Why is it cloudy when the radar is empty?
Because radar doesn't see clouds. It sees falling rain and snow. Cloud droplets are hundreds of times smaller than raindrops, far too small to show up, so a gray sky with no rain looks blank on radar.
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A weather radar sends out pulses of microwaves and listens for echoes. The echo from a drop grows with the sixth power of its diameter, so a drop ten times wider returns about a million times more signal. Cloud droplets are around 0.01 to 0.02 mm across; raindrops are 1 to 5 mm. The clouds are effectively invisible.
The beam also misses low clouds
The radar beam leaves the dish angled slightly upward, and the Earth curves away beneath it. Fifty miles from the radar the beam is already a few thousand feet up; a hundred miles out it's even higher. A low gray deck, like a coastal marine layer, can sit entirely underneath it.
Use satellite for clouds
Satellites look down from above and see cloud tops directly. Infrared satellite images show cold, high clouds as bright and warm, low clouds as dim, so a marine layer can be hard to spot at night, but thick storm clouds stand out clearly. Air Story's “Around you” chapter flips between rain (radar) and clouds (satellite) so you can compare.
Questions
Why does radar show rain but it's not raining?
The beam may be seeing rain high up that evaporates before reaching the ground (virga), especially in dry air. It can also pick up birds, insects or ground clutter close to the radar.
Can radar see drizzle?
Barely. Drizzle drops are small, so light drizzle from a marine layer often doesn't show up at all.