# What is the Doppler effect?

The Doppler effect refers to the way sound and light waves behave, depending on the motion of the transmitting and receiving objects. As objects move away from each other, waves become stretched out; and as objects move toward each other, waves become compressed. When it comes to sound waves, the result is that one hears a higher-pitched sound if an object making noise comes closer; if that object is moving away, the result is a lower-pitched sound. When it comes to light waves, compressed visible light is shifted toward the blue spectrum, while it is red-shifted if objects are moving away. Astronomer Edwin Hubble (1889-1953) used this information about light waves to form his theory of an expanding universe.

The Doppler effect gets its name from Austrian physicist and mathematician Johann Christian Doppler (1803-1853), a figure in history whose real name has caused considerable confusion. His baptismal certificate indicates that he was actually born Christian Andreas Doppler, but on his gravesite his name is presented as Christian Johann Doppler. Depending on sources, he has also been cited as Christian Andreas Doppler and Johann Christian Andreas Doppler! Let's just go with his last name. Interestingly, the Doppler effect was not confirmed by its theorizer. Rather, it was a Dutch scientist named Christoph Hendrik Diederik Buys Ballot (1817-1890) who proved it by conducting an experiment in which he was trying to refute Doppler's theory. Buys Ballot set up a series of trials by a railroad track in Utrecht, the Netherlands. An accomplished horn player road a train while a second highly trained musician stood by the tracks, listening to the first musician play a note on the horn. After two trials called on weather, a third trial confirmed that Doppler was correct about how sound waves change with motion.

# How do radars use the Doppler effect to measure the weather?

Doppler radar—unlike conventional radar systems that could only indicate where a storm was and how intense it was—can be used to detect wind speed and the direction storms are taking. Because one Doppler radar can only tell if a storm is heading away or toward it, a network of radars is needed to chart a storm's precise course.

# What do the colors shown on television radars indicate?

Colors on weather maps shown on television news programs are indications of precipitation levels. Cooler colors (blue, green, etc.) indicate light precipitation, while warmer colors (yellow, orange, red) are used for heavier precipitation.

# What is a hook echo?

A hook echo—also called a "hook pattern"—is a warning sign that a tornado has possibly formed. On radar, the echo looks kind of like the number "6" and is associated with a mesocyclone within a storm, so it is called a tornadic mesocyclone. The distinctive hook shape was first seen on April 9, 1953, by electrical engineer Donald Staggs, who was monitoring the radar system at the Illinois State Water Survey in Champaign. A hook echo does not necessarily indicate a tornado has definitely formed, but it is a strong indication that it has. On the other hand, tornadoes can and do form without the tell-tale hook echo ever being seen.

A 1988 radar image of clouds near Norman, Oklahoma, shows the distinctive hook echo that is often a precursor to tornado activity. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)