Surface obscuration caused by fog has troubled navigators on land, sea and in the air ever since travel began – and it is still a serious cause of aircraft accidents. Fog appears to be a non-threatening form of cloud, without turbulence or precipitation (usually), but because it is in contact with the ground surface it seals off ground facilities and mountain passes with little or no warning. Fog dense enough to restrict visibility to a mile or less can form quite rapidly. All fog tends to look alike to pilots, but it is formed and dissipated under a variety of conditions with which airmen should be familiar. Radiation Fog More commonly called ground fog, radiation fog is the result of ground cooling on clear, calm nights. The ground cools the air to the dew point temperature. It is restricted to land areas because water areas do not have much daily variation in temperature. It forms almost exclusively late at night or in the early morning, and usually disappears or burns off within a few hours. Ground fog favors flat land areas – such as airports. Advection Fog This type of fog forms when moist air moves over colder ground or water. Very common along coastal areas, it is called sea fog when occurring at sea. It frequently forms offshore as a result of very cold water from the ocean depths rising to the surface. The fog is then carried inland by the wind. Advection fog results from moist air moving over a cold area. Therefore, in most areas it is more common in the winter than in the summer. Upslope Fog This fog results from moist, stable air being cooled by being moved up a sloping land surface. An upslope wind is necessary for its creation and its continued existence. Steam Fog The movement of cold air over much warmer water causes intense evaporation. This usually adds enough water vapor to the cold air to saturate it, forming steam fog. Steam fog rises from the water surface like smoke, and it is sometimes referred to as sea smoke. Since there is heating from below, turbulence often occurs in this type of fog. Structural icing is also a potential hazard. Precipitation-Induced Fog The addition of moisture to the air through evaporation of rain causes this type of fog. Evaporation can occur while the precipitation is falling through the air, or after it reaches the ground. It is frequently associated with warm fronts.5. Which of the following is true according to the passage?