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    Contrails, what are they?


    What are contrails?

    They are the white trails from aircraft exhausts at the rear of the engine. They add to the local humidity of the air the aircraft is flying through which tends to super saturate the air. As the gases coming from the engine are high this helps to raise the temperature of the surrounding air, thus the air can hold more water vapour(yes it really does for the doubters!), before it becomes saturated. There are actually two opposing forces at work. (1) the water vapour in the exhaust trying to saturate the air, and (2) the hot gases of the exhaust trying to decrease the relative humidity. When an exact balance is created between the outside air temperature and the local humidity then condensation trails will occur. Called CONTRAILS and coded in meteorology as COTRA(COndensation TRAils)

    The trails may last very briefly or for quite long periods. They can have dimensions of several kilometres across and several hundred metres in depth. They spread because there is turbulence due to the aircraft moving through the air, and differences in wind speed along the path of the aircraft. Some believe there may be some input from solar heating. If they last a long time then they can tend to mask or at least decrease the sunshine striking the ground adjacent to the trails.

    Forecasters can use upper air diagrams to predict when and where they will occur. This technique was developed during the second World War when being seen was not something enemy aircraft wanted.

    As a forecast guide the following is what is used. I am unable to reproduce a t-phi diagram which has it plotted. If the actual airmass temperature is below these values at the heights given then contrails can occur.

    -24C at around sea level, in a linear line to -45C at 50,000ft. This is called the MINTRA line (MINimum TRAil line)

    So on the skew-t for Watnall (Nottingham) below for 12z today, there could be contrails, assuming its moist enough, above about 24-25,000ft.


    Unfortunately the satellite photographs from this afternoon do not show this very clearly. There are some but not good enough for teaching purposes, so I’ll wait until the two coincide.

    John Holmes

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