Which factors are crucial for the development of fog?

The development of fog is significantly influenced by humidity, temperature, and calm winds. Humidity is essential because fog forms when the air is saturated with moisture. When the relative humidity reaches 100%, tiny water droplets condense, leading to the formation of fog. Temperature also plays a critical role; fog commonly develops when the temperature of the air drops at night or in the morning, which can reduce the air's capacity to hold moisture.

Calm winds are equally important, as they allow the moisture to settle in a localized area without being dispersed. Strong winds can mix the air layers and prevent the air from reaching saturation, thus inhibiting fog formation. This combination of high humidity, lower temperatures, and still conditions creates the ideal environment for fog to develop.

The other options incorporate factors that either do not directly contribute to fog formation or pertain to entirely different meteorological phenomena. For instance, sunlight generally helps to warm the atmosphere, which would counteract the cooling necessary for fog formation. Additionally, aspects such as temperature inversions can influence fog development but are not as fundamental as the calm winds, humidity, and temperature relationship.