Professional Pilot, March 2016

Image from the International Space Station of a contrail produced by a Russian missile test in October 2013 The massive quantity of water vapor in the exhaust of a rocket is generally sufficient to saturate air at any altitude producing vertical contrails that may extend to the edge of space and can clearly show wind patterns with altitude though discerning a vertical from a horizontal contrail at distance can be tricky PROFESSIONAL PILOT March 2016 91 Contrail formation US Army Captain Ward Wells serving in the Argonne in the fall of 1918 first reported Our attention was drawn to the sky by the sudden appearance of several strange and startling clouds long graceful looping ribbons of white On close observation we noticed some distance ahead of each cloud point the tiny speck of an airplane An article appearing in the Monthly Weather Review just 2 years later confirmed that these new type of clouds were being produced by the condensation of aircraft exhaust into cold and nearly saturated air As clouds contrails have even been given their own Latin taxonomy in cloud classification Cirrus aviaticus Because gasoline or petrol is a hydrocarbon that is combined with oxygen during the combustion process the primary byproducts are water H 2 O and carbon dioxide CO 2 Combusting a liter of gasoline in an aircraft engine produces roughly a liter of water vapor But within the normal range of exhaust temperatures the exhausted air is more than capable of holding all of that water as vapor Once the water vapor rich exhaust is expelled into the surrounding atmosphere it begins to cool as heat is drawn from it And if the exhaust column held together as it cooled it is likely that a contrail would form and persist for a while However most of the time the exhaust quickly mixes into the surrounding air due to the turbulent flow generated both by the velocity with which the exhaust is expelled and by the wake of the aircraft itself Under most circumstances this mixing is sufficient to dissipate the water vapor into the surrounding air And since a cruising aircraft can cover several km in the process of burning that liter of fuel the amount of water vapor pushed into the atmosphere at any given point is very low perhaps only a few grams This means that contrails are really a relatively rare occurrence Humidity control The control on contrail formation and persistence is the humidity of the atmosphere into which the exhaust is expelled There are 2 humidity factors that are important 1 The amount of water vapor that the air could hold without having to condense some of the water This capacity humidity is commonly measured in grams of water vapor per kg of dry air and its a function of the temperature of the air The colder the temperature the less moisture the air can hold At 20 C air can hold up to 15 g of water per kg of dry air At 20 C it can hold less than 1 g If we assume that an aircraft exhaust might pump 05 g of water vapor into a kg of 20 C dry air it would not be enough to force condensation at that temperature Given the speed and fuel efficiency of modern jets a rough estimate is that most contrails will form in air that is below around 35 C However a modifier is how much water vapor is already in the air 2 The amount of water vapor that is actually in the air The airs dew point can be used to estimate this value in g kg This measure subtracted from the capacity humidity tells you how much water vapor youd need to add to the air to saturate it and start net condensation If theres enough water vapor being produced by the engine exhaust as it passes through that kg of air a contrail will form A look at most atmospheric soundings reveals that naturally occurring water vapor tends to be concentrated near the surface where relatively warm temperatures keep the mixing ratio of actual to capacity humidity low But some moisture does mix into higher levels often through thunderstorms or orographic flow At these cold temperatures aloft just a little bit of moisture 01 or 02 g can be enough to saturate the air into cirrus clouds Contrails can also form due to a pressure drop behind the aircraft often aft of the wing tips although these types of contrail tend to be minor and ephemeral Because pressure is directly related to temperature when one drops so does the other This drop in pressure temperature in the air behind the aircraft lowers the capacity humidity of the air occasionally to the point where it equals the actual humidity and moisture is condensed These sort of contrails are most commonly seen as wake vortices but also occur as vapor streamers from propeller tips or extended flaps and contrary to exhaust contrails are normally encountered at low altitudes in near saturated air Chemtrails Conspiracy theorists argue that contrails are actually a deliberate and sinister effort by governments and industries to inject toxins into the atmosphere in the form of so called chemtrails While combustion of petroleum does indeed produce a number of toxic compounds they represent a minor proportion of the exhaust and are generally well dispersed by the atmosphere But importantly many of them aid in the formation of contrails Among the byproducts of combustion are sulfur and carbon particulates that Well defined contrails of 2 USAF F15s as they escort 2 Soviet Mikoyan Mig29s enroute to the Abbotsford Airshow While visually interesting contrails easily give away an aircrafts position Photo courtesy NASA