What is an initial indication that a pilot may be flying into restricted visibility conditions at night?

When flying at night, one of the most critical factors to consider is visibility. The gradual disappearance of lights on the ground serves as a strong indicator that the pilot may be entering restricted visibility conditions. This phenomenon occurs because as a pilot approaches fog, haze, or precipitation, the light from ground sources becomes more diffuse and is increasingly obstructed. Refuge on the ground, such as streetlights, city lights, and other illuminated features, become progressively less visible, alerting the pilot to a potential reduction in visibility.

In contrast, while dark clouds and changes in wind speed are indeed important meteorological indicators, they do not provide a direct indication of visibility degradation in the same way that ground lights do. The presence of dark clouds might suggest stormy conditions, and increased wind speed could indicate approaching weather systems, but these factors do not necessarily correlate with immediate visibility issues at night. The focus on ground lights directly relates to the perception of visibility, making it the most relevant factor in this context.