I frequently see lines like this (the red arrows in the image below point to what I am referring to) on doppler radars. They are usually propagating outward from a line of thunderstorms.
What are they, what do they indicate, and how do they form?
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If they propagate outward from thunderstorms, they are outflow boundaries. Cold air flows down from the thunderstorm with precipitation and spreads outward upon hitting the ground. This air moves ahead of the storm and forces warmer air upward, suspending clouds and other particles. That's what is visible on radar. More: https://en.wikipedia.org/wiki/Outflow_boundary