A fog is most likely to develop when a warm moist wind blows over a cold current.
Fog is a type of low-lying cloud that forms when the air near the ground cools and reaches its dew point, causing water vapor to condense into tiny water droplets. In order for fog to form, there needs to be moisture in the air and a cooling mechanism.
When a warm moist wind blows over a cold current, the warm air is cooled by the cold water below. This cooling process can cause the air to reach its dew point and form fog. This type of fog is called advection fog and is commonly seen near coastal areas where warm ocean currents flow over cold water.
In contrast, when a cold dry wind blows over a cold current, there is little moisture in the air to form fog. Similarly, when a cold dry wind blows over a warm current or a warm moist wind blows over a warm current, the air is not cooled enough to reach its dew point and form fog.
In summary, a fog is most likely to develop when a warm moist wind blows over a cold current because the warm air is cooled by the cold water below, causing the air to reach its dew point and form fog.