contrails versus altitude versus temperature
08-07-2022, 03:54 PM
#1
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Joined APC: Jan 2016
Position: usually middle seat on 737
Posts: 44
contrails versus altitude versus temperature
...living beneath a common track for US to EU flights, my eldest flies for a major carrier so I simply enjoy identifying what's flying over (thanks Flightaware). I'm curious that a flight from DFW to FCO just passed over at FL370...but there was zero contrail. GOG (Good 'Ol Google) says it requires -40F for contrails to show. So I wonder if air temps at higher FL's are happening...it's pretty warm here this week!
08-08-2022, 08:38 AM
#3
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,019
There is nothing magic that happens at -40 with respect to the formation of condensation trails. Formation dependson several factors, and there are several forms which condensation trails (contrails) take or b which they are created. Relative humidity of the surrounding atmosphere is part of the factor, as well as outside air temperature. The internet is full of wild ideas about contrails, from the temperatures needed to form to irrelevant ideas about ram temperature rise, etc. The notion of -40 being any kind of pre-requisite, is equally nonsense.
-40 is generally considered the dividing line where all moisture in the atmosphere is frozen; water and precipitation no longer exists in liquid form below that temperature, in theory (For some time, that temperature was believed higher; I happen to know the individual who recorded water at -40, initially, in atmospheric research). What we knew yesterday, we know tomorrow, but differently, and so it goes.
I did for seeding in a former life; in low visibility conditions, we'd dispense powdered dry ice while flying at low altitude down a runway, preceding the arrival of a commercial aircraft on an instrument approach; this was done in low visibility conditions when the visibility was too low to legally accept the instrument approach. Visibility came up following our pass, allowing following aircraft to get in. This worked under specific conditions and was one of several methods in use; it was a type of "distrail," or dissipation trail, in this case an artificial introduction of condensation nuclei, or something for visible moisture to coalesce on and fall out of the air. Think of it as a lint roller for freezing fog.
Sometimes when we'd launch, flying twin piston airplanes, it was -15 degrees and crystal clear; the exhaust trail behind the airplane would create a fog that acted as a catalyst, and spread such that on the takeoff roll, a condensation trail formed behind the airplane, and slowly spread outward until the entire field was solid instrument conditions.
The first observed condensation trails formed behind piston airplanes at lower altitudes during the first world war. They were a common site by the time of the second world war, happening behind piston aircraft at altitudes much lower than 37,000' and temperatures much warmer than -40. In a standard atmosphere, -40 is reached by 28,000' but we know that standard seldom exists; it's often warmer, sometimes colder than standard, and given that -40 is not a magic number for contrail formation, it happens at lower altitudes, and often doesn't happen or happens intermittently at higher altitudes.
A lot of time and effort has gone into researching preventing the formation of both contrails and distrails, primarily with respect to military applications, as both provide clues to the presence of aircraft, visually. Contrail formation isn't always predictable, and it's very difficult to eliminate. On long flights behind other aircraft, we frequently see contrails stop and start as the temperature of relative humidity of a parcel of air changes (though relative humidity at high altitudes plays a minor role in contrail formation; temperature plays a more significant role.
Distrails can sometimes be seen in which the passage of aircraft through a cloud causes the cloud to disappear or appear "cut;" the cloud formation is dissipated by the aircraft passage.
For temperatures to be above -40 at 37,000 would be unusual.
Condensation trails form at warmer temperatures at lower altitudes, and are also a function of air density.
-40 is generally considered the dividing line where all moisture in the atmosphere is frozen; water and precipitation no longer exists in liquid form below that temperature, in theory (For some time, that temperature was believed higher; I happen to know the individual who recorded water at -40, initially, in atmospheric research). What we knew yesterday, we know tomorrow, but differently, and so it goes.
I did for seeding in a former life; in low visibility conditions, we'd dispense powdered dry ice while flying at low altitude down a runway, preceding the arrival of a commercial aircraft on an instrument approach; this was done in low visibility conditions when the visibility was too low to legally accept the instrument approach. Visibility came up following our pass, allowing following aircraft to get in. This worked under specific conditions and was one of several methods in use; it was a type of "distrail," or dissipation trail, in this case an artificial introduction of condensation nuclei, or something for visible moisture to coalesce on and fall out of the air. Think of it as a lint roller for freezing fog.
Sometimes when we'd launch, flying twin piston airplanes, it was -15 degrees and crystal clear; the exhaust trail behind the airplane would create a fog that acted as a catalyst, and spread such that on the takeoff roll, a condensation trail formed behind the airplane, and slowly spread outward until the entire field was solid instrument conditions.
