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To pilots, the cumulonimbus cloud is perhaps the most dangerous cloud type. It appears individually or in groups and is known as either an air mass or orographic thunderstorm. Heating of the air near the Earth’s surface creates an air mass thunderstorm; the upslope motion of air in the mountainous regions causes orographic thunderstorms. Cumulonimbus clouds that form in a continuous line are nonfrontal bands of thunderstorms or squall lines.
Knowledge of principal cloud types and the factors that affect them helps the pilot visualize expected weather conditions, and to recognize potential weather hazards.
4-18
Ceilings and Visibilities
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.
Ceiling
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.
Visibility
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.
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.Temperature/Dew Point Relationship
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 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:
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
Example:
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.
Explanation:
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.
This relationship is useful in determining the height of the overlying cloud base when completing preflight preparations.Fog
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.
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
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Balloon Flying Handbook(49)
