RadarScope Mac

RadarScope Mac

What is RadarScope

RadarScope is a specialized display utility for weather enthusiasts and meteorologists that allows you to view NEXRAD Level 3 radar data and severe weather warnings. It can display the latest reflectivity, velocity, and other radar products from any NEXRAD radar site in the United States, Guam and Puerto Rico. These aren’t smoothed PNG or GIF images, this is real Level 3 radar data rendered in its original radial format for a high level of detail.

Whether you are scanning reflectivity for a mesocyclone’s tell-tale hook echo, trying to pinpoint the landfall of a hurricane’s eye wall, or looking for small features like velocity couplets in the storm relative radial velocity product, RadarScope gives you the power to view true radial NEXRAD weather radar on your Mac.

RadarScope displays tornado, severe thunderstorm, and flash flood warnings issued by the National Weather Service. You can browser the list of active warnings in the information sidebar, select a warning to view the details, and even zoom to the selected warning on the map.

Zoom in and out on the map using the mouse or pinch gestures on multi-touch trackpads. Click and drag to scroll around the map. Select one of the 155 different radar sites using buttons on the map, the full radar list in the sidebar, or your list of saved favorites in the menu bar. Tap the play button to download and animate up to 20 radar frames. Display the names of over 25,000 cities and towns on the map as you zoom and scroll. Move the cursor over the color legend to see the data value associated with each color. You can also save the currently displayed map several common image formats.

Meanwhile, RadarScope will retrieve and display updated data automatically and intelligently (approximately every 5 to 10 minutes, depending on the radar scan strategy).

You can display Level 3 radar data from NOAA’s public access web site or your Allison House subscriber account.

Using the RadarScope Map

The main display area contains a map of the United States, including Hawaii, Alaska, Puerto Rico, and Guam. The map shows state and county borders. There are also options to show cities, interstates, and warnings.

Moving Around the Map

  1. Click and hold the mouse button on the map
  2. Move the mouse to pan around the map

You may need to move far to the west on the map to locate Hawaii, Guam, or Alaska depending on your current zoom level.


Zooming

  • Double-click on the map to zoom in the map
  • Hold the Control key and double-click on the map to zoom out of the map
  • Use the mouse scroll wheel to zoom in or out of the map
  • Pinch or stretch your fingers on the trackpad to zoom in or out of the map. This feature requires that multi-touch is enabled in the system preferences.

City names and county borders will disappear as you zoom out on the map. As you zoom in, the county borders and city names will begin to appear showing your more detail.


About the Data

The top bar contains information about the radar data that you’re currently viewing. The top left displays your currently selected radar and city name. Below that is the information about the data, such as current tilt and operating mode. The top right shows the actual time of the last radar data scan along with the time that RadarScope last updated the data.


The Toolbar

The bottom toolbar contains the buttons to interact with the map. The bottom left contains the play buttons for animating the radar image. The bottom right contains the buttons for changing radars and viewing additional information about radar and current warnings.

 

Changing Radars

Click on the Radars button on the toolbar to show radar station buttons and hide all the city names. Use your mouse to pan around the map to find a new radar. Click on the radar button on the map to change to that radar. Click on the Radars button on the toolbar again to hide the radar station buttons on the map. The screenshot below show the blue radar station buttons on the map after clicking on the “Radars” button on toolbar.

You can also click on the info button at the bottom right of the toolbar to bring up a list of radars. Select “Radars” from the pulldown menu at the top right. The radars are arranged alphabetically by state. Double-click on the radar to automatically move the map to that radar location. Tip: this is the easiest way to locate radars in Guam, Hawaii, Puerto Rico, and Alaska.


Favorites

You can add radars to your favorites list to easily switch to them later.

Adding Favorites

  1. Click the main menu item “Favorites”
  2. Select Add “Radar”

Removing Favorites

  1. Click the main menu item “Favorites”
  2. Select Remove “Radar”

Radar Status

Click on a radar to view the current status of the radar. At times, a radar station may be down for maintenance. This is usually documented in the radar status text.


