If visibility is reported as "000" in a METAR, what does this indicate?

When visibility is reported as "000" in a METAR, it indicates that there is no visibility, which means that the observer cannot see anything at all. This situation often arises in severe weather conditions, such as fog, heavy precipitation, or snow, where visibility is so severely restricted that it cannot be quantified.

The term "000" is a standard representation used in aviation reporting to denote that visibility is effectively zero. This can greatly impact flight operations, as pilots must be particularly cautious when navigating in such conditions.

Other options do not accurately represent the scenario. Unlimited visibility would suggest that there are no obstructions to sight, which is the opposite of what "000" indicates. Visibility less than 1 mile might encompass a range of visibility issues but does not specify a complete lack of sight. Dense fog could lead to "000" visibility, but "000" itself directly specifies no visibility without attributing it to a specific cause.