch a whole region from a tower. It is very good at telling you a tornado is somewhere inside a storm, and very bad at telling you how fast the wind is turning at the bottom of it. The Doppler on Wheels, usually shortened to DOW, was built to solve exactly that problem by putting the radar dish on a truck and driving it to the storm. It is the instrument behind the highest wind speed ever reliably measured on Earth, and the one you have seen parked in a field on Storm Chasers. Here are the answers to the questions people most often ask about it.
What Is the Doppler on Wheels?
The Doppler on Wheels is a mobile Doppler weather radar mounted on the back of a flatbed truck. Rather than a permanent installation on a tower, the whole instrument drives to the storm, levels itself on hydraulic jacks and begins scanning within minutes of arriving. The DOW units operate on X-band, a much shorter radar wavelength than the fixed national network uses. A shorter wavelength and a close deployment together produce a far finer picture, letting the radar resolve structures inside a tornado only tens of metres across. Later additions to the fleet include a Rapid-Scan DOW, which transmits several beams at different elevations simultaneously so a violently evolving tornado can be sampled every few seconds instead of every couple of minutes.
Who Invented the Doppler on Wheels?
The DOW was conceived by atmospheric scientist Joshua Wurman, who built the first unit in 1995 with colleagues in the American research community. The fleet was subsequently operated by the Center for Severe Weather Research in Boulder, Colorado, and supported largely by the National Science Foundation as a shared facility that visiting researchers could request. The idea behind it was simple and slightly reckless: if the radar cannot see the bottom of the tornado from sixty miles away, stop trying to improve the radar and start closing the distance instead. The DOWs went on to become the core instrument of VORTEX2 in 2009 and 2010, the largest tornado field campaign ever mounted.
What Is the Fastest Wind Speed the DOW Has Measured?
On 3 May 1999, a DOW sampled the Bridge Creek-Moore tornado in Oklahoma and recorded a wind speed of roughly 301 mph about 100 feet above the ground. That measurement is generally accepted as the highest wind speed ever reliably recorded anywhere on Earth. You will see the figure quoted variously as 301, 302 or 318 mph, and the disagreement is not sloppiness on anyone's part. The reading carries an uncertainty of around 20 mph either way, and different analyses of the same raw data settle on slightly different points inside that band. The DOW recorded a comparable measurement in the 2013 El Reno tornado, the same storm whose 2.6-mile width made it the widest ever documented.
It is worth being precise about what this number is not. It is not a wind speed at the surface, and it is not what the tornado was rated on. Ratings on the Enhanced Fujita scale are assigned from damage found on the ground, not from radar, which is why a tornado can be measured at over 300 mph and still not carry the rating you would expect.
How Is the Doppler on Wheels Different From NEXRAD?
The difference is distance, and everything follows from it. NEXRAD is a fixed national network of radars on towers, and a radar beam does two inconvenient things as it travels. It spreads, so what starts as a narrow beam becomes thousands of feet wide by the time it is sixty miles out. And it travels in a straight line while the Earth curves away beneath it, so at that range the beam is sampling the storm thousands of feet up rather than near the ground.
The result is that a distant NEXRAD sees an average of a large volume of air well above the tornado, which is enough to issue a warning and nowhere near enough to measure the wind where houses are. The DOW parks a few miles away and samples the lowest few hundred feet directly. NEXRAD covers the country continuously and the DOW covers one storm intensely, and neither substitutes for the other.
Why Does the DOW Have to Drive So Close to the Tornado?
Because beam spread and Earth curvature cannot be engineered away, only driven around. Every mile of extra distance makes the sample coarser and higher, so the only route to a measurement near the ground is to shorten the range.
That is a deliberate trade rather than bravado. Crews position on paved roads with an escape route, work from forecast storm motion, and aim to be beside the tornado's path rather than in it. Being close enough for a useful measurement and being close enough to be hit are separated by a margin that has to be managed on every deployment, which is why DOW teams spend far more time on road networks and escape planning than the television footage suggests.