Offline Radars

NEXRAD radars are large, multi-million dollar pieces of machinery that are maintained and operated by the National Weather Service. They are prone to mechanical failure just like any other piece of complex machinery. In general, they operate 24 hours a day, 7 days a week. But sometimes they must be taken offline for maintenance and sometimes one of the million moving parts just breaks. The NWS makes every effort to keep them running, especially when severe weather is in the area, and when there are problems they try fix them as fast as they can. RadarScope will display offline radars by a red button on the map.

Changing Products

The center menu on the toolbar contains a list of available radar products. Click on the menu to view a list and click on the product in the list to view the data for that product. For more information about the available products, please click here.


Change Products

  1. Click on the Product menu list on the toolbar
  2. Select a new product from the menu list

Available products include:


Expert Mode

Expert mode displays the lower range of the radar. See Clear Air Mode.


Old Data

At times, you will see the “Last Updated” text turn red and warn you that the data is old. Many different things can account for the delay in the data being displayed in RadarScope.

See also:

Map Settings

The Inspector Panel

All RadarScope preferences are set in the inspector panel. The inspector displays the current latitude and longitude of your cursor along with the range and beam height of your selected radar.

Show the Inspector Panel

  • Click the Info button (bottom right corner).
  • Select “Inspector” from the pulldown menu on the top right of the window.

Map Overlays

RadarScope offers multiple overlays to assist in determining the location of storms. In addition to state and county borders, RadarScope can display city names and interstate highways. Warning polygons can also be displayed on the map.

Display City Names

  1. Click on the “Info” button in the toolbar.
  2. Select the “Inspector” menu item from the menu list on the top right
  3. Click the Cities checkbox to toggle city names

Note: the city names will always be hidden when the radar buttons are enabled on the map. See Changing Radars.

Display Interstates

  1. Click on the “Info” button in the toolbar.
  2. Select the “Inspector” menu item from the menu list on the top right
  3. Click the Interstates checkbox to toggle interstates

Display Warnings

  1. Click on the “Info” button in the toolbar.
  2. Select the “Inspector” menu item from the menu list on the top right
  3. Click the Warnings checkbox to toggle Warnings

Display Watches

  1. Click on the “Info” button in the toolbar.
  2. Select the “Inspector” menu item from the menu list on the top right
  3. Click the Watches checkbox to toggle Watches

Display Spotters

    1. Click on the “Info” button in the toolbar.
    2. Select the “Inspector” menu item from the menu list on the top right
    3. Click the Spotters checkbox to toggle Spotters

See Also

Display Storm Tracks

    1. Click on the “Info” button in the toolbar.
    2. Select the “Inspector” menu item from the menu list on the top right
    3. Click the Storm Tracks checkbox to toggle Warnings

Storm tracks provider can be set to NOAA or AllisonHouse. (AllisonHouse requires AllisonHouse integration)

See Also

Display Storm Reports

    1. Click on the “Info” button in the toolbar.
    2. Select the “Inspector” menu item from the menu list on the top right
    3. Click the Storm Reports checkbox to toggle Warnings

Requires AllisonHouse integration.

See Also

Display Lightning

    1. Click on the “Info” button in the toolbar.
    2. Select the “Inspector” menu item from the menu list on the top right
    3. Click the Lightning checkbox to toggle Warnings

Requires AllisonHouse integration.

See Also

Display Day 1 Outlook

RadarScope can display the Day 1 Outlooks for tornadoes, hail, wind, and thunderstorms.

    1. Click on the “Info” button in the toolbar.
    2. Select the “Inspector” menu item from the menu list on the top right
    3. Select the outlook from the Day 1 Outlook pulldown menu.

Requires AllisonHouse integration.

See Also


Expert Mode

  1. Click on the “Info” button in the toolbar.
  2. Select the “Inspector” menu item from the menu list on the top right
  3. Click the Cities checkbox to toggle Expert Mode

Expert mode displays the lower range of the radar. By default, RadarScope hides the colors associated with lower reflectivity values. But if you enable “Expert Mode” in the preferences, it reveals the full color scale. This reveals more of the detail seen by the radar, but it means that what you often see is a big plume of dust, insects, and other clutter surrounding the radar. See Clear Air Mode.


