How does weather radar work? I assume it works in the traditional sense of radar: send out an EM signal, it bounces off rain or clouds (moisture) and you get an idea of what is around. But based on that, wouldn’t a nearby storm create a “shadow” that prevents the radar signal from “seeing” weather on the same radial line?
Good thinking but not quite.
Weather Radar 101 …
You’re 100% right that it’s simply an EM transmission which is reflected by the moisture in the air. Modern weather radar can’t see clouds = water vapor. It can only see liquid or solid water. Rain is seen best, then wet snow & wet hail. Dry hail & dry snow are seen poorly but modern digital signal processing can detect the weird way EM is refracted by the dry stuff. Doppler processing can determine radial windspeeds from the precip’s motion. Localized horizontal shear in the doppler values imply rotation.
Most rainfall is not completely opaque to the radar. So a close-in storm partially hides storms further out, rather than obscuring them completely. Modern radars include logic to adjust the presentation of distant returns to take account of the fact they appear weaker than they really are due to to closer weather. The technical terms are “attenuation” & “attenuation compensation”.
At the extreme, a very dense thunderstorm can in fact all but blind modern airborne radar to rain cells farther out. Ground based radar can be vastly more powerful and see decently through the worst storms. As well, the weather service uses overlapping radar sites so they can often times see the same weather from more than one point of view. When they stitch the mosaic together they can resolve teh attenaution problem.
This Southern Airways Flight 242 - Wikipedia is a famous (within in the industry) aircraft accident which was caused by extremely strong rainfall causing so much attenuation that the primitive radar showed what looked like a thin spot in a wall of weather they needed to get past. The “thin spot” was actually a fully tornadic supercell with incredible rainfall density. The rain was so thick that only partway into the cell the radar was completely attenuated and didn’t show the rest of the cell, much less anything behind it. The crew attempted to penetrate the “thin spot” & it didn’t work out so well.
I just skimmed this Weather radar - Wikipedia and it’s a very nice intro to everything you might want to know about weather radar, including attenuation.
Further to that, there’s a mode on aircraft weather radar called “RCT” which is short for rain echo attenuation correction technique or something similar. Anyway, it basically gives you an idea of how far the radar is penetrating. If you turn it on you get blue on the radar in areas where it’s not penetrating very well, this can help you decide whether what is painting is all there is out there, or just the edge of something nasty.