Wavelength: 1,000,000× difference
A typical radar uses 3 cm waves. A typical LiDAR uses 905 nm (0.000905 mm). LiDAR's tiny wavelength gives centimetre resolution at hundreds of metres; radar's longer wavelength gives metre resolution but punches through rain, fog and dust.
Weather
LiDAR struggles in heavy rain, snow and fog — the laser scatters off droplets and you lose your signal. Radar barely notices. This is why no serious self-driving stack relies on LiDAR alone.
Resolution
A modern automotive LiDAR makes a 3D point cloud with millions of points per second. Radar gives you maybe a few hundred targets per scan, each just a position and velocity. LiDAR sees that the object is a pedestrian; radar sees that something is moving 50 km/h at 80 metres.
Speed measurement
Radar measures velocity directly via Doppler shift. LiDAR has to compare two consecutive frames to estimate velocity — slower and less accurate. This is why police use radar guns, not laser guns, for speed traps.
When you'd choose which
LiDAR: archaeology, forestry, autonomous vehicle perception layer, indoor mapping. Radar: weather, aviation, ships, all-weather automotive ADAS, missile defence.