Animation Settings

RadarScope can animate the radar image by displaying the last several radar scans. The length and speed of the animation can be adjusted through the preferences panel. The default duration of the animation is six frames, meaning that RadarScope will play the last six scans of the radar. The duration can be adjusted to play twelve or twenty frames of animation. The length of time of the animation can vary even with the same number of frames. The radars scan more frequently during severe weather.

Adjust Animation Duration

  1. Click on the Settings button on the toolbar
  2. Select the number for frames under the Loop Duration menu list

Adjust Animation Speed

  1. Click on the Settings button on the toolbar
  2. Adjust the Loop Speed slider to increase or decrease animation speed

Data Providers

RadarScope offers multiple data sources.

  • NOAA
  • AllisonHouse

See Also

Watches & Warnings

The following watches and warnings are available in RadarScope:

  • Severe Thunderstorms
  • Tornadoes
  • Flash Floods

Warning polygons are displayed directly on the map. You can zoom out and pan around the map to view these warning polygons. A list of warnings is also available by clicking on the info button on the toolbar. Click on a warning on the list to read the warning text in the box below. Double-click the warning in the list to center the map to the warning polygon.


Read Warnings

  1. Click on Info button on toolbar
  2. Click Warnings tab
  3. Click on a warning in the listThe warning text will appear in the box below the warnings list

Find Warnings on Map From List

  1. Click on Info button on toolbar
  2. Click Warnings tab
  3. Double-click a warning in the listThe map will then center on the selected warning.


Watches require AllisonHouse integration.

Read Watches

  1. Click on Info button on toolbar
  2. Click Watches tab
  3. Click on a watch in the listThe watch text will appear in the box below the watch list

Find Watches on Map From List

  1. Click on Info button on toolbar
  2. Click Watches tab
  3. Double-click a watch in the listThe map will then center on the selected watch.

See Also

Storm Tracks

Storm tracks display the predicted path of the storm. Each tick on the path show the approximate time in 15 minute intervals along the path of the storm. Storm tracks are updated every two minutes.

Saving Images

RadarScope can save the current display as an image file.

  1. File -> Save As
  2. Enter a file name for the image under “Save As”
  3. Select a destination folder under “Where”
  4. Select an image format under “Format”
  5. click “Save”

RadarScope Extras

Spotter Network

The Spotter Network brings storm spotters, storm chasers, coordinators and public servants together in a seamless network of information. It provides accurate position data of spotters and chasers for coordination/reporting and provides ground truth to public servants engaged in the protection of life and property.

Members of the Spotter Network can use RadarScope to plot the locations of other spotters on the map. Follow these steps to activate your Spotter Network account in RadarScope:

  • Open http://spotternetwork.org/rs using your web browser.
  • Log in using your Spotter Network username and password.
  • Tap the “Enable Radarscope Integration” link.

Once your account has been activated, follow these steps to enable location reporting:

  • Open RadarScope and click the Info button (bottom right corner).
  • Select “Inspector” from the pulldown menu on the top right of the window.
  • Click on “Spotters”.

Spotter locations are updated every two minutes

To learn more about the Spotter Network, visit their web site at http://spotternetwork.org.

Data Providers

  • iMapWeather
    The iMapWeather data feed is provided by Weather Decision Technologies. This is the default data provider for RadarScope.
  • NOAA
    NOAA data feed is provided by the national weather service. This is a free public data server operated by the US Government. CAUTION: For serious consumers of NEXRAD radar data, this server will likely not be sufficient as it routinely fails during widespread severe weather outbreaks. Use of another data provider is strongly recommended for high reliability (see below).
  • AllisonHouseAllisonHouse is a data aggregation and integration company that specializes in weather and weather related data. They provide various forms of weather data as part of their subscription plans. AllisonHouse can be used as a NEXRAD Level 3 radar data provider in RadarScope and requires an AllisonHouse subscription. For all questions about their services and subscription fees accompanying their service, please refer to AllisonHouse.com.

Acknowledgments

Portions of this Base Velocity software utilize the following copyrighted material, the use of which is hereby acknowledged.

