Thursday, 20 October 2016

Question: What do meteorologists, police and television baseball announcers have in common? Answer: Doppler radar, for one thing.

Radar, RAdio Detection And Ranging, uses a pulse of microwave energy sent out and echoed back by a distant target. Using radar, we can spot targets at various distances and directions. Tracking airplanes was the first goal of radar units. Meteorologists have long used radar to spot the locations, intensities and movements of precipitation that are associated with storm systems. Within the last ten years, the National Weather Service installed a network of new radar units across the U.S. that can also detect motions of raindrops, snowflakes, and hail using the Doppler principle. When a moving target echoes back part of the radar pulse, the frequency of the electromagnetic energy is shifted slightly from the original signal value. The amount of shift relates to the speed of target movement directly toward or away from the receiver. With this type of weather radar, we can infer the wind speeds and directions that move the raindrops and other particles in the storm.

Police agencies have also used Doppler radar units in enforcing speed limits. Traffic radar aimed at a moving car will tell the car's speed directly toward or away from the unit. The sports media have been quick to use this technology to impress viewers with the speed of a baseball pitch or tennis serve. While the speed of a pitch from the mound to home plate can be "measured" because it travels toward the radar, a throw from third to first base would not because it is moving across the radar beam. Therefore, Doppler weather radar interpretation is much more complex than baseball and chasing speeders. Storm motions are scanned across the entire atmosphere surrounding the radar site for hundreds of kilometers out and several kilometers up, with no "seventh inning stretch"!

Another additional feature of the new Doppler radar units operated by the National Weather Service is the capability to estimate the accumulated rainfall over a given area near the radar unit and then alert the radar operator to the possibility of flooding if some critical threshold were reached. As a result, flash flood watches and warnings can be issued more promptly.

For more information describing interpretation of DataStreme Atmosphere radar summary products, you may consult the Supplemental Information…In Greater Depth found below.


To be submitted on the lines for Thursday on the Investigations Manual, Week 7 Chapter Progress Response Form, under section B. Daily Summary.

  1. Weather radars do not detect precipitation's [(location) (intensity) (temperature)].
  2. Doppler radars only measure speeds [(along) (perpendicular to)] a line from the target to the radar.



In the DataStreme Atmosphere Project, current radar imagery is available in several formats so we can better visualize where precipitation is falling across the nation. You have become acquainted with one such product, which is an overlay of the composite radar summary that is placed over the surface analysis that you retrieve from the current "Isobars, Fronts, Radar & Data" link on the RealTime Weather Portal. This feature that includes various color-coded regions on the surface analysis allows us to see the precipitation areas and intensity for the entire coterminous U.S. Several other charts are available that provide additional detail. A separate radar summary chart, with a link identified as "Radar", or a radar summary with fronts chart ("Radar & Fronts") can be similarly accessed from the Radar portion of the homepage.


These various composite radar summary charts are prepared hourly at 35 minutes past the hour from the data supplied by National Weather Service radar reports originating from the national network of primary weather radar units. These radar units are designated WSR-88D for Weather Surveillance Radar, which was developed in 1988 as a Doppler radar.

The intent of the national radar summary chart is to graphically depict the large-scale distribution of precipitation. Precipitation intensities are indicated on the chart by color codes. Some adjustment of the data may be needed, especially if precipitation echoes are reported by more than one radar site.


The reflectivity or intensity of the reflected radar signal returned to the radar receiver depends upon several factors to include the size, shape, density (the number of targets per unit volume), and state (rain, snow or mixture) of the hydrometeor target. Usually, the intensity of the reflected echo is assumed proportional to the rain drop density, thus indicating the precipitation rate or intensity.

Levels of echo reflectivity are plotted on the radar charts as a color-coded display. The colors representing each level provide an indication of the precipitation intensity and the size encompassed by the color depicts the areal extent of the detected precipitation within the lowest 10,000 feet of the atmosphere. A color bar on the left side of the various radar charts identifies 15-color levels measured in dBZ (a logarithmic ratio of return to transmitted energy in a "decibels" scale of Z, which is the letter radar meteorologists use to describe reflectivity). These colors cover a wide range of signals, from very weak (5 dBZ) to very strong (75 dBZ). Based on empirical studies, the reflectivity can be related to approximate rainfall rates:

Radar Precipitation

Rainfall Rates (dBZ)

Intensity Description

Inches per hour

 5 - 15

Light Precipitation


15 - 29

Light Precipitation

0.01 - 0.09

30 - 39

Moderate Precipitation

0.10 - 0.40

40 - 44

Heavy Precipitation

0.50 - 0.90

45 - 49

Heavy Precipitation

1.00 - 1.90

50 - 54

Extreme Precipitation

2.00 - 3.90

55 - 65

Extreme Precipitation

> 4.00

Typically, the blue colors indicate light rain or snow; the green colors identify light thunderstorms/moderate rain showers; yellows identify moderate thunderstorms, while the colors of magenta grading to red are reserved for the severe thunderstorms that can cause flooding rains. On a local scale (not possible on the national summary), an echo with red can also indicate highly reflective water-coated hail.


The radar summary chart helps fill in the precipitation regions between surface observation stations. Hence, the meteorologist can obtain an additional perspective of precipitation types, intensities and movements that are not provided by the conventional surface analysis. However, care must be exercised and surface observations should be inspected, because radar summaries may contain echoes of precipitation that does not reach the ground. This condition, called "virga", may exist because the air would be too dry and evaporation of the rain droplets has taken place. The inclusion of current severe weather watch boxes on the chart helps identify regions having weather conditions that are potentially conducive for severe weather.


While not shown on AMS weather maps with displays of radar information, a link is provided in the webpage Radar section to the NWS Radar Page. By clicking on the location of an individual radar site, usually at your local NWS Forecast Office, a radar display surrounding that office is shown. Any areas encompassed by an official severe weather watch issued by the Storm Prediction Center are plotted on the local NWS radar summary display. These areas appear outlined by a rectangular "box", conforming to the map coordinates of the affected area. Weather watch numbers associated with each box are placed in the box.

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2016, The American Meteorological Society.