A: A weather radar consists of a transmitter and a receiver. The transmitter emits pulses of radio waves outward in a circular pattern. Precipitation scatters these radio waves.
“Reflectivity” is the amount of transmitted power returned to the radar and measured by its receiver. The intensity of this received signal indicates the intensity of the precipitation.
Measuring the time it takes for the radio wave to leave the radar and return tells us how far away the storm is. The direction pointed by the radar locates the storm.
A hook echo is a pattern in a reflectivity image. It looks like a spiral rotating outward clockwise, with the precipitation “thickness” increasing – or, a hook shape. This pattern suggests that the storm is rotating and may produce a tornado. A tornado could be found at the narrow top of the spiral.
Doppler radars measure the speed at which particles in the cloud are approaching or moving away from the radar. Returning radio waves have a higher frequency if the particles in the cloud are moving towards the radar, and a lower frequency if the particles are moving away. This allows Doppler radars to identify severe weather phenomena. For example, a spinning vortex would have particles no longer moving towards the Doppler radar and then moving away from it a small distance.
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A radio wave is an electromagnetic wave and therefore has electric and magnetic fields oriented perpendicular to each other. This is called polarization. Dual polarization radars measure this polarization and can distinguish between heavy rain, hail, snow and sleet, as well as debris from a tornado.
When a tornado is on the ground, it throws dirt, plant matter, and other debris into the atmosphere. Because the radar is designed to detect the presence of airborne objects, it can show meteorologists where the debris is present and therefore the tornado.
Photos: See the deadly destruction of tornadoes over the years
May 22, 2011: Joplin, Missouri
April 2011: Southeastern United States
February 5, 2008: “Super Tuesday” outbreak
April 2014: Southeast and Midwest
May 20, 2013: Moore, Oklahoma
March 18, 1925: Missouri, Illinois and Indiana
May 11, 1953: Waco, Texas
November 6, 2005: Evansville, Indiana
May 10, 2008: Southwestern Missouri
May 25, 2008: Iowa
February 29, 2012: Illinois
February 11, 2009: Oklahoma
April 28, 2011: Virginia
June 8, 1984: Barneveld, Wis.
May 1955: Udall, Kansas
March 2, 2012: Indiana
October 2013: Nebraska
May 4, 2003: Missouri
June 11, 2008: Iowa
July 8, 2014: Upstate New York
December 10-11, 2021: Kentucky, Tennessee, Ohio Valley, Southern United States
“Weather Guys” Steve Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at the University of Wisconsin-Madison.
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