Note from the author:

The content of this page is intended for educational purposes. While nearly all of the content comes directly from reputable sources (radar manufacturer manual, National Weather Service, my own collaboration with radar operators, etc), you should use this information at your own risk and always rely on trustworthy & reliable sources for your weather alerts.

-Eric

Weather Radar

History of Radar

Early discoveries and experiments date back to the late 19th Century . However, the word “radar” has a relatively young existence.

While there’s some debate on the etymology , popular believe is that RADAR is actually a quassi-acronym, dating back to World War II.

It’s believed the U.S. Navy coined the term, a shorter version of the technology they used for detecting/measuring distance of enemy ships/planes.

and so… RAdio Detection And Ranging became just… RADAR

Regardless of the origin, the technology remains the same. Radar has many applications in our day-to-day lives, especialy in weather awareness.

Doppler Radar

Doppler Radar is a unique type of radar that analyzes objects using the Doppler effect .

It is the type of radar used by most weather operations, including the National Weather Service (NWS). Its technology is what allows the measurement of things like wind speed and rotation, in addition to the detection and analysis of rain.

The specific type of Doppler Radar used by NWS is the WSR-88D.

Image 1. WSR-88D Radar at NWS Melbourne, FL
Image 1. WSR-88D Radar at NWS Melbourne, FL  Source:  NWS

Weather Radar Principles

The radar operates by sending out a pulse of radio wave. If that radio wave encounters something in its path, it will reflect a radio wave back to the radar.

Image 2. Animation of Doppler radar signal and response
Image 2. Animation of Doppler radar signal and response  Source:  NWS

Radar Terms

Here is a list of basic radar terms:

Echo - name for the reflected signal back to the radar. The characteristics of the Echo determines the size/shape/density/movement of the object.

Radial - the angle or degree of the beam, left/right relative, to center. The radar completes a full 360-degree rotation, analyzing the atmosphere in all directions.

Tilt - describes the angle of elevation the radar is sending/receiving it’s signals at. Each rotation scans at a specific tilt. Multiple rotations/tilts are used to survey different parts of the atmosphere.

Beam Characteristics

When the beam leaves the radar, it travels away in a straight line.

However, two very important characteristics impact how the radar can see objects.

  • The beam increases size as it travels away from the radar (see beam in image below)

    • Less signals to cover more area? This results in lower resolution (quality) the further the beam is from the radar.

  • The curvature of Earth results in the beam center increasing in elevation as it travels away.

    • At long distances, radar can actually overshoot objects and miss them altogether.
Image 3. Radar beam increases naturally over distance because of Earth's curve
Image 3. Radar beam increases naturally over distance because of Earth's curve  Source: MN Chaser

Products

The radar rotates, sending out signals, and receiving responses. This data is analyzed into several products that make it easier for us to interpret.

Reflectivity

Reflectivity analyzes the Echo data to determine the size, shape, and density of objects in the scanned area.

There are two types of Reflectivity products.

  • Base Reflectivity: reflectivity data (Echoes) scanned at a single tilt
  • Composite Reflectivity: a merge of scans from multiple tilts

Reading Reflectivity

The table below shows examples of common precipitation types with their corresponding reflectivity dBZ values.

Image 4. Radar reflectivity dBZ measurements with common precipitation
Image 4. Radar reflectivity dBZ measurements with common precipitation  Source:  NOAA

Weather radar can see things other than rain too. Birds, insects, debris, etc. are all detected by the radar.

Velocity

In addition to detecting the type/size of objects, Doppler Radar can also detect motion.

There are two main types of Velocity products:

  • Base Velocity - speed/direction from a single tilt - helpful for telling overall motion of winds. Velocity here represents the total wind speed. Meaning, the speed of the storm and the winds it produces.

  • Storm Relative Velocity - subtracts the overall motion of a storm from the measured velocity values - shows what the winds are doing within the storm as it moves along.

Reading Velocity

Velocity products use different colors to describe values.

For example, often…

  • Green is used for motion towards the radar
  • Red indicates motion moving away from the radar

Consider the conceptualized velocity scan below. With green values on the left (moving towards the radar) and red values on the right (moving away from the radar), we can easily infer that winds are moving west to east.

Brightness of the colors indicates the strength of wind. So, in this case our strongest winds are near the radar site (center)…with bright green and red.

Image 5. Simple example of velocity values
Image 5. Simple example of velocity values  Source:  NOAA

Unfortunately, things are never that simple.

