Definition
In a METAR, VV indicates an indefinite ceiling, meaning the sky is obscured (by fog, heavy precipitation, or similar) and the cloud base cannot be determined. The digits that follow VV give the vertical visibility into the obscuration in hundreds of feet above ground level. For example, VV004 means vertical visibility is 400 feet AGL.
Plain English
VV in a weather report means the sky is so obscured that there is no normal cloud base to measure. Instead, the report tells you how far up you could see if you were looking straight up from the ground.
Context Anchor
Seen in METAR aviation weather reports, especially during fog, heavy snow, blowing dust, or other conditions that hide the sky.
Derivation
VV stands for Vertical Visibility. It is used instead of a cloud layer code because there is no defined layer to report — only how far a person on the ground can see upward into the obscuration.
Why Pilots Care
VV reports an indefinite ceiling, which counts as a ceiling for IFR and approach minimum purposes. A METAR showing VV002 (200 feet) means very low IFR conditions and may put the airport below approach minimums.
Grounding Statement
If you stand on the airport and fog hides the sky, VV is the reported height you can see upward before the view disappears.
Intuition Check
VV is not forward visibility along the runway. It is visibility straight upward into a hidden sky.
Example Sentence 1
The METAR reported VV003, indicating an indefinite ceiling with vertical visibility of 300 feet in fog.
Example Sentence 2
With VV001 reported, the field was below landing minimums for the approach.