The first observed condensation trails formed behind piston airplanes at lower altitudes during the first world war. They were a common site by the time of the second world war, happening behind piston aircraft at altitudes much lower than 37,000' and temperatures much warmer than -40. In a standard atmosphere, -40 is reached by 28,000' but we know that standard seldom exists; it's often warmer, sometimes colder than standard, and given that -40 is not a magic number for contrail formation, it happens at lower altitudes, and often doesn't happen or happens intermittently at higher altitudes.
A lot of time and effort has gone into researching preventing the formation of both contrails and distrails, primarily with respect to military applications, as both provide clues to the presence of aircraft, visually. Contrail formation isn't always predictable, and it's very difficult to eliminate. On long flights behind other aircraft, we frequently see contrails stop and start as the temperature of relative humidity of a parcel of air changes (though relative humidity at high altitudes plays a minor role in contrail formation; temperature plays a more significant role.
Distrails can sometimes be seen in which the passage of aircraft through a cloud causes the cloud to disappear or appear "cut;" the cloud formation is dissipated by the aircraft passage.
For temperatures to be above -40 at 37,000 would be unusual.
Condensation trails form at warmer temperatures at lower altitudes, and are also a function of air density.
08-10-2022, 06:06 AM
#5
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,019
Temperature is perhaps the primary factor, but there are many others; -40 (and a few other numbers in s similar range (eg, -35, -37, etc) are sometimes thrown out as being the point after which (below which) ice formation will occur without condensation nuclei or surface tension disruption on the water droplets, but that's over-simplified. Were it not so, the government(s) wouldn't have spent ridiculous sums of money over decades attempting to eliminate tell-tale contrails.
The oversimplification stem from the principle that all water vapor below a certain point freezes, indicating that regardless of other factors, the condensation trail should form at or below that absolute value....most hold that at -40 and below, all water moisture is at or below freezing in the atmosphere, which isn't necessarily true, nor does it necessarily mean a visible contrail will form. One of the byproducts of combustion of fuel in an engine is water vapor, and in high concentrations in the exhaust stream, it can be visible; in many cases, rapidly freezing and forming a visible contrail. The key isn't necessarily ice formation, as much as being visible. There are two types (broadly speaking) of condensation trails; we frequently see the visible formation of a trail around propellers, wingtips, etc, in high humidity, where air pressure drops enough to cause condensation; this typically dissipates or evaporates shortly after it forms and doesn't remain visible; it is also a form of condensation trail, and often takes place in temperatures well above freezing.
Fuel doesn't burn cleanly; it has material which is also expelled into the atmosphere as smoke or very small particles which form the nucleus of condensation drops, which can start the condensation process; water vapor coalesces around these small particles, aiding in condensation and increasing droplet size; these are called "condensation nuclei." In freezing conditions, the ability of a water droplet to remain supercooled, or liquid at a temperature less than freezing, depends on the chemical composition of the water, as well as the surface tension of a droplet that forms; anything that breaks that surface tension or changes the freezing point of the water droplet will affect the temperature at which the droplet turns to an ice crystal (often making it visible, and reflective). The concept of all droplets freezing and no longer remaining supercooled at or below a certain temperature is why -40 is often thrown out as the threshold for contrail formation, but again, that's not a correct assertion, and contrails frequently do not form at colder temperatures, and frequently form at warmer temperatures, too.
The oversimplification stem from the principle that all water vapor below a certain point freezes, indicating that regardless of other factors, the condensation trail should form at or below that absolute value....most hold that at -40 and below, all water moisture is at or below freezing in the atmosphere, which isn't necessarily true, nor does it necessarily mean a visible contrail will form. One of the byproducts of combustion of fuel in an engine is water vapor, and in high concentrations in the exhaust stream, it can be visible; in many cases, rapidly freezing and forming a visible contrail. The key isn't necessarily ice formation, as much as being visible. There are two types (broadly speaking) of condensation trails; we frequently see the visible formation of a trail around propellers, wingtips, etc, in high humidity, where air pressure drops enough to cause condensation; this typically dissipates or evaporates shortly after it forms and doesn't remain visible; it is also a form of condensation trail, and often takes place in temperatures well above freezing.
Fuel doesn't burn cleanly; it has material which is also expelled into the atmosphere as smoke or very small particles which form the nucleus of condensation drops, which can start the condensation process; water vapor coalesces around these small particles, aiding in condensation and increasing droplet size; these are called "condensation nuclei." In freezing conditions, the ability of a water droplet to remain supercooled, or liquid at a temperature less than freezing, depends on the chemical composition of the water, as well as the surface tension of a droplet that forms; anything that breaks that surface tension or changes the freezing point of the water droplet will affect the temperature at which the droplet turns to an ice crystal (often making it visible, and reflective). The concept of all droplets freezing and no longer remaining supercooled at or below a certain temperature is why -40 is often thrown out as the threshold for contrail formation, but again, that's not a correct assertion, and contrails frequently do not form at colder temperatures, and frequently form at warmer temperatures, too.
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