Dew and Frost  Fog  Clouds  12-15  Figure 12-21.  Radiation fog.  Fog is a cloud that is on the surface. It typically occurs when  the temperature of air near the ground is cooled to the air’s  dew point. At this point, water vapor in the air condenses and  becomes visible in the form of fog. Fog is classified according  to the manner in which it forms and is dependent upon the  current temperature and the amount of water vapor in the air.  On clear nights, with relatively little to no wind present,  radiation fog may develop.  [Figure 12-21]  Usually, it forms  in low-lying areas like mountain valleys. This type of fog  occurs when the ground cools rapidly due to terrestrial  radiation, and the surrounding air temperature reaches its  dew point. As the sun rises and the temperature increases,  radiation fog lifts and eventually burns off. Any increase in  wind also speeds the dissipation of radiation fog. If radiation  fog is less than 20 feet thick, it is known as ground fog.  When a layer of warm, moist air moves over a cold surface,  advection fog is likely to occur. Unlike radiation fog, wind  is required to form advection fog. Winds of up to 15 knots  Low clouds are those that form near the Earth’s surface and  extend up to about 6,500 feet AGL. They are made primarily  of water droplets but can include supercooled water droplets  that induce hazardous aircraft icing. Typical low clouds  are stratus, stratocumulus, and nimbostratus. Fog is also  classified as a type of low cloud formation. Clouds in this  family create low ceilings, hamper visibility, and can change  rapidly. Because of this, they influence flight planning and  can make visual flight rules (VFR) flight impossible.  Cloud type is determined by its height, shape, and  characteristics. They are classified according to the height of  their bases as low, middle, or high clouds, as well as clouds  with vertical development.  [Figure 12-22]  Clouds are visible indicators and are often indicative of  future weather. For clouds to form, there must be adequate  water vapor and condensation nuclei, as well as a method by  which the air can be cooled. When the air cools and reaches  its saturation point, the invisible water vapor changes into  a visible state. Through the processes of deposition (also  referred to as sublimation) and condensation, moisture  condenses or sublimates onto miniscule particles of matter  like dust, salt, and smoke known as condensation nuclei. The  nuclei are important because they provide a means for the  moisture to change from one state to another.  Ice fog occurs in cold weather when the temperature is  much below freezing and water vapor forms directly into ice  crystals. Conditions favorable for its formation are the same  as for radiation fog except for cold temperature, usually –25  °F or colder. It occurs mostly in the arctic regions but is not  unknown in middle latitudes during the cold season.  Steam fog, or sea smoke, forms when cold, dry air moves over  warm water. As the water evaporates, it rises and resembles  smoke. This type of fog is common over bodies of water  during the coldest times of the year. Low-level turbulence  and icing are commonly associated with steam fog.  Upslope fog occurs when moist, stable air is forced up sloping  land features like a mountain range. This type of fog also  requires wind for formation and continued existence. Upslope  and advection fog, unlike radiation fog, may not burn off with  the morning sun but instead can persist for days. They can  also extend to greater heights than radiation fog.  allow the fog to form and intensify; above a speed of 15 knots,  the fog usually lifts and forms low stratus clouds. Advection  fog is common in coastal areas where sea breezes can blow  the air over cooler landmasses.  On cool, clear, calm nights, the temperature of the ground  and objects on the surface can cause temperatures of the  surrounding air to drop below the dew point. When this  occurs, the moisture in the air condenses and deposits itself on  the ground, buildings, and other objects like cars and aircraft.  This moisture is known as dew and sometimes can be seen  on grass and other objects in the morning. If the temperature  is below freezing, the moisture is deposited in the form of  frost. While dew poses no threat to an aircraft, frost poses a  definite flight safety hazard. Frost disrupts the flow of air over  the wing and can drastically reduce the production of lift. It  also increases drag, which when combined with lowered lift  production, can adversely affect the ability to take off. An  aircraft must be thoroughly cleaned and free of frost prior  to beginning a flight. 