Behind the Forecast: How radar works

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Published: Nov. 6, 2020 at 9:26 AM EST
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LOUISVILLE, Ky. (WAVE) - A weather radar image is something we’ve grown used to seeing. Whether on television or our phones, weather radar images have become ubiquitous. So, how does it work?

At the most basic level, a radar sends out a beam of radio waves from an antenna. As the radio waves hit various objects in our atmosphere, they are scattered and returned to the radar. The scattering allows us to see precipitation or other items that may be in our skies. The amount of time it takes for radio waves to be transmitted and returned to the radar allows meteorologists to know how far away that object is from the radar and how/where it is moving.

An object’s movement, whether away or toward the radar, is calculated by measuring the change, or shift, in phase between the broadcasted beam and received echo. A positive phase shift indicates the object is moving towards the radar, while a negative phase shift shows movement away from the radar.

The National Weather Service’s Doppler radar antenna automatically rises higher in preset increments, called elevation slices, as it rotates. These elevation slices create what’s called a volume coverage pattern (VCP). The radar completes a volume scan every four to six minutes, according to the National Weather Service (NWS).

Initially developed for military use in the 1930s, radar played a vital role in World War II. The National Weather Service installed NEXRAD 159 Doppler radars were at major airports across the United States through the early and mid-1990s. That was an impressive upgrade compared to the old models; it improved severe weather warning due to increasingly detailed and storm detection.

A look at the radar tower in Fort Knox, Kentucky from the National Weather Service.
A look at the radar tower in Fort Knox, Kentucky from the National Weather Service.(National Weather Service Louisville)

From 2011 to 2013, NWS radars received dual-polarization (dual-pol) upgrades. Dual-polarization required a hardware attachment to the radar dish and new software. Dual-pol gives meteorologists a more comprehensive two-dimensional image of potential precipitation. Instead of just horizontal pulses, dual-polarization allows the radar to send and receive pulses horizontally and vertically. The addition of the vertical orientation lets forecasters identify differences between rain, snow, hail, and ice pellets; it even makes it easier to see where the rain to snow transition occurs.

Dual-pol also detects other objects. From birds to bats to smoke to the wind, the radar can see it all.

The ability to detect tornado debris thrown into the air was another significant improvement that came with dual-polarization. Seeing a debris ball on radar improves a meteorologist’s ability to warn the general public, especially at night.

All modern radars are digital. That sweeping line that you see on television radars is computer generated; it doesn’t apply anymore.

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