Environment Canada has since ended the fog advisory for parts of Simcoe County Thursday morning.
The weather agency said persistent fog has led to near-zero visibility in Barrie, Collingwood and Hillsdale.
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The conditions are expected to dissipate later in the morning.
Environment Canada warns drivers that travel is expected to be hazardous due to reduced visibility in some locations.
If visibility is reduced while driving, turn on your lights and maintain a safe following distance, the weather agency suggests.