to the dew point, the air is completely saturated and moisture  begins to condense out of the air in the form of fog, dew,  frost, clouds, rain, hail, or snow. As moist, unstable air rises, clouds often form at the altitude  where temperature and dew point reach the same value. When  lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet  and the dew point temperature decreases at a rate of 1 °F  per 1,000 feet. This results in a convergence of temperature  and dew point at a rate of 4.4 °F. A pilot can determine the  height of the cloud base by applying the convergence rate  to the reported temperature and dew point in the following  manner: Ceilings and visibilities have an important role in the  classification of sky conditions, and are critical for the  definition of flight restrictions. It is necessary to define these  terms to make those distinctions clear for the balloon pilot. For aviation purposes, a ceiling is the lowest layer of clouds  reported as being broken or overcast, or the vertical visibility  into an obscuration like fog or haze.  Observations are made using the concept of the “celestial  dome,” the hemisphere of sky which can be seen from a  specific point on the ground. Cloud coverage is reported  as the total cloud cover at and below a specific layer, and  is reported in one-eighth increments (octals). A ceiling is  reported as broken when five-eighths to seven-eighths of the  sky is covered with clouds. Overcast means the entire sky is  covered with clouds. Current ceiling information is reported by the aviation routine  weather report (METAR) and automated weather stations of  various types. Ceilings are reported in height AGL. Closely related to cloud cover and reported ceilings is  visibility information. Visibility refers to the greatest  horizontal distance at which prominent objects can be  viewed with the naked eye. Visibilities reported in standard  weather reports are horizontal surface visibilities and are  generally considered linear. Predominant visibility is the  greatest horizontal distance over which objects can be seen  and identified over at least half of the horizon. In the United  States, prevailing visibilities are reported in statute miles  and portions thereof.  With an outside air temperature (OAT) of 85 °F at the surface,  and dew point at the surface of 71 °F, the spread is 14 °F.  Divide the temperature dew point spread by the convergence  rate of 4.4 °F, and multiply by 1,000 to determine the  approximate height of the cloud base. Example: This relationship is useful in determining the height  of the overlying cloud base when completing preflight  preparations. Fog is a cloud that begins within 50 feet of 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. Since prevailing visibility is used for reporting purposes,  three miles visibility does not mean that a pilot must have  one and one half miles visibility in front of and behind the  balloon, but that the predominant visibility in most quadrants  must be three miles. Current visibility is reported in METAR and other aviation  weather reports, as well as automated weather stations.  Visibility information is available during a preflight weather  briefing. The relationship between dew point and temperature defines  the concept of relative humidity. The dew point, given in  degrees, is the temperature at which the air can hold no  more moisture. When the temperature of the air is reduced  Fog is composed of minute droplets of water or ice crystals  suspended in the atmosphere with no visible downward  motion. It is one of the most common and persistent weather  hazards encountered by balloonists. Similar to stratus clouds,  the base of fog is at the Earth’s surface while the base of  4-18 Fog Temperature/Dew Point Relationship Visibility Explanation: Temperature (T) = 85 °F Dew point (DP) = 71 °F Convergence Rate (CR) = 4.4° T – DP = Temperature Dew Point Spread (TDS) TDS ÷ CR = X X  x  1,000 feet = height of cloud base AGL 85 °F – 71 °F = 14 °F 14 °F ÷ 4.4 °F = 3.18 3.18  x  1,000 = 3,180 feet AGL The height of the cloud base is 3,180 feet AGL. Ceilings and Visibilities Ceiling