  • recall, the radar beam increases elevation as it travels away from the radar
  • wind typically changes direction at different elevations in the atmosphere

Putting these two parameters in practice, look at the image below. It is a more accurate, conceptualized view of how velocity might look.

Image 6. Common 'S' pattern change of wind speed/direction with height
Image 6. Common 'S' pattern change of wind speed/direction with height  Source:  NOAA

Notice the “S” shape of the radar. This is a very common pattern we see in Minnesota (and Northern Hemisphere).

  1. at the radar site (center) and along line 1, we can see surface winds SW -> NE
    This represents the area closest to the ground.

  2. moving away from the radar towards arrows #2, winds take on a more W,SW -> E,NE direction
    Depending on the distance and tilt of the radar, this area could be 4,000ft+ off the ground.

  3. near arrows #3, the outer edge of our scan, we can see winds are taking almost a W -> E path
    Again, depending on distance/tilt, this reading could be from 2,000ft+ above ground.

Notice, as you move away from the center, the direction of the wind change is uniform. E.g. the numbered arrows point in the same direction. From this analysis, one could quickly determine broad wind shear occuring with change in wind direction SW to W with elevation!

Spectrum Width

Spectrum Width is a critical and often over-looked product from the radar. It measures the turbulence of the atmosphere. Or said another way, how consistent or not the velocity values are to one another.

Image 7. Low vs. High Spectrum Width Turbulence
Image 7. Low vs. High Spectrum Width Turbulence  Source: MN Chaser

Some of the best uses for Spectrum Width are for validating the other two products.

For example, the location of the leading edge of a storm may be difficult to determine using Reflectivity alone.

Radar In Practice

Let’s check out some actual radar values from a 2024 chase across Iowa and Wisconsin.

  • Date/Time : June 22, 2024 5:34pm
  • Location : just north of Aurora, IA
  • Radar : apprx 85mi SW from KARX (LaCrosse, WI)
  • Beam Height : 7.1k ft - tilt 1, 13.6kft - tilt 2

Reflectivity

Below is a screenshot of side-by-side Reflectivity and Velocity. With a visual of the storm within a minute or two of the radar scan.

Image 8. Screenshot from Radar (Reflectivity and Velocity)
Image 8. Screenshot from Radar (Reflectivity and Velocity)  Source: MN Chaser
Image 9. Photograph of the storm from June 22, 2024
Image 9. Photograph of the storm from June 22, 2024  Source: MN Chaser

Velocity

Now, compare the two veolocity images below. They were pulled from the archive during the same volume scan.

Notice how Tilt 1 shows some nice storm features, but Tilt 2 has significantly reduced values (brightness) and definition.

This is because, at this distance, Tilt 2 is nearly 14kft above ground level. (almost twice as high as Tilt 1).

However, this doesn’t mean we avoid higher tilts altogether…in fact, the opposite! Here, Tilt 2 is giving us a good indication of how ‘deep’ (high) the rotation is occurring.

image image

The point of comparing these two images shows us how analyzing just a single tilt can only tell us part of the story. And looking at the wrong tilt, can tell us a completely different story about the storm entirely.

Spectrum Width

Lastly, look at the Spectrum Width scan below.

Remember that:

  • areas of high spectrum width (bright orange here) = high turbulence
  • areas of low spectrum width (gray colors) = low turbulence, uniform winds

It’s clear to see from the Spectrum Width values:

  1. large area of uniform wind (yellow circle) - which validates where we were hypothesizing the storm’s inflow
  2. large area of turbulence (orange pixels) which sits smack dab over the broad area of rotation we noted
  3. clearly denoted leading edge of the storm
  4. Rear Downdraft - appears in a gray section in the NW corner of the turbulent area. These were the strongest winds of the storms.
Image 10. Spectrum Width showing amount of wind turbulence.
Image 10. Spectrum Width showing amount of wind turbulence.  Source: MN Chaser

Weather Radar Apps

The information provided by most weather radars is available for 'free' .

There are a number of apps and websites that provide this information at no cost, starting with the National Weather Service .

Other free websites & apps provide forecast data and basic radar reflectivity information. Here are a few:

There are a number of paid apps that focus only on radar. If you are interested in severe weather awareness, I highly recommend checking one of them out.

RadarScope is my pick and likely my #1 overall must have app for weather awareness and storm chasing.

I’m planning some additional content specific to RadarScope, so stay tuned for that in the future.

Summary

Resources

NWS WSR-88D - Additional info on WSR-88D Radar used by NWS.

NWS Radar Operations Center - mission, history, other interesting info