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Continuing Contrail Controversy |
R. W. Stuart To examine: does a contrail derive its moisture from exhaust gases or from the atmosphere through which the airplane is flying or from both? Four meteorology authors failed to specifically answer this question, and after research I can understand why. It is a complicated problem. Contrail per author "A": "is a cloud like streamer frequently observed behind aircraft flying in clear, cold, humid air and caused by the addition (note "addition"-RWS) to the atmosphere (already containing some water vapor-RWS) of water vapor from engine exhaust gases". Contrail per author "B"- "aircraft flying at high altitudes may produce a cirrus-like trail of condensed vapor called a contrail. The condensation may come directly from the water vapor added to the air from the engine exhaust, or it may be formed by condensation of airborne water vapor by cooling resulting from reduced pressure as air flows over the wings, or it may result from nucleation around particulate matter (smoke and sulphur dioxide) produced by engine combustion, or it may form by a combined exercise of all of these phenomena. The contrail may disappear quickly or last for many minutes and those that last may stay put or drift as they are influenced by upper level winds". Contrail per author "C"- "contrails are sometimes ephemeral and dissipate as quickly as they form. Sometimes they persist and grow wide enough to cover a substantial portion of the sky with a sheet of cirrostratus. Sometimes they do not occur at all. Condensation occurs when the moisture content of the air at a particular temperature exceeds a critical amount and this condition may be attained by adding water vapor to the air from an external source or by cooling the air thus reducing the amount of water vapor it can hold- or both. Also at a given temperature air can hold slightly more water vapor over a water surface than over an ice surface". Contrail per author "D"- "A streamer cloud of water drops or ice crystals that forms in the wake of an aircraft, resulting from condensation of the moisture added to the air by the combustion of fuel". We will consider contrails over the Cleveland area laid down by commercial jets flying at 32,800 feet to 42,600 feet where the temperature ranges from -22 degrees F to -85 degrees F. This is the region of the high troposphere or low stratosphere. It takes very little water to produce a condition of saturation or supersaturation under these conditions. Moisture (vapor) at high elevations often advances through the sky in tongues or uneven patterns both large and small thus producing contrails in uneven segments of growth and dissipation. Nominal values per Bill Riley and Pat Long, the only LCRC working jet pilots that I know: "33,000 feet elevation (9900 meters) and 555 mph (480 knots) for commercial jets". Pressure at 33,000 feet altitude is approximately 350 millibars and is near the top edge of the troposphere over the Ohio latitude. Temperature decreases at a steady rate to the top of the troposphere after which we enter the stratosphere where the temperature remains more or less constant for the next 30,000 feet of elevation. A great deal of vertical mixing of the atmosphere occurs in the troposphere including the formation of clouds and weather. Little or no mixing and no clouds exist in the stratosphere. The temperature at 33,000 feet may range from -20 to -58 degrees F depending partly on surface temperature, but is always very cold. The clouds (cirrus) at the upper elevations are thin and wispy- composed of ice crystals and ice crystal precipitation. From North American Mfg. Co. combustion handbook: #2 oil properties will be used for jet fuel properties-- the two oils are very similar. The combustion products are: carbon dioxide- 3.20 lb. per lb. oil ; water- 1.12 lb. per lb. oil (extra weight results from the oxygen combined with fuel). Some sulphur dioxide will be present in combustion products, and may have later consequence in this discussion as a pollutant for nucleation of condensing water. It is estimated that a jet burns approximately 9000 pounds of jet type oil per hour. This information provided by a pilot from U S Air. 9000 lb./ hr. of fuel will produce 168 lb. water vapor per minute. Any degree of incomplete combustion will reduce the weight of water produced per minute. More than complete combustion is not possible. A nominal altitude and speed must be established in order to complete the analysis. Also the variations in water vapor dissolved in the air at a selected altitude and temperature should be considered as well as degree of possible supersaturation. Altitude will be considered only for pressure and temperature to calculate estimates of the actual properties of the contrail cloud, and must be averaged for the "typical" contrail. Humidity concerns itself with the amount of water vapor dissolved in the air. Water vapor in air is invisible until condensed at which time in becomes clouds, fog, snow, ice or rain. Warm air can hold more water vapor than cold air at a given pressure and when the amount of water vapor for a given temperature and pressure cannot be increased the air is said to be saturated. As a parcel of air is warmed at ground level it rises, cools at higher altitudes, and condenses some vapor which forms clouds. The condensing vapor releases latent heat of vaporization which causes the parcel of air to "thermal" and climb even higher, cooling, and wringing out even more condensation. This process may continue to near the top of the troposphere where very little water vapor is left in the parcel of