Weather Radar: How 'Listening to the Sky' Helps Us Track Precipitation

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Radio Detection and Ranging (RADAR)

A Dual-polarization doppler radar tower. NOAA

Much like other great scientific discoveries of our time, the benefits of radar technology in weather prediction were discovered unintentionally. First used during World War II as a defense mechanism, radar was identified as a potential scientific tool when military personnel happened to notice "noise" from precipitation on their radar displays.

Today, radar is an essential tool for forecasting precipitation associated with thunderstorms, hurricanes, and winter storms, and is perhaps the most widely recognized weather graphic.

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How Radar Works

The radar sending and receiving process. The COMET Program (UCAR)

Have you ever shouted down a long empty hallway and heard your voice repeat itself? This echo is heard because sound waves in your voice bounce off of the hall's end and travel back to your ears.

Well, radar works by a similar principle...

First, a device called a transmitter generates an electromagnetic form of energy known as a radio wave. Next, this energy is broadcast into the atmosphere at short intervals, or pulses, by an antenna that rotates in a 360° circular sweep. The radio waves travel through the air at the speed of light and either (1) continue traveling or (2) intercept the surface of an airborne object (a raindrop, snowflake, hailstone, bird, or bug) and are scattered back toward the antenna. Only a fraction of the original energy is received back by the radar; the rest is lost to the surrounding air.

Back at the radar site, a computer then computes the object's coordinates via the following:

  • Target presence (or lack thereof) is detected from whether or not a return signal is received
  • Target direction is taken to be that in which the antenna aimed
  • Target distance is calculated using the standard formula D = R * T, (where R is the speed of light, and T is the time it took the radio wave to return to the radar site).

The information for a coordinate is plotted for every point within a site's scanning range. The compilation of these plots creates an image.

This basic type of radar is commonly known as "pulse radar" because rather than transmitting a constant stream of energy, it transmits at intervals and waits for a return response.

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WSR-88D (Weather Surveillance Radar - 1988 Doppler)

A visual representation of the NEXRAD weather radar scanning process; each black circle represents one complete scan at increased scan angles. The COMET Program (UCAR)

The fleet of WSR-88D radar used by the National Weather Service, collectively called NEXRAD (Next Generation Radar), are "pulse-Doppler" radar. This means they detect the location of precipitation as previously described, but also its movement, thanks to the Doppler effect.

If you’ve ever heard a train whistle change in tone from high as it approaches your location, to low as it passes and travels down the track, then you've experienced the Doppler effect. This change occurs due to sound waves being compressed as an object speeds towards you, then stretched as it moves away. Similarly, a radio wave pulse that is reflected off of a raindrop from a storm moving toward the radar site will be reflected at a higher frequency than it was originally broadcast at, while a pulse reflected off of a raindrop moving out of the area will be reflected at a lower frequency. A computer measures this frequency difference or "shift" and converts it to velocity--either toward or from the radar site.

3-Dimensional Scanning

In addition, WSR-88D radar are equipped with the capability to scan at incremental angles. The antenna begins at a low angle of 0.5°, sends out a pulse, listens for scattered energy, then continues onward. Once a full circular revolution is completed in this way, the antenna angle increases and repeats the process. A maximum scan angle of 19.5° above the horizon is reached before the tilt must begin decreasing back to its original 0.5° position.

This process allows a volumetric "pie slice" of the atmosphere to be scanned. Without this capability, precipitation located high within clouds would fail to be detected.

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How to Read Radar

Reflectivity dBZ scales. NOAA

Recall that radar measures the intensity of energy reflected back to the radar site. Intensity values are measured in units of dBZ, or decibels of "Z" (where Z = reflectivity), then assigned a color based on whether they fall within a low or high measurement range.

Blues and greens correspond to low dBZ values and indicate light precipitation, while more intense colors (i.e. reds, yellows, oranges) mean greater dBZ values and moderate to heavy precipitation.

Clear Air and Precipitation Modes

There are two dBZ scales, one for each radar operation mode. The scale on the left is used during clear air mode, an energy efficient mode that's used when precipitation isn't occurring. The scale on the right is used during precipitation mode; this is an active mode of operation that's put into use during heavy rain or snow events. (In this mode, values of 45+ and 60+ dBZ are associated with thunderstorms and hail cores, respectively.

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Reflectivity Images

Base (top and bottom left) vs. Composite (top and bottom right) reflectivity. Notice the added detail of the composite radar return. NOAA

Look familiar? These are the images and loops commonly used in news forecasts.

The term reflectivity describes how good an object is at returning the energy aimed at it. (The object's size, shape, and composition each determine how well it does this.) Reflectivity images, then, are a mirror image of the precipitation that's "out there."

There are 2 types of reflectivity images: base reflectivity, which displays the results of a 0.5° elevation scan, and composite reflectivity, which displays the results of multiple elevation scans blended together as one image.

What do the colors mean?

  • Light Green: light rain
  • Dark Green: light to moderate rain
  • Yellow: moderate rain
  • Orange: heavy rain
  • Red: very heavy rain or a hail core
  • White or Blue: snow
  • Pink: ice (freezing rain, sleet, or a wintry mix)

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Velocity Images

Base velocity (left) vs. Storm relative motion (right). NOAA

Velocity images show storm motion. They're helpful in detecting wind events like downbursts, derechoes, and the passage of fronts.

There are 2 types of velocity images: base velocity, which displays surface wind speed and direction toward/away from the radar site, and storm relative motion, which displays surface wind flow versus the wind flow of embedded storms*.

What do the colors mean?

  • Red: wind moving away from the radar (outbound)
  • Green: wind moving toward the radar (inbound)
  • Purple: areas where the radar cannot determine velocity (known as "range-folded")
  • Gray: an area where winds are changing direction

*Note: If inbound and outbound winds appear side-by-side on a storm relative motion image, this typically indicates a rotating thunderstorm (i.e., a possible tornado).

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Precipitation Images

1-hour precipitation (left) vs. Storm total precipitation (right). NOAA

These images show accumulated precipitation (in inches) at locations within 143 miles (230 km) of the radar site. They're used when issuing flash flood warnings and flood statements.

There are 2 types of precipitation images: one-hour precipitation, which displays the amount of precipitation that occurred within the last hour, and storm total precipitation, which displays the amount of precipitation for a continual rain event*.

What do the colors mean?

These images share the same color key as reflectivity, however, the dBZ values also indicate precipitation rates.

  • Light Green: Trace
  • Dark Green: 0.10"
  • Yellow: 0.25"
  • Orange: 1.25"
  • Red: 2.5"
  • Pink: 15+"

*Note: A continual rain event is defined as as rain with breaks lasting under an hour; if precipitation stops for an hour or longer, the current storm event ends and the accumulation is reset to zero.