When can fog form, negatively impacting visibility for pilots?

Fog forms when the air near the ground cools to its dew point, which is the temperature at which air becomes saturated with moisture. This saturation leads to the condensation of water vapor in the air, producing tiny water droplets that create a haze, significantly reducing visibility.

The cooling process can occur through various mechanisms, including radiational cooling at night or the presence of cooler air masses. Once the air temperature reaches the dew point, the excess moisture in the air condenses, resulting in fog formation. This phenomenon can create dangerous flying conditions due to low visibility, making it essential for pilots to understand these conditions.

The other scenarios mentioned would not lead to fog formation. Heating surface air tends to increase its capacity to hold moisture, reducing the likelihood of fog. Removing moisture from the air also decreases the chances of reaching saturation. A sharp rise in temperature at night typically does not contribute to fog; rather, it can result in a stabilization of the air that may prevent cooling to the dew point.