Instrument flight rules
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In aviation, instrument flight rules is one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other is visual flight rules.
IFR flying with clouds below
In aviation, instrument meteorological conditions are weather conditions that require pilots to fly primarily by reference to flight instruments, and therefore under instrument flight rules, as oppose
A pilot's view of the runway just before landing in thick fog at night
Summary of United States VMC minima (14 CFR 91.155), providing requirements for minimum visibility and separation from clouds for different airspace classes. In this example, for airspace classes C/D/E up to an altitude of 10,000 ft (3,000 m) AMSL, whether day or night, VMC minima require 3 miles (5 km) of visibility (denoted "3 SM") and cloud clearance of 1,000 ft (300 m) above, 500 ft (150 m) below, and 2,000 ft (610 m) horizontally ("152").