University of Oklahoma
Copyright © 2008-2011 University of Oklahoma Board of Regents. All Rights Reserved.
Used under license.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Terms of Use

BASE VELOCITY, LLC SOFTWARE LICENSE AGREEMENT FOR RADARSCOPE
PLEASE READ THIS SOFTWARE LICENSE AGREEMENT (“LICENSE”) CAREFULLY BEFORE USING THE BASE VELOCITY SOFTWARE. IF YOU DO NOT AGREE TO THE TERMS OF THIS LICENSE, DO NOT USE THE SOFTWARE.

1. General. The software, documentation and any fonts accompanying this License, whether on disk, in read only memory, on any other media or in any other form (collectively the “Software”) are licensed, not sold, to you by Base Velocity, LLC (“Base Velocity”) for use only under the terms of this License, and Base Velocity reserves all rights not expressly granted to you. The rights granted herein are limited to Base Velocity’s and its licensors’ intellectual property rights in the Software and do not include any other patents or intellectual property rights. You own the media on which the Software is recorded but Base Velocity and/or Base Velocity’s licensor(s) retain ownership of the Software itself. The terms of this License will govern any software upgrades provided by Base Velocity that replace and/or supplement the original Software product, unless such upgrade is accompanied by a separate license in which case the terms of that license will govern. Title and intellectual property rights in and to any content displayed by or accessed through the Software belongs to the respective content owner. Such content may be protected by copyright or other intellectual property laws and treaties, and may be subject to terms of use of the third party providing such content. This License does not grant you any rights to use such content.

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13. Complete Agreement; Governing Language. This License constitutes the entire agreement between the parties with respect to the use of the Software licensed hereunder and supersedes all prior or contemporaneous understandings regarding such subject matter. No amendment to or modifications of this License will be binding unless in writing and signed by Base Velocity. Any translation of this License is done for local requirements and in the event of a dispute between the English and any non-English versions, the English version of this License shall govern.

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Radar Products

Base Velocity

Base velocity indicates storm motion toward or away from the radar, measured in m/s. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation.

Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn’t a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading.

Since the beam is sent out at an angle to the ground, it is looking higher up in the atmosphere as it gets farther from the radar. So the data you see in a radar image are often thousands of feet above the ground. At that height, wind speeds are often higher than they are on the ground. Doppler velocity products are valuable tools for meteorologists to use to determine motion in storm systems. But if you’re interested in surface level winds, your best bet is to look at data from weather stations on the ground. There are several other sources on the web which provide such information.

You can learn more about base velocity products on this National Weather Service page:

Classic Velocity

Base velocity indicates storm motion toward or away from the radar, measured in knots. One knot is equal to one nautical mile per hour, or about 1.15 miles per hour. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation.

Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn’t a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading.

Since the beam is sent out at an angle to the ground, it is looking higher up in the atmosphere as it gets farther from the radar. So the data you see in a radar image are often thousands of feet above the ground. At that height, wind speeds are often higher than they are on the ground. Doppler velocity products are valuable tools for meteorologists to use to determine motion in storm systems. But if you’re interested in surface level winds, your best bet is to look at data from weather stations on the ground. There are several other sources on the web which provide such information.

You can learn more about base velocity products on this National Weather Service page:

Composite Reflectivity

Composite reflectivity combines data from all elevation scans, or tilts, to create a single product. The resulting image shows the highest reflectivity value from the vertical cross section at that location. Composite reflectivity can reveal important features in a storm’s structure that might not be seen in the base reflectivity product.

Because it combines data from all the tilts, the composite reflectivity product is one of the last to be produced during a volume scan. As with all NEXRAD products, it’s important to remember that the data displayed in the image depict conditions that have already happened rather than what is happening right now.

Learn more about composite reflectivity from this National Weather Service web page:

Classic Reflectivity 248 mmi

The 248 nautical mile classic reflectivity product is an older product that shows the same data as the base reflectivity tilt 1 product, but with less spatial and color resolution. For most situations, we recommend the use of base reflectivity tilt 1 instead.

