Definition
On a NEXRAD weather radar display, shadows are areas of false weakened or absent returns that appear behind a strong echo because the radar beam has been partially or fully attenuated as it passed through heavy precipitation. The display shows little or no precipitation in the shadowed region even though significant weather may actually be present there.
Plain English
When a radar beam pushes through a heavy storm, much of its energy is absorbed or scattered. Anything beyond that storm shows up weaker on the screen, or doesn't show at all -- like a shadow cast behind the storm. The weather there might still be bad; the radar just can't see it clearly.
Context Anchor
Seen when interpreting NEXRAD weather images, especially near strong rain or thunderstorms.
Derivation
From the everyday word 'shadow' -- an area shielded from light by something in the way. Here the radar beam plays the role of light, and a heavy storm cell plays the role of the object blocking it. The area beyond is in the storm's radar 'shadow.'
Why Pilots Care
Failure to recognize a shadow can lead to underestimating storm coverage and inadvertently flying toward undetected hazardous weather.
Analogy
It is like standing behind a thick wall during a flashlight beam. The dark area behind the wall does not mean nothing is there; it means the light did not reach it well.
Grounding Statement
If a strong storm sits between the radar and the area beyond it, the display behind that storm may not show the full picture.
Intuition Check
Do not assume a radar shadow is only a harmless dark spot on the screen. In this context, a shadow means the radar may be showing less weather than is actually present behind strong precipitation.
Example Sentence 1
The instructor pointed out a suspicious clear patch on the NEXRAD image and explained it was likely a shadow behind the line of thunderstorms, not actual clear air.
Example Sentence 2
Mountainous terrain created persistent shadows on the NEXRAD image, masking weather on the far side of the range.