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Weather radar is one of your most important sources for weather information. But do you know what it can't see?
Radar beams reflect off nearly everything, including clouds. But before NEXRAD radar images are sent to your iPhone, non-precipitation items are filtered out, including clouds. Unless it's rain or snow, chances are it's not showing up on your radar image. Of course, there are exceptions for everything, and in this case, it's roll clouds. Because of the motion of roll clouds, doppler radar can pick them up and show them on a return echo.
Because light rain is typically made of small, widely spaced drops, it often times doesn't return a strong enough echo to show up on radar. The problem gets worse for precipitation far from the radar station. The greater the distance from the station, the less likely it is to show up on a radar image.
Virga is precipitation that falls from the sky, but evaporates before it hits the ground. This can create false radar images where it appears significant precipitation is occurring, but in reality, nothing is hitting the ground.
There are two problems with drizzle. First, the tiny drops have very low radar reflectivity. Second, drizzle usually occurs in a shallow layer close the ground, which can get filtered out with ground clutter.
What's harder to detect than light rain? Light snow. That's because snow doesn't reflect radar beams as strongly as rain. And when you see those bands of heavy snow on the Weather Channel, chances are there's rain or sleet mixed with it at altitude.
While doppler radar can detect funnel clouds as they form, it can't tell when they touch the ground. And because of that, tornados get crossed off the list.
There are two reasons why radar struggles to see distant thunderstorms. 1) if the majority of thunderstorm precipitation is below the horizon, nothing shows up on radar, and 2) even if precip is above the horizon, it tends to appear weaker than it actually is, because of radar beam spread at long distances.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at email@example.com.