How does radar see through clouds, fog and darkness?

Radar sees through clouds, fog and darkness because it doesn't use light at all: it uses radio waves, and it makes its own. Fog, clouds, rain and falling snow block visible light, but they are usually transparent to radio waves, which the medium they pass through absorbs only weakly. And because a radar sends out its own radio waves, a process called illumination, instead of relying on sunlight or moonlight, night makes no difference to it.

Those waves bounce back especially well off good electrical conductors, like the metal of an aircraft or a ship, seawater or wet ground. Time the round trip of a short pulse, multiply by the speed of the signal and halve it, and you know how far away the object is. If the object is moving toward or away from the radar, the echo's frequency shifts slightly, the Doppler effect, which reveals its speed too.

Making it practical started with a rumour. In 1935 Robert Watson-Watt was asked to judge reports of a German radio "death ray" and handed the question to Wilkins, who calculated it was basically impossible but remembered that aircraft could cause radio interference. On 26 February 1935, in the Daventry Experiment, a powerful BBC shortwave transmitter served as the source, a receiver sat in a field, and a bomber flew around the site. The plane was clearly detected, and funding followed immediately.

That work led to Chain Home, a network of detection stations along England's East and South coasts, ready for the outbreak of World War II. Its advance warning helped the Royal Air Force win the Battle of Britain: without it, Britain would have needed fighters it didn't have constantly in the air.