What’s my relationship with radar? Well, sometimes it has trouble putting the pieces together, and other times it blows things out of proportion. It plays games by saying it’s in a certain spot when it isn’t, and even stands me up on occasion by not bothering to show up! Like most relationships, we go through ups and downs; and while there are many great points to using radar, there is also a good deal that can elude the interpreter.
In general, radar works by sending waves out into the lower atmosphere from a rotating antenna. If these waves encounter precipitation, they scatter, and a portion of that wave is reflected back toward the antenna. Once received, the information is used to create an image depicting where precipitation is (for more info see “Weather Radars Explained”)..
There are 159 Doppler radar stations in the United States that independently receive and interpret data. When viewing the national radar, you are seeing many different radar images pieced together in a mosaic displaying precipitation in one cohesive product. Misinterpretation happens due to the potential lag time between radars since each station works separately. Also, scheduled maintenance or an error can cause a mosaic to jump from one frame to the next when it lacks data, making it inconsistent with surrounding information.
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Another caveat to interpreting radar stems from liquid water being more reflective than frozen water. When snow mixes with or transitions to rain, radar will “bright band”. This is an area of enhanced reflectivity (warmer colors) which usually implies heavy rain. In actuality, it is indicating a transition from snow to rain. As a snowflake begins to melt, it becomes coated in a layer of liquid water that is essentially “puffed up” over the frozen portion of the flake. Since the liquid area is more reflective than its frozen counterpart, radar blows things out of proportion, allowing for misinterpretation of higher reflectivity.
Radar can also imply precipitation is falling in a certain spot when it isn’t. This phenomenon is called Virga which occurs when precipitation evaporates before reaching the ground. Here, it is crucial to understand radar emits waves at an angle into the sky and as they travel away from the station, the Earth being circular curves away from the beam. Therefore, 100 miles away from the station, radar may be seeing precipitation that is thousands of feet above the surface allowing plenty of time for evaporation to occur. Therefore it is important to use radar in conjunction with surface weather information to confirm what is making it to the ground.
Piggy-backing off of that idea, consider the consequences of precipitation occurring below the radar beam. This situation is most common in instances of drizzle and what happens, is that saturation takes place at such a low level of the atmosphere that the beam travels over the top of it and therefore nothing shows up on radar. Again, we see the benefit of incorporating data from surface weather stations.
Radar has many benefits, and understanding where misconception can occur will keep you from being fooled. What’s my relationship with radar? You could say it’s complicated.
