In the simplest of terms, fog is a cloud at the earth's surface. Technically, fog is a suspension of small water droplets in the air, reducing horizontal visibility at the earth's surface. Fog forms when the air near the ground becomes saturated and condensation occurs on tiny particles suspended in the air. Fog forms in stable air with light winds, high relative humidity and conditions which bring the air to its dew point. The resulting layer of fog is normally only a few hundred feet thick.
Radiation fog is common over land in late fall and winter.
Advection Fog forms when wind moves warm, moist air over a cooler surface. The cold surface cools the warmer air and lowers it to its dew point. Condensation occurs, resulting in the formation of a layer of fog. The layer of fog will deepen as wind speed increases up to about 15 knots. With winds stronger than 15 kts the fog will lift as stratus cloud.