Weather Theory

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