fog may exist as super-cooled liquid droplets. This situation may produce delicate needle or platelike ice crystals on exposed surfaces, known as hoarfrost. Rime ice, a smooth, milky, white ice coating, or glaze ice, a smooth coating of clear ice, may also be produced by fog when the temperature is below freezing. Fog is sometimes identified by the physical process by which it forms. Examples are radiation fog, formed by radiational cooling; advection fog, formed by moist air moving over a cooler surface; steam fog, formed when cold air moves over a warm body of water; upslope fog, caused by air cooling as it rises up a hill or mountain; and frontal fog, formed by the evaporation of rain in a colder air mass. You will study these later in preparation for AG2. To observe the presence of fog, you need not know how fog forms-only if it is present. The terms used to record fog that you, as the observer, must be familiar with are as follows: Fog—The vertical depth of fog is greater than 20 feet and the prevailing visibility is reduced to less than 5/8 mile (1,000 meters OCONUS). Mist—A  fog  condition  that  reduces  prevailing visibility to between 5/8 mile (1,000 meters) and 6 miles (9,000 meters). The vertical depth of mist is greater than 20 feet. Ground fog—This term applies to fog that has little vertical extent, i.e., normally greater than 6 feet but less than 20 feet. This is a local phenomenon, usually formed by radiational cooling of the air. Ground fog can further be described as shallow, partial, or patchy. Shallow fog—This descriptor of fog applies to ground fog that covers the station and visibility at eye level is 7 miles or more, but the apparent visibility in the fog layer is still less than 5/8 mile. Shallow fog does not extend above 6 feet. Partial fog—This descriptor of fog applies to ground fog that covers a substantial part of the station and visibility in the fog is less than 5/8 mile, and visibility over the uncovered parts of the station is 5/8 mile or more. The vertical extent ofpartial fog is greater than 6 feet but less than 20 feet. This type of ground fog may be coded even when the prevailing visibility is 7 miles or more. Patchy fog—This descriptor of fog applies to ground fog that covers portions of the station, the apparent visibilty in the fog patch or bank is less than 5/8 mile, and visibility over the uncovered portions of the station is 5/8 mile or greater. The vertical extent of patchy fog is greater than 6 feet but less than 20 feet. This type of ground fog may be coded even when the prevailing  visibility  is  7  miles  or  greater. DEW.—Dew is moisture that condenses directly on surfaces. Dew will form during the evening or late at night, usually when the winds are light. After the sun sets, the ground and objects near the ground cool by radiational cooling; they radiate heat energy as infrared radiation. When the ground or objects cool to the dew-point temperature, water vapor condenses out of the air onto the object’s surface. White dew is dew that has frozen after the water condenses. It is recognizable as small beads or a beaded layer of clear ice, or sometimes milky-colored ice on surfaces. FROST.—Frost  is  a  layer  of  milky  white  ice crystals that sublime directly on the ground or on other surfaces. The crystals are commonly in the shape of needles, scales, feathers, or fans. Frost forms when radiational cooling lowers the temperature of objects below the freezing level. Since many objects cool faster than the air surrounding the objects, frost may form with ambient air temperatures above freezing, as high as 37°F. For frost to form, the object must be cooled to the frost-point temperature, which is also referred to the "dew-point temperature with respect to ice." This is the dew-point   temperature   calculated   on   the   "low temperature" side of the Psychrometric Computer. Thicker deposits of needle or platelike frost, up to several inches thick, form in fog with ambient air temperatures below freezing. This form of frost is known as hoarfrost. Q36. Q37. Q38. Q39. Q40. Q41. Q42. Q43. Q44. REVIEW  QUESTIONS What type of lithometeor produces a yellow or orange tinge when viewed against a brighter background? Where  are  dust/sand  whirls  most  likely  to develop? What does the term “heavy sandstorm“ mean? Define sublimation. What three factors are necessary for cloud formation? Fog may form when the temperature-dewpoint spread is how many Celsius degrees? Fog formed by moist air moving over a cooler surface is known by what term? Define  the  term  "mist." Explain the formation of frost. 1-32