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Water vapor con denses around these microscopic solid particles. In order for fog to form, dust or some kind of air pollution needs to be in the air. There has to be a lot of water vapor in the air for fog to form. You can see fog because of these tiny water droplets.

During condensation, molecules of water vapor combine to make tiny liquid water droplets that hang in the air. Fog shows up when water vapor, or water in its gaseous form, condenses. Even monuments like London Bridge, in London, England, or the Golden Gate Bridge, in San Francisco, California, are almost impossible to see in thick fog. In some conditions, fog can be so thick that it makes it hard to drive safely because it obscures the road and other cars. Fog can be thin or thick, meaning people have difficulty seeing through it. 5-7 AG5f0502 RADIATION LAND COOLING Figure 5-2.—Radiation fog.Fog is a cloud that touches the ground. These conditions permit maximum radiation cooling. Radiation fog is common in high-pressure areas where the wind speed is usually low (less than 5 knots) and clear skies are frequent. Before noon, heat radiated from the warming surface of Earth destroys the inversion and the fog evaporates into the warmed air. After the Sun rises, Earth is heated. Radiation from the warming surface heats the lower air, causing evaporation of the lower part of the fog, thereby giving the appearance of lifting. Wind of low speed (3 to 5 knots) causes slight, turbulent currents. This turbulence is enough to spread the fog through deeper layers. As the nocturnal cooling continues, the air temperature drops further, more moisture is condensed, and the fog becomes deeper and denser. If winds increase to 5 to 10 knots, the fog will usually thicken vertically. Winds greater than 10 knots usually result in the formation of low scud, stratus, or stratocumulus. 5-2.) In case of a calm wind, this cooling by conduction affects only a very shallow layer (a few inches deep), since air is a poor conductor of heat. After sunset, Earth receives no heat from the Sun, but its surface continues to reradiate heat. The surface begins to cool because of this heat loss. As Earth cools, the layer of air adjacent to the surface is cooled by conduction (the transfer of heat from warmer to colder matter by contact). This causes the layer near Earth to be cooler than the air immediately above it, a condition called an inversion. If the air beneath the inversion layer is sufficiently moist and cools to its dew point, fog forms. It is primarily a nighttime occurrence, but it often begins to form in the late afternoon and may not dissipate until well after sunrise. It never forms over a water surface. Radiation fog usually covers a wide area. —Radiation fog, which generally occurs as ground fog, is caused by the radiation cooling of Earth’s surface. There are several classifications of fog: radiation fog, advection fog, upslope fog, and frontal fog.

The former results from the addition of water vapor to the air by evaporation from water surfaces, wet ground, or rain falling through the air. The latter results from the cooling of the air by contact with a cold surface underneath. Temperature and dew point may be made to coincide either by raising the dew point until it equals the temperature of by lowering the temperature to the dew point. The more water vapor present, the higher the dew point. Thus, the dew point is really an index of the amount of water vapor present in the air at a given pressure. The temperature to which air must be cooled, at a constant pressure and a constant water vapor content, in order for saturation to occur is the dew point. This is a variable, based upon the amount of water vapor present in the atmosphere.

Air drainage (gravity induced, downslope flow of relatively cold air) may be enough to prevent fog formation, or a sudden shift in the wind direction may cause fog to cover an airfield.