What do radars show

Radar basics and the doppler shift. The radar emits a burst of energy green in the animated image. If the energy strikes an object rain drop, snowflake, hail, bug, bird, etc , the energy is scattered in all directions blue. Note: it's a small fraction of the emitted energy that is scattered directly back toward the radar. Learn about the Radar Beam here.

This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned pulse, time it took to travel to the object and back, and phase, or doppler shift of the pulse. This process of emitting a signal, listening for any returned signal, then emitting the next signal, takes place very fast, up to around times each second!

When the time of all the pulses each hour are totaled the time the radar is actually transmitting , the radar is "on" for about 7 seconds each hour. The remaining 59 minutes and 53 seconds are spent listening for any returned signals. Learn about the different scanning modes of the Radar here.

The phase of the returning signal typically changes based upon the motion of the raindrops or bugs, dust, etc. This Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it. You have most likely experienced the "Doppler effect" around trains. As a train passes your location, you may have noticed the pitch in the train's whistle changing from high to low.

As the train approaches, the sound waves that make up the whistle are compressed making the pitch higher than if the train was stationary. Likewise, as the train moves away from you, the sound waves are stretched, lowering the pitch of the whistle. The faster the train moves, the greater the change in the whistle's pitch as it passes your location.

The same effect takes place in the atmosphere as a pulse of energy from NEXRAD strikes an object and is reflected back toward the radar. The radar's computers measure the phase change of the reflected pulse of energy which then convert that change to a velocity of the object, either toward or from the radar.

Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to "see" the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice. In the image above, the grey line is the transmitted signal. You can see how the returned energy changes its wavelength characteristics when it hits a target moving away or toward the radar red and green line, respectively.

There are two main types of data, Velocity and Reflectivity. Reflectivity data shows us the strength of the energy that is returned to the radar after it bounces off precipitation targets.

Other non-precipitation targets will return energy, but for now, we will only deal with the precipitation. In general, the stronger the returned energy, the heavier the precipitation. Learn more about Reflectivity here. Velocity data is derived from the phase, or doppler shift of the returned energy. The radar's computers will calculate the shift and determine whether the precipitation is moving toward or away from the radar, and how fast, then apply a corresponding color to those directions and speeds.

Red is typically a target moving away from the radar, while green is applied to targets moving toward the radar. The intensity of these colors determines its estimated speed. Learn more about Velocity here. Weather radar is an incredible piece of technology, and knowing how to interpret the colors on the map can keep you safe as we enter severe weather season.

Troops on the front lines during World War II discovered that the radar they used to track incoming enemy aircraft also detected precipitation, giving them the ability to keep watch over both storms and airplanes.

Meteorologists studied this phenomenon once the war was over, and developed this technology into a tool we use every day. The United States has more than weather radar sites around the country continuously keeping an eye on the skies to keep us safe no matter what pops up on the horizon. Weather radar consists of a rotating dish protected by a large white dome; this dish sends pulses of energy the radar beam into the atmosphere to detect objects like rain or hail.

If the radar beam encounters an object, some of the radiation will bounce off of it and return to the radar site. The strength of the return beam and the time it takes for the pulse to return to the radar dish allows us to see how heavy the precipitation is and how far away it is from the radar site.

The resulting data is displayed on a map using a rainbow scale that typically spans from light blue to dark red and purple, with cooler colors indicating lighter precipitation and warmer colors showing heavy precipitation.

Solid batches of oranges, reds, and purples on a radar image usually indicate an intense thunderstorm. A recent development in radar technology called "dual polarization" allows the radar to send out two beams of energy—one that is oriented horizontally and another oriented vertically. This dual radar beam allows us to see the size and shape of the objects falling through the atmosphere.

As the vehicle or train passes your location, the siren or whistle's pitch lowers as the object passes by. Doppler radar pulses have an average transmitted power of about , watts. By comparison, a typical home microwave oven will generate about 1, watts of energy. Yet, each pulse only lasts about 0. Therefore, the total time the radar is actually transmitting a signal when the duration of transmission of all pulses, each hour , are added together , the radar is transmitting for a little over 7 seconds each hour.

The remaining 59 minutes and 53 seconds are spent listening for any returned signals. The NWS Doppler radar employs scanning strategies in which the antenna automatically raises to higher and higher preset angles, called elevation slices, as it rotates. These elevation slices comprise a volume coverage pattern VCP.

Once the radar sweeps through all elevation slices a volume scan is complete. In precipitation mode, the radar completes a volume scan every minutes depending upon which volume coverage pattern VCP is in operation, providing a 3-dimensional look at the atmosphere around the radar site.

Take it to the MAX! Volume Coverage Patterns: Turn it up! An addition to the NWS Doppler radar has been of dual-polarization of the radar pulse. The "dual-pol" upgrade included new software and a hardware attachment to the radar dish that provides a much more informative two-dimensional picture. Another important benefit is dual-pol more clearly detects airborne tornado debris the debris ball - allowing forecasters to confirm a tornado is on the ground and causing damage so they can more confidently warn communities in its path.

This is especially helpful at night when ground spotters are unable to see the tornado. These two images show how dual-polarization helps the NWS forecaster detect a tornado producing damage. The left image shows how the Doppler radar can detect rotation.