air, but the air is still saturated for the existing pressure and temperature. Since no further cooling with increasing elevation is occurring (top of troposphere), no more clouds are formed and the vertical motion stops. The parcel of air becomes very quiet and clear, but saturated. When a jet travels through this air it causes pressure to change locally including the production of eddies. Decreasing pressure lowers the temperature causing condensation of the saturated or supersaturated air which freezes into lingering ice clouds or contrails. Vapor from the exhaust may add to this condensation and particulates from the exhaust may provide condensation nuclei to enhance the process. Latent heat from the condensation will slow the process. Some parcels of air do not produce contrails, so the exhaust moisture alone is not enough to trigger contrails in very dry air. Consider: The airplane cruises at a more or less constant altitude and speed, which means the air temperature and pressure are practically constant and obviously the engines are running. If you watch contrails once in a while you will see one that blinks- on a while and then off a while. If temp, press, and engine operation are all constant, what is blinking?- could it be variations in the water vapor and saturation conditions of the parcel of air through which the plane is flying- moment by moment? Might be! Consider: Saturday, June 22, 1996 at about 9 PM on the far west side of Cleveland an aging contrail was observed which obviously had "blinked". As the "on" sections of the trail aged more, they grew in size , became more random in shape and increased in whiteness- indicating an increase in density rather than a thinning as they grew. Finally they looked more like respectable evening clouds than a contrail. Where did this increasing moisture come from? -- the jet was long gone. Did it come from a parcel of atmosphere that was saturated or supersaturated and provided moisture for the contrail in the first place? Might be! Consider: In watching jets make their final turn low over my house on humid days, I notice little "jet trails" tailing off the edges of the spoilers and ailerons- obviously the result of turbulence at the edges of these surfaces; could it be that the water vapor and saturation conditions through which the plane is flying are being upset by the turbulence and velocity of the air as it is stressed by the control surfaces? Same as a contrail? Might be! Cloud droplets do not freeze at 32 degrees F. as expected. Pure water droplets suspended in air (so small and so rare that they are not visible) may not freeze until the temperature reaches nearly -40 degrees F. Smokey gray haze, 0.3 to 1 micron diameter droplets, results from condensation of water molecules. Under dew point conditions fog or cloud droplets are about 10 microns and become readily visible. Water in the liquid state below 32 degrees F is referred to as supercooled. Supercooled water will readily freeze if sufficiently agitated by the turbulence behind a jet airplane. When air is saturated with respect to water droplets (100% relative humidity) it is supersaturated with respect to ice crystals (110% relative humidity). Ice crystals, once started, will grow at a high rate in air that is supersaturated with respect to ice crystals and has a temperature well below freezing. As the ice crystals grow they become visible as contrails and the contrails can continue to grow after the airplane has passed. Most cirrus clouds (high, thin, whispie) are formed of ice crystals. Clouds producing halos and sun dogs are generally ice crystals. Contrails often spread out to produce streaks or sheets of cirrus clouds. Contrails may age into many different forms from an instantly vanishing smudge behind the airplane, through a long compact streak to a messy set of eddies which might eventually produce a large cirrus cloud. The contrail pattern will be determined by the moisture content of the air, the temperature of the air, the moisture introduced into the air by the combustion of the fuel, the vertical stability of the air above, at, and below the aircraft, the cloud condensation and ice nuclei count of the air, the mixing that takes place between the environmental air and the exhaust gasses and finally the turbulence and pressure changes caused by the aircraft moving through the air. An established contrail several minutes behind the aircraft, covering a one minute distance, conservatively contains at least 1868 pounds of water, but the aircraft exhaust over the same distance contains only 168 pounds of water, which means the exhaust gasses alone can not supply sufficient moisture to form and maintain a respectable contrail. The analytical method involves calculating the volume of the contrail plume over one minute of distance at the speed of the aircraft, converting to pounds of air corrected for the elevation, and then calculating the corresponding amount of water vapor at saturation and - 40 degrees F. A vcr by NASA shows the shuttle coming in through the upper troposphere with an interval of contrail- obviously no engines are running. On the same tape, video of an experimental aircraft taken from a chase plane clearly shows contrails from the wing tips. WW2 footages of airplanes taken at close range from other planes show many tip contrails. One can only conclude that atmospheric vapor provides most of the moisture which exists in contrails and engine exhaust only enhances the contrail by providing additional moisture and condensation nuclei and that contrails may be formed by turbulence alone without engine gasses. |
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