What do you mean by surface inversion?

Surface temperature inversion is developed when air is cooled because of contact with a colder and becomes cooler than the overlying atmosphere. This occurs most often on clear nights when the ground cools off rapidly by radiation. If the temperature of surface air drops below its dew point, it may result in the formation of fog. It is very common in higher latitudes. In lower and middle latitudes, it occurs during cold nights and gets destroyed during the daytime.

Further Reading:

• Factors Controlling Temperature Distribution
• Geography Notes