Base Reflectivity

NEXRAD radars work by bouncing radio waves off particles in the air. Those particles could be raindrops, hail, snow, or even dust and insects. The amount of energy that bounces off of those particles and returns to the radar is called “reflectivity” and is represented by the variable “Z”. Reflectivity covers a wide range of signal strength, from very weak to very strong, so it is measured on a decibel (logarithmic) scale in units of dBZ, or decibels of Z. The higher the dBZ value, the larger the number and/or size of the particles the radar beam is seeing.

The dBZ values increase as the strength of the signal returned to the radar increases. The scale of dBZ values is related to the intensity of rainfall. It is important to remember, however, that the radar shows only areas of returned energy and not necessarily precipitation. So the presence of a return, especially a very weak return below 20 dBZ, doesn’t always mean that it’s raining.

The colors along the bottom of the map correspond to precipitation types and intensities. When you move your cursor across the squares, RadarScope will display a value for each color. NEXRAD radars can’t distinguish between different types of precipitation with absolute certainty. However, reflectivity values can be somewhat roughly associated with different precipitation types:

  • 10 dBZ (blue) – Very light rain or light snow
  • 20 dBZ (green) – Light rain or moderate to heavy snow
  • 30 dBZ (yellow) – Moderate rain or sleet showers
  • 40 dBZ (orange) – Moderate to heavy rain or sleet showers
  • 50 dBZ (red) – Heavy thunderstorms
  • 60 dBZ (pink) – Intense to severe thunderstorms with hail

Like in the movie “Pirates of the Caribbean,” these reflectivity values are more like guidelines than rules. This is a rough guide only. The atmosphere is a complex system, so you can’t always associate particular values with precise conditions or events. As a general rule, the higher the dBZ value, the heavier the concentration of objects at that location in the atmosphere.

You can learn more about base reflectivity products on this NWS web page:

Classic Reflectivity

NEXRAD radars work by bouncing radio waves off particles in the air. Those particles could be raindrops, hail, snow, or even dust and insects. The amount of energy that bounces off of those particles and returns to the radar is called “reflectivity” and is represented by the variable “Z”. Reflectivity covers a wide range of signal strength, from very weak to very strong, so it is measured on a decibel (logarithmic) scale in units of dBZ, or decibels of Z. The higher the dBZ value, the larger the number and/or size of the particles the radar beam is seeing.

The dBZ values increase as the strength of the signal returned to the radar increases. The scale of dBZ values is related to the intensity of rainfall. It is important to remember, however, that the radar shows only areas of returned energy and not necessarily precipitation. So the presence of a return, especially a very weak return below 20 dBZ, doesn’t always mean that it’s raining.

The colors along the bottom of the map correspond to precipitation types and intensities. When you move your cursor across the squares, RadarScope will display a value for each color. NEXRAD radars can’t distinguish between different types of precipitation with absolute certainty. However, reflectivity values can be somewhat roughly associated with different precipitation types:

  • 10 dBZ (green) – Very light rain or light snow
  • 20 dBZ (green) – Light rain or moderate to heavy snow
  • 30 dBZ (yellow) – Moderate rain or sleet showers
  • 40 dBZ (orange) – Moderate to heavy rain or sleet showers
  • 50 dBZ (red) – Heavy thunderstorms
  • 60 dBZ (pink) – Intense to severe thunderstorms with hail

Like in the movie “Pirates of the Caribbean,” these reflectivity values are more like guidelines than rules. This is a rough guide only. The atmosphere is a complex system, so you can’t always associate particular values with precise conditions or events. As a general rule, the higher the dBZ value, the heavier the concentration of objects at that location in the atmosphere.

You can learn more about base reflectivity products on this NWS web page:

Storm Relative Velocity

Storm relative velocity is simply base velocity with the average storm motion subtracted out. When storms are moving quickly, this makes it easier to spot green/red velocity couplets that are indicative of rotation and which might be masked out by the motion of the storm. As with base velocity, green is motion towards the radar and red indicates motion away.

It’s also worth noting that the above rotation images are ideal cases. We aren’t always lucky enough to get such prominent radar signatures from tornadoes. The radar isn’t looking at ground level, so it can’t actually see the tornado itself. It’s seeing rotation higher up in the storm covering an area that is several miles wide. The height and width of the radar beam increases with its distance from the radar. So the farther away a storm is from the radar, the higher up the radar is seeing and the wider the beam, making it is less likely to detect the rotation associated with a tornado.

