How Do You Use the Map Key on a Radar Map?

RADAR MAPS

MAP:

QUESTIONS:

•What does radar show?

•How do you use the map key on a radar map?

•How do we get radar images?

•What are some of the limitations of radar maps?

BACKGROUND

Understanding Weather Radar

We all use radar to assist in planning outdoor activities. Should we bring an umbrella? Is a severe thunderstorm on its way or will we be dry the rest of the day? While radar helps us forecast upcoming weather, few understand exactly how it works.

What is Radar and What do Radar Maps Show?

RADAR stands for Radio Detecting And Ranging and as indicated by the name, it is based on the use of radio waves. Radars send out electromagnetic waves similar to wireless computer networks and mobile phones. The signals are sent out as short pulses, which may be reflected by objects in their path, in part reflecting back to the radar. When these pulses intercept precipitation, part of the energy is scattered back to the radar. This concept is similar to hearing an echo. For example, when you shout into a well, the sound waves of your shout reflect off the water and back up to you. In that same way, the pulse reflects off precipitation and sends a signal back to the radar. From this information the radar is able to tell where the precipitation is occurring and how much precipitation exists.Radar shows rain snow and sleet reflectivity. Some radar maps also show wind direction and speed. Radar images are color-coded to indicate precipitation intensity. On the one to the right, light rain is green and heavy rain is red. Light pink to purple colors show mixed rain and ice with purple being heavy precipitation. Light snow is shown as grey and heavy snow as white.

History of Radar

Radar was slowly developed over time starting way back in the late 1800s. By the start of World War II, many countries used it to detect enemy ships and aircrafts. When radar operators discovered that precipitation caused ‘false’ echoes on their screen (masking potential enemy targets) they realized the new found potential of radar. Soon after the war surplus radars were used as precipitation detectors. A weather radar consists of a parabolic dish (it looks like a satellite dish) encased in a protective dome and mounted on a tower. About 150 Doppler radars were installed throughout the U.S. in the 1990s. Weather radar, also called weather surveillance radar (WSR) and Doppler radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail, etc.).

The Doppler Effect

By utilizing the Doppler Effect, Doppler radars provide information regarding the movement and positions of targets. After the radar emits a pulse of radio waves, it tracks the phase shift between the transmitted radio wave and the received echo. This phase shift shows whether the target is moving directly toward or away from the radar, called its radial velocity. A positive phase shift implies motion toward the radar and a negative shift suggests motion away from the radar. The phase shift effect is similar to the "Doppler shift" observed with sound waves. If an object emits sound waves as it approaches a location, the waves are compressed leading to a higher frequency. As the object moves away from a location, the sound waves are stretched leading to a lower frequency. This is often experienced when an emergency vehicle drives past with its siren blaring.

How Modern Weather Radar Works

A modern Doppler radar system consists of a large radar dish housed inside an even larger hexagonal dome to protect it from the elements. The radar dish can rotate 360 degrees in the horizontal and approximately 20 degrees in the vertical. As the radar antenna turns, it emits extremely short bursts of radio waves, called pulses and waits for these pulses to return during the "listening period". Each pulse lasts about 0.00000157 seconds with a "listening period" of 0.00099843 second. The transmitted radio waves move through the atmosphere at around the speed of light. Once it hits a target such as a raindrop or snowflake, the radio waves are scattered with some of the energy returning back to the radar. Radar observes all of this information during the “listening period” with the process repeated up to 1,300 times per second. Observing the time it takes the radio waves to leave the antenna, hit the target, and return to the antenna, the radar can calculate the distance and direction of the target using the “Doppler effect” (hence the title Doppler radar). In addition, the returned energy the radar receives provides information on the target’s characteristics including size, intensity and with the newest Dual Polarized radars, even precipitation type.

Objects that reflect the beam back to the radar include rain, snow, sleet and even insects.

If more of the beam is sent back, the object is said to have a high reflectivity and is indicated by a bright color. Objects which return a small part of the beam have a low reflectivity and are indicated by darker colors.

Limitations of Radar

Weather radar does have its limitations. For one thing, lighter precipitation may sometimes not be detected. Also, other clutter can show up on the screen or block the signal (like hills and buildings). Sometimes radar images show precipitation that isn't actually reaching the ground. Snow doesn’t reflect as well and might not show up, and sleet is very reflective, so may be misinterpreted as very heavy precipitation.