When a tornado begins to form, high-speed winds several kilometers above the ground begin to move in a tight rotation known as a “Tornado Vortex Signature” (TVS). Detecting this raises a warning about a possible tornado well before the tornado touches down on the ground. But how can we tell this distant TVS is happening? Tornadoes often form in thunderstorms, and therefore carry droplets of rain with them. Which leads us to the secret sauce: radar signals bounce off rain droplets, and how they bounce off the droplets is affected by the motion of the droplets. That can be leveraged into using radar to measure the speed of the droplets, and detect the TVS.

How is that leveraging done? In 1973, researchers in Norman, Oklahoma, working for the United States Weather Bureau (which later became part of NOAA) figured out how to do it using a principle known as the Doppler effect. You know how a siren on an ambulance sounds high-pitched if the ambulance is driving towards you, but sounds lower-pitched once the ambulance is past you and is driving away? The very same applies to radar and rain droplets: the radar tower sends out a signal at some frequency. If it bounces off a droplet that is moving away, the radar signal that comes back to the radar tower will have a lower frequency. But if the droplet is moving towards the radar tower, the radar signal that comes back to the tower will have higher frequency. How much higher or lower depends on the droplet’s speed. Using this, radar can be used to measure storm wind speed and direction. When two close-by locations are measured to have very different wind speeds, or even opposing wind directions, that suggests the tight rotation that is a TVS.

This principle forms the basis of modern tornado early-warning systems, and helps to keep millions of people safe.