Originally posted by dotgov101
Well, simce contrails are made of the same thing that clouds are, (i.e. ice crystals) then why don't con'trails behave the same way as
clouds? Clouds don't dissapear after a few minutes, why should contrails?
Perhaps it's a reaction with the burning jet fuel. Heat melts ice.
Oh, my. Where do I start.
First, let’s clarify exactly how and why contrails form.
To begin with, contrails formation is not limited to just jet engine exhaust, contrails were commonly observed during WWII being produced by both
bombers and fighters.
However, since the era of the 24 cylinder piston engine aircraft has gone the way of the steam powered trains, we will stick to the jet engines.
You are right in one respect, jet engines burn jet fuel. Jet fuel is basically a form of kerosene, you can even think of it as a clean version of
When jet fuel burns, three things happen. It produces CO2 gas, It produces H2O vapor, It produces heat (it also produces a trace amount of soot and
other byproducts from the combustion of various fuel additives, more on this later).
The heat of the combustion raises the pressure of the gas (CO2 and H20) which is basically squirted out the back end of the jet engine providing
thrust for the plane. Once that exhaust gas exists the bag of the engine, it begins to expand since it is no longer constrained by the sides of the
engine. When you expand any gas in such a manner, thermodynamic laws state that the temperature of the gas has to go down, i.e. it cools off.
Furthermore, the exhaust gas mixes with the ambient outside air as it expands. Since the outside air temperature at 30,000 feet is typically about 47
degrees F below
zero, it loses heat fairly quickly. Thus, the temperature of the exhaust gas quickly drops to well below freezing.
Now, at this point, there is one overriding factor that determines if contrails form at all, if they are short, medium, if they persist for hours, or
even if they expand into an overcast cloud cover.
That is the relative humidity
, or more specifically, the relative
humidity with respect to ice (RHi).
If the RHi is low, the contrails will be short or the may not even form at all. If the RHi is around, 100%, the contrails will persist for awhile,
but will eventually dissipate. If the RHi is over 100%, the contrails will persist. If the RHi is in the 150% range, the atmosphere is
supersaturated with water vapor and the contrails will be very persistent and they may even expand into cirrus clouds.
This process is aided by the fact that no combustion process is 100% effective and that the exhaust gas also contains traces of soot particles which
provide nuclei for ice formation
Clouds form based on the same criteria: the RHi, and the present or absence of nucleation particles.
Note that if you see two planes in the sky at one time and one is producing a short, dissipating contrail, and the other a big, persistent contrail,
it means that the planes are flying at different altitudes.