During the day, air rises as the sun heats the atmosphere. Whether a storm is coming or going, air is always rising during the day. This rising of the air actually affects the radar. A radar works by sending out a wave in all directions. This 'wave' then bounces off any clouds and back to the radar, telling the radar that it caught something. This 'catch' by the radar wave is how we see storms on the radar.
At night, the radar is still working, sending out beams in all directions. However, at night, air sinks if high pressure is in the area. This sinking of the air will take the radar 'waves' and actually push them to the ground. When they are pushed down, they hit houses, trees, anything and everything on the ground. This gives the radar a false idea that there is something in the area, hence the colorful display like the one above.
So when you're having an outdoor party and check the radar to see something like the above but see a clear sky, it's the radar beams hitting the ground thanks to sinking air.