Those droplets are so small, they can remain suspended in the air. If you get millions and millions of those tiny droplets suspended in the air, you have problems because now you can't see where you are going. Upslope fog is pretty self-explanatory. As the air rises, it expands and cools to reach the dewpoint. Upslope fog is most common over the Great Plains where the terrain gradually rises to meet up with the Rocky Mountains.
Advection refers to the horizontal movement of air.
So advection fog forms when moist air blows over a cool surface. The air then cools to the dewpoint and fog forms. In the Winter, advection fog is common in the Midwest when a Southerly wind from the Gulf of Mexico blows over the cold land. If you have ever been to San Francisco in the Summer, you know what coastal advection fog is. Radiational cooling is most efficient on clear nights with light winds.