When does steam fog typically form?

Steam fog typically forms during cold weather conditions, especially over bodies of water. This phenomenon occurs when the air above the water is warmer and more humid than the air above it, causing the warm air to rise. As this warm, moist air comes into contact with the colder air above, it condenses into tiny water droplets, creating a foggy appearance. This is particularly common in the early morning or late at night when temperatures are cooler, often leading to a dramatic visual effect over lakes, rivers, or oceans.

The other options describe conditions that do not lead to the formation of steam fog. For instance, hot summer days would generally cause any fog to dissipate rather than form, as warmer air absorbs more moisture. In desert conditions, the low humidity and high temperatures also prevent fog from forming. Lastly, while nocturnal cooling can lead to other types of fog, such as valley fog, mountainous areas do not specifically create the conditions necessary for steam fog, as it requires a significant temperature contrast between warmer water and cooler air.