Fine line

From Glossary of Meteorology

fine line

A narrow radar echo indicating a boundary (such as a frontal boundary, gust front, or dryline) across which a density or moisture discontinuity exists.

The reflectivity is apparently explained by scattering from the refractive index gradients across the discontinuity and from insects and insect-eating birds that are concentrated along the line.
See clear-air echo.

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