Fog blankets France's plains, creating rare optical phenomena and posing driving challenges as anticyclonic conditions persist through late autumn.
Fog lingers over French plains
Fog continues to affect the plains of France this weekend, with no signs of clearing. This has led to grey and humid mornings, while the mountains and the Mediterranean coast remain clear.Optical phenomenon caused by fog
The fog has created a rare optical phenomenon: a white arc, sometimes called a fogbow. This occurs when light interacts with very small fog or mist droplets, typically less than 0.05 mm in diameter, causing diffraction and forming a pale white arc instead of a rainbow.Anticyclonic weather pattern continues
A powerful anticyclonic block is responsible for the weather pattern, with pressure reaching as high as 1030 hPa. Models predict this configuration will continue, with pressure anomalies expected to remain high for the following weeks.Fog expected to persist into late autumn
The current anticyclonic configuration suggests foggy mornings will persist through late autumn, which extends until November 30. Conditions are ideal for observing the white arc, especially in valleys and wet areas where thick fog has been reported.Driving precautions in foggy conditions
Caution is advised on the roads, as dense fog remains the leading cause of reduced visibility in autumn. If visibility drops below 50 meters, the maximum allowed speed is 50 km/h, and high beams should not be used to avoid creating a blinding white wall effect.
