Nowadays this is encoded “0SM FG VV000.”įrom the 1920s to the 1950s, weather observers released “pilot balloons” to estimate ceiling height. adopted the METAR format, this essentially meant an obscured zero vertical visibility (ceiling) with prevailing visibility zero miles in fog-the worst it got. You may have seen or heard the term “W0X0F”, pronounced wawks-off. When the sky is blocked by obscurations like fog, snow, or rain, we simply measure the vertical visibility, beyond which a ceilometer beam is extinguished. Always make sure you know which height is being reported.Ī special form of ceiling is vertical visibility. The exceptions are in area forecast products, any cloud height reported as MSL, or PIREPs where clouds are encoded as given by the pilot and can be either (usually MSL). Remember, ceilings and values marked CIG are always reported in height AGL. The stratocumulus in this example is considered to cover 5/8 of the sky, even though we can’t observe it all. This is defined by the base (the undersurface) of each cloud layer.īut, this is a cumulative measurement that includes all lower layers, so if the sky is half covered with stratus at 500 feet, and a higher 1/8 layer of stratocumulus can be observed at 2000 feet, we consider the ceiling to be 2000 feet. As we’ve discussed previously, automated stations like ASOS estimate this figure with a spot check of a one-meter volume of air at the sensors, but in much of the world prevailing visibility is taken the old-fashioned way: looking all around at the horizon.Ĭeiling is the lowest height, above ground level, at which a cloud layer covers more than half of the sky. Prevailing visibility, which is the primary visibility measurement we see in METAR reports, is the maximum visibility figure valid for half of all directions around an observer. Visibility is the maximum distance in a specific direction at which objects can be seen and identified-an important distinction just seeing objects isn’t good enough. In the United States, the bible is the Federal Meteorological Handbook #1, published by the Federal Coordinator of Meteorology. Surprisingly, the definitions haven’t changed much since those early days. This increase led to widespread standardization of ceiling and visibility measurements. Instrument flight grew rapidly as a practical reality. And true instrument flying arrived in the mid-1930s with the low-frequency radio range (LFR) system, promoted by Jimmy Doolittle in 1929. By the 1930s the airlines all had their own meteorology departments. This sparked massive advances in weather observation, data dissemination, and forecasting. This system wasn’t good enough for the emerging airline industry, whose airplanes flew at higher altitudes across varied routes. If the pilot felt things were deteriorating, he could be on the ground in less than 10 minutes. Weather could be avoided because the planes flew at low altitudes, and illuminated diversion airfields were placed at frequent intervals. Simply by following the route from beacon to beacon, a pilot could arrive safely at the destination. These facilities, operated by the federal government, used lamps powered by cylinders of acetylene.
These early pilots depended on a network of rotating beacon towers at four-mile intervals, like lighthouses in the sky.
Early in the 1920s it was calculated that a pilot’s odds of dying were 1:1 after completing 100,000 cross-country miles. This put aviators in dangerous new territory to meet the deadlines. And a schedule meant that encounters with bad weather were inevitable. The nature of the business required that airplanes be flown at night for a New York package to be delivered to Chicago the next day. Air Mail service grabbed national headlines and promised to be one of the biggest advances since the Pony Express.
Instrument flying got its start in the 1920s. Pilots stuck to clear air and avoided clouds where possible. It was a simpler time when the Red Baron appeared in the French skies during World War I. These are the main parameters that determine whether or not we’re flying in instrument conditions, and if we’re unprepared, low ceilings and visibility can get us into serious trouble. This article is being prepared in the middle of the winter, and there’s probably no better time to talk about ceiling and visibility.
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