Radar Basics, part one:
By: Chris Gatsis
My latest blog deals with Radar concepts and radar imagery interpretation. I decided this was necessary to write since here at Zoomradar, we deal with radar for our media clients and for weather enthusiasts and it would be a good idea to explain the basics of radar and the typical interpretation errors of radar for the public. Since there is a great deal of information regarding radar, I will be splitting this blog into two parts, with this first blog discussing radar basics and interpretation errors in an upcoming second blog. All images here explaining radar are courtesy of the National Weather Service.
Quick history of radar:
First, I will briefly discuss the history of radar development for the 20th century. Radar is an acronym which stands for Radio Detection and Ranging. This was created in the early 1940’s by the British and American military to detect enemy planes as an early warning system during the Second World War. However, it was noticed precipitation (like rain and snow) was being viewed on radar screens. Coincidentally, both the British and American military were working on it at the same time, but without their knowledge of each other. So after the war, in the early 1950’s, radar stations were being created throughout the US and started being used in short term weather forecasting.
Your common radar system is composed of a parabolic antenna, which focuses a pulsed radio frequency beam out in the atmosphere (think of a lighthouse light) sweeping the sky at 360 degrees around the radar site. Each time the radar antenna makes a 360 degree sweep, it’s being pointed at different elevation angles in the atmosphere which gives the forecaster an image. When the radar beam being emitted from the antenna strikes a particle, such as water droplets, snowflakes, ice pellets or hail, a part of the beam is reflected back to the radar dish, which is called the echo. The intensity of what gets reflected back as the echo will tell the forecaster how close or far the precipitation is, what type of precipitation it is, which way it is moving and how much (in inches per hour) the precipitation is. Some interesting side notes regarding radar coverage in the US, the radar range for most NWS radar stations are 80 miles and there are currently 155 weather radar stations in continental US, including Puerto Rico and Guam, but we also have continental coverage as well at Zoomradar!
Analysis of Radar Basics:
The main colors for analysing base reflectivity (or precipitation reflectivity) are green for rain, blue for less intense of rain while the radar is in precipitation mode for the warmer months of the year (however it changes to Clear Air Mode or ‘CAM’ for the winter months to pick up snowflakes, ice pellets etc). The reflectivity will tell what type of precipitation is present. For the image below, this is during the warmer months of the year and is showing rain with different intensities. You will see on the side of a radar image a bar with colors ranging from -30 to +75 for NWS radar images, this is what shows you the reflectivity in units of dBz (decibels of z). If you see small bright red pockets, that’s usually a good sign of thunderstorm cells. If you see small purple pockets, it is a good sign of present hail. A good rule of thumb is, the higher the dBz value, the higher the amount of precipitation present. Even though we do not have a dBz legend at Zoomradar, all of what was written above applies to our radar images. Here is a good example:
For snow, This image is courtesy from Environment Canada’s Radar station in King City, just outside of Toronto for January 14th, 2012. As you can see blue is for light snow, and green for heavier snow, but just like with the NWS, this radar station is in Clear Air Mode to catch snowflakes. The intensity scales actually change for the winter months, for rain it’s mm/hr and for snow it’s cm/hr for this image. The decibel level scale from Canadian Radar also applies here for US radar.
For base velocity colors, its noted as green for coming towards the radar station and red for moving away from the station and purple is for area which the radar is unable to distinguish radial velocity. Note, precipitation velocity only tells how fast the precipitation band is moving, and what direction it is heading to and from the radar site. See image below:
For composite reflectivity, it can reveal important storm structure features and intensity trends of storms. This is important because often during the development of strong to severe thunderstorms, rain-free areas (or areas with light rain) develop as a result of strong updrafts. See picture below:
Radar is a very important forecasting tool since it can show precipitation intensity, precipitation type, the speed and direction of it as well. You can also see updrafts and even tornadic signatures as well. One important fact I must stress is when viewing radar images, they deal with what has happened and not which will happen, so always check the time stamp of the image you are viewing. In some cases, the radar cannot distinguish between real echoes (precipitation) and false echoes (trees, hills, tall buildings etc), so this and some common interpretation errors will be discussed in part two of my radar blog, so stay tuned!