You can learn more about storm relative velocity on this National Weather Service page:

Estimated Rainfall

The rainfall products are estimates of how much rain has fallen at a particular location. The National Weather Service has computers that analyze the reflectivity values returned by the radar and estimate how much rain has fallen. It is not, of course, perfectly accurate but it usually gives you a good idea of the relative amount of rainfall at various locations within the radar’s coverage area. The One Hour Surface Rainfall product provides an estimate of how much rain has reached the ground in the past hour. The Storm Total Surface Rainfall product does the same thing for an arbitrary period of time specified by the radar operator, usually corresponding to the beginning of a rainfall event. Since this product is based on the relationship of reflectivity (Z) to rainfall rate (R), it is important to note that it is not an indicator of snowfall accumulation.

You can learn more about precipitation estimates on these National Weather Service pages:

Vertically Integrated Liquid

The vertically integrated liquid (VIL) product estimates the amount of water in a column of air. High values for VIL can indicate heavy rainfall or the presence of hail. When VIL values fall rapidly, it may indicate a downburst. VIL is subject to radar limitations and seasonal dependencies, so it’s a tricky product to interpret.

Learn more about vertically integrated liquid from this Wikipedia web page:

Echo Tops

The echo tops product shows the maximum height of precipitation echoes detected by the radar between 5,000 and 70,000 feet that exceed 18 dBZ. Higher echoes are often associated with stronger areas of a storm. This product is useful for identifying strong updrafts, and a sudden drop can indicate the onset of a downdraft. Some storms are too close to the radar for the beam to see the top, so echo tops is often underestimated for strong storms near the radar.

Clear Air Mode

When there’s no precipitation in the area, it’s common for the radar to be operating in what is called “clear air mode.” In this mode, the radar is scanning more slowly so that it can be more sensitive and pick up much weaker returns. This allows it to see more details and detect finer particles in the atmosphere, including things like dust and insects.

This more sensitive mode of operation allows meteorologists to see what’s going on in the atmosphere even though no rain is falling. Clear air mode gives meteorologists the ability to see things like cold fronts and subtle airmass boundaries. When conditions are right, these boundaries can become the focal point for storm initiation, so being able to see them is extremely important. Clear air mode is also useful for detecting very light drizzle and light snow. Sometimes these phenomena do not generate a strong enough return signal to be detected in precipitation mode, but are clearly visible in the more sensitive clear air mode. For this reason, the NWS will sometimes leave a radar in clear air mode when it’s snowing.

RadarScope supports a couple of display options for clear air mode. By default, RadarScope hides the colors associated with lower reflectivity values. But if you enable “Expert Mode” in the preferences, it reveals the full color scale. This reveals more of the detail seen by the radar, but it means that what you often see is a big plume of dust, insects, and other clutter surrounding the radar. The following two images provide an example of this using the same clear air mode base reflectivity product. The first image uses RadarScope’s default color scale. The second image uses RadarScope’s expert mode color scale.

When precipitation begins within the coverage area of a particular radar, the NWS usually switches to precipitation mode. This mode looks more like what you’d expect when looking at radar images on various web sites.

You can learn more about clear air mode on this National Weather Service page:

Tilts

The radar beam is sent into the air at varying angles, or tilts, from the horizon. The lowest angle (tilt 1) is about 0.5 degrees for most radars. The highest angle (tilt 4) is between 3 and 4 degrees from horizontal. Higher tilts allow you to see higher levels of the storm structure. With any tilt, the farther the beam gets from the radar the higher it is looking in the air. Because of the steeper angle, that effect is more pronounced in the higher tilts. The curvature of the Earth also comes into play, so even if there were no tilt to the radar beam whatsoever, it is looking higher above the ground the further it gets away from the radar. Meteorologists use the higher tilts to get an idea of the vertical structure of a storm. But because of the steeper angle, those products can be a little more difficult to interpret.

For most purposes, the casual user will want to stick with tilt 1, which is closest to the ground. But keep in mind, even the lowest tilt can be sampling at thousands of feet above the ground depending on the distance from the radar. There can still be a lot of weather happening in that lowest few thousand feet beneath the beam, even for the lowest tilt.

