§ 107.51 sets the weather floor Chapter 1 introduced: at least 3 statute miles of flight visibility, measured from the location of your control station, and the aircraft kept at least 500 feet below and 2,000 feet horizontally from any cloud. The reason to return to them here is that they do not relax when the light goes. The same three miles and the same cloud clearances apply at dusk and at night as at noon — the rule offers no allowance for darkness. What changes is not the number but how hard the number is to judge.

The rule actually builds the low-light case into its own definition. § 107.51 defines flight visibility as the average slant distance at which prominent unlighted objects can be seen and identified by day, and prominent lighted objects by night — so after dark you are gauging three miles against lit references, and that is a judgment call, not an instrument readout. In daylight you have a full scene to calibrate against; at dusk and at night, haze that was invisible an hour earlier scatters light, and distant lights fool the eye about how far off they are. The three-mile figure does not move, but your margin for misjudging it narrows, so the honest reading is the conservative one: if you cannot be sure you have three miles, you treat it as though you do not.

The cloud clearances protect the same see-and-avoid margin — a crewed aircraft dropping out of a cloud has no time to pick out a drone tucked against it — and at night you frequently cannot make out the cloud base at all. That is precisely why the clearance matters: a clearance you cannot confirm is a clearance you do not have. Stacked with the rules from earlier in the chapter, the after-dusk go or no-go becomes a short, unforgiving checklist — is the pilot qualified for night, is the aircraft lit to the standard, and can you actually confirm three miles of visibility and the cloud clearances? A single no on that list grounds the flight.