General Info About Radar:
A radar is an instrument designed to measure movement, intensity, type, and position of different types of precipitation (i.e. rain, hail, sleet, or snow). Radar works by sending radiation pulses in directions around it, and when it lands on a form of precipitation it bounces back toward the radar. When the radar picks up these return signals it measures time taken to return, distance, and height as well as size.. There is more detail here on how radar works, going into wavelength sizes and more.
http://en.wikipedia.org/wiki/Weather_radar#How_a_weather_radar_works
http://www.srh.noaa.gov/radar/radinfo/radinfo.html (Click right one for animation)
(images from the National Weather Service at http://www.srh.noaa.gov/radar/radinfo/radinfo.html)
Basic Data Types:
Reflectivity: Reflectivity data measures precipitation intensity to determine rate and volume of the precipitation. It is measured in Decibels of Z, or DbZ.
How to read: the intensity and volume of precipitation in a shower or storm is usually monitored by color. A blue/green represents very little precipitation or radar interference. A orange/red represents heavy rain, and a purple/white represents extremely heavy rain, but more likely hail. More advanced radar readings regarding reflectivity will be explained later.
Here is a base reflectivity radar image from http://www.nssl.noaa.gov/divisions/warning/swat/Cases/990503/A_images/tlx-53_ref31.gif We will discuss the hook echo in this image later on.
Velocity (both base and storm relative): Velocity radar measures the movement of precipitation. Precipitation moving toward the radar shows up as green (negative), and precipitation moving away from the radar shows up as red (positive). Please note that it does not measure the intensity of the precipitation, just its velocity. Also, colors may vary depending on the color table. look at the color guide for velocities.
Base Velocity measures all precipitation direction
Storm Relative Velocity measures precipitation motion within the storm complex itself, and may be different that base images.
This is a velocity radar image from http://www.crh.noaa.gov/images/dvn/events/20060413tor/v_0124.png
This is a storm cell and its precipitation direction and velocity.
Special Radar Observations
Hook Echos: A hook echo is a section of thunderstorm that wraps inward to create a hook, and is visible in reflectivity mode. A hook echo indicates a strong mesocyclone (a large area of rotating thunderstorm), an ingredient in the development of tornadoes. [PLEASE NOTE: just because there is a hook echo it does NOT always mean there is a tornado.]
The image below is that of a famous hook echo, recorded in Moore, OK 1999. This storm produced a tornado that was well over a mile wide, and rated at an F-5 it was one of the most destructive tornadoes ever recorded. Radar recorded astonishing wind speeds of 318 MPH. Image by the National Weather Service’s Storm Prediction Center. (http://www.spc.noaa.gov/faq/tornado/radscel.htm)
Click to enlarge, it has some helpful observations on it.
Tornado Vortex Signature: Now, this is the velocity image of the Moore, OK 1999 tornado. Now this brings us into another radar phenomenon called a tornado vortex signature, or abbreviated TVS. A TVS is seen when there are two opposing colors of velocity (green/red) forming a tight couplet. That couplet indicates tight rotation inside of a storm, a good indicator of a tornado. [PLEASE NOTE: just because there is a TVS it does NOT always mean there is a tornado!!] Here is an example, and it has the reflectivity with the hook echo side by side with the velocity image. click to see a larger version.
(http://www.nssl.noaa.gov/news/may3rd/wdss.jpg)
Note the red and green making a small, tight couplet.
More special radar observations will come… mainly discussing gust-fronts.
Notes:
Radar has its flaws and it takes the trained eye and wisdom of the observer to determine what is real, and what is not. Just because there is red on green it does not mean there is rotation, and same goes for a TVS. Just because it is there it does not mean with certainty there is a tornado. My biggest tip: USE COMMON SENSE! If there is no thunderstorm, chances are there is no tornado! If you would like to learn more about how radar works, how to read it, and where to find the data, click on the links below.
http://en.wikipedia.org/wiki/Weather_radar Radar info
http://www.srh.noaa.gov/radar/radinfo/radinfo.html Radar info – MUST READ!
http://www.wunderground.com/radar/help.asp?MR=1 Information on NEXRAD
http://www.weather.gov/radar_tab.php US National Weather service Radar.
http://www.wunderground.com/radar/map.asp US NEXRAD – contains velocity and other advanced products
