Which of the following is not reliably detected by weather radar?

• A. Clear-air turbulence (CAT)
• B. Clouds
• C. Precipitation
• D. Lightning

Answer: (A)

Weather radar works by sending out radio waves and listening for echoes from particles in the air. It reliably shows where there are droplets and ice crystals—precipitation and clouds—because these hydrometeors reflect the radar signal well. Clear-air turbulence occurs in regions with little to no moisture or particulates, so there’s hardly anything for the radar to bounce off. The result is a weak or absent radar return, making CAT difficult to detect reliably. Lightning isn’t something radar detects directly either; it’s an electrical discharge, not a radar target, although a strong storm cell that could produce lightning may appear on radar due to its associated precipitation. So the phenomenon not reliably detected is clear-air turbulence.