 

Base Reflectivity Tilt 1

Base Reflectivity Tilt 2

Base Reflectivity Tilt 3

Base Reflectivity Tilt 4

You can learn more about radar tilts on this National Weather Service page:

Resolution

RadarScope renders NEXRAD Level 3 data that it receives from the National Weather Service at its true resolution. Level 3 reflectivity data is 1 kilometer per gate (or radar pixel) radially as you move away from the radar, and about a 1 degree angle as the radar rotates. Like a flashlight beam, the radar beam widens as it gets farther from the radar itself and the width of the pixels increases as a result. So while the data pixels are still 1 kilometer radially, they become significantly wider and are thus lower resolution as you move away from the radar. RadarScope displays images at the true resolution of this data, so what you see is the best that NEXRAD Level 3 data can provide.

Below is a sample image from the State College radar. In this case, the radar is located just west of the “Unionville” label. You’ll notice that the radar pixels become wider as you move away from the radar.

Update Interval

Collecting data is not an instantaneous process for NEXRAD radars. It takes a certain amount of time to rotate the antenna and collect data for all the different tilts. Collectively these tilts make up what is called a volume scan. Depending on whether the radar is operating in clear air mode or precipitation mode, each volume scan takes a different amount of time. When operating in precipitation mode, a volume scan takes 5-6 minutes. In clear air mode, since the antenna is rotating more slowly, a volume scan takes about 10 minutes.

As a radar collects a volume scan, it first collects a 360 degree sample at an elevation angle of 0.5 degrees (tilt 1), then a scan for tilt 2, and so on, increasing elevation angle with each revolution. Once all the tilts for a given volume scan have been collected, it will recycle back down to tilt 1 and do it all over again.

This is why NEXRAD radar images update on a 5-10 minute interval.

RadarScope is tuned to the NEXRAD volume scan strategy and only checks for new data at times defined by the current operating mode. Checking for updates more often than that is an unnecessary waste of resources because new data will not exist until the volume scan is complete.

More Information

As you can see, there’s a lot of useful information in radar images, but interpreting them can be a tricky prospect. It takes a good understanding of how the radar works as well as how the atmosphere behaves to make sound judgements during severe weather events. The NEXRAD network offers high density coverage of the U.S., but it still can’t see everything. RadarScope is one of many tools you can use to stay informed. But it should always be used in conjunction with official information from the National Weather Service, local emergency management officials, and your local news media.

The National Weather Service has some good information on its web site about NEXRAD radar products. Here are a couple of good pages that provide starting points for learning more about NEXRAD radar:

Terminal Doppler Weather Radar

Terminal Doppler Weather Radar (TDWR) were developed to detect hazardous wind shear conditions for aviation. There are currently 48 TDWR stations in the United States. They are generally located on or near major airports.

The TDWRs have a different product set than the standard NEXRAD Radars.

Dual-Pol Products

Differential Reflectivity (ZDR)

The Differential Reflectivity (ZDR) product shows the difference in returned energy between the horizontal and vertical pulses of the radar. Differential Reflectivity is defined as the difference between the horizontal and vertical reflectivity factors in dBZ units. Its values can range from -7.9 to +7.9 in units of decibels (dB).

Positive values indicate that the targets are larger horizontally than they are vertically, while negative values indicate that the targets are larger vertically than they are horizontally. Values near zero suggest that the target is spherical, with the horizontal and vertical size being nearly the same. Differential Reflectivity is available in two resolutions: 8-bit at 1 degree x 0.25 km resolution and 4-bit at 1 degree x 1.0 km resolution.

Differential Reflectivity values are biased toward larger particles. Stated differently, the larger the particle, the more it contributes to the resulting reflectivity factor. Hence while raindrops are normally wider than they are tall which would tend to yield a positive ZDR value, a scattering of large hailstones in the same volume of air being observed will yield a ZDR value closer to 0, because the spherical shape of the larger objects contributes more to the final reflectivity value. If the base reflectivity product is indicating high dBZ values whereas differential reflectivity is returning values near zero, then the volume in question is likely filled with a mixture of hail and rain.

Correlation Coefficient (CC)

The Correlation Coefficient (CC) product is defined as the measure of how similarly the horizontally and vertically polarized pulses are behaving within a pulse volume. Its values range from 0 to 1 and are unitless, with higher values indicating similar behavior and lower values conveying dissimilar behavior. The CC will be high as long as the magnitude or angle of the radar’s horizontal and vertical pulses undergo similar change from pulse to pulse, otherwise it will be low. It is available in two resolutions: 8-bit at 1 degree x 0.25 km resolution and 4-bit at 1 degree x 1.0 km resolution.

Correlation Coefficient serves well at discerning echoes of meteorological significance. Non-meteorological echoes (such as birds, insects, and ground clutter) produce a complex scattering pattern which causes the horizontal and vertical pulses of the radar to vary widely from pulse to pulse, yielding CC values typically below 0.8. Hail and melting snow are non-uniform in shape and thus cause a scattering effect as well, but these meteorological echoes have more moderate CC values ranging from 0.8 to 0.97. Uniform meteorological echoes such as found in rain and hail yield well-behaved scatter patterns, and their CC from pulse to pulse generally exceeds 0.97.

The accuracy of the Correlation Coefficient product degrades with distance from the radar. The CC will also decrease when multiple types of hydrometeors are present within a pulse volume, thus a volume with rain and hail will yield a lower CC than the same volume with solely rain.

Specific Differential Phase (KDP)

Differential phase shift in general (technically classified as propagation differential phase shift) is the difference between the horizontal and vertical pulses of the radar as they propagate through a medium such as rain or hail and are subsequently attenuated (slow down). Due to differing shapes and concentration, most targets do not cause equal phase shifting in the horizontal and vertical pulses. When the horizontal phase shift is greater than the vertical the differential phase shift is positive, otherwise it is negative. Stated differently, horizontally oriented targets will produce a positive differential phase shift, whereas vertically oriented targets product a negative differential phase shift.

While this correspondence between positive values (horizontal) and negative values (vertical) is analogous to Differential Reflectivity (ZDR), there is a key distinction: differential phase is dependent on particle concentration. That is, the more horizontally oriented targets are present within a pulse volume, the greater the positive differential phase shift. Thus a high concentration of small raindrops could yield a higher differential phase value than a smaller concentration of larger raindrops. Differential phase shifting is largely unaffected by the presence of hail, and shifts in snow and ice crystals are typically near zero degrees. Non-meteorological echoes (birds, insects, and so forth) produce highly variable differential phase shifts.

Specific Differential Phase (KDP) is defined as the range derivative of the differential phase shift along a radial. Its possible values range from -2 to 7 in units of degrees per kilometer. It is available in two resolutions: 8-bit at 1 degree x 0.25 km resolution and 4-bit at 1 degree x 1.0 km resolution. It is best used to detect heavy rain. Areas of heavy rain will typically have high KDP due to the size or concentration of the drops. Hail and snow/ice crystals have no preferential orientation and will yield KDP values near zero degrees. Non-meteorological echoes will result in noisy KDP values. KDP is not calculated for areas in which the Correlation Coefficient (CC) is less than 0.9, which will result in gaps in the rendered data.

Hydrometeor Classification (HC)

Hydrometeor Classification (HC) is an algorithm to identify the predominant hydrometeor in the radar beam. The pre-defined categories recognized under this classification are as follows:

  • BI- Biological (birds, insects)
  • GC – Ground clutter (buildings, trees)
  • IC – Ice crystals
  • DS – Dry snow
  • WS – Wet snow
  • RA – Light/moderate rain
  • HR – Heavy rain
  • BD – Big drops
  • GR – Graupel (soft ice, snow pellets)
  • HA – Hail-rain
  • UK – Unknown
  • RF – Range folded

The Hydrometeor Classification product should be used in conjunction with other data for proper interpretation, as it is merely an algorithm and not an absolute indicator of what is occurring at a particular location. As currently implemented, the algorithm determines only the most likely type of hydrometeor, omitting information pertaining to the likelihood of other categories.