Dont forget...wether or not a contrail will or will not be made is strictly dependant on a few factors. One of the most important factors is temperature. When you are standing on th eground in 40 degree weather...and you look up at the vast sky...you basically assume that the temperature is a uniform 40 dgrees EVERYWHERE you look. But that is far from the case. Temps and pressures are VERY fluid. Havent you even been outside only to notice a breeze roll in and the temp drop 5 degrees in a several minutes? The sky is NOT a uniform mass of temperature and pressure. it varies greatly even in the areas visible to you. Many times when you are looking at a contrail forming in real time...the aircraft and contrail are 50-75-100-200+ miles away and traveling a mile every 8-10 seconds or so. So if you see an airplane in the distance that has a consistant contrail...and then for 20-30 seconds does not have a contrail...and then for 20-30 seconds does again....all you need is for the temps, or humidity to change in those few mile stretches by a VERY SMALL margin for this to happen. 1 degree of temp is enough. Its basically the same as dew point.

Have you ever gone outside early in the morning to see dew on grass....but not on trees. Or maybe dew on the roof of your car but no where else? Those are the same principle. Slight variations in temps and humidity cause different effects in different areas of close proximity. It is actually to be expected.

So contrails turning "off" or "on" is really just the aircraft flying through non uniform patches of temperature and humidity. No biggie really.

Same goes for two aircraft that appear to be in close proximity....one with contrail and one without. They could easily be 1000 feet apart in altitude and you would barely be able to tell. I can tell you that in my 6 years in the Marines on helos I got pretty good at calling out enemy aircraft distances and altitudes. But I would never say that I could determine position at 20 miles within 1000 feet. And 1000 feet is PLENTY of distance to be in contrail weather or be out. Heck..a difference of 3-4 feet is all thats needed for dew on the lawn/car example.

Here is something that may surprise you. Next time it gets cold...like tonight or tommorrow night...and if you have a good laser thermometer....take it outside at night when its cold and temp is really dropping and take measurements of various things at various heights off the ground. Car included. Measure tires, bumper hood and roof. Measure grass vs trees/leaves. You will be very very surprised to see what is cold and what is not as cold and how much the variance is. I did this during the last cold snap to check potential plant damage. Its amazing that the grass may be 32....but just a few feet away things may dip to 25. And then at 10 feet temps could be 40. Its all over the map. Granted...as temps are changing...that rate of change of the ambient air is affected by things with mass such as...THE GROUND...lol. Or a car or a large tree...but its just goes to show you that temps are rarely uniform.....even though when we say its 40 dgrees outside...we tend to think everything is 40 degrees.

Ever notice plant damage from freezing temps rarely seems uniform? One plant dies while one (of the same species) in a different location survives just fine. Its typically because the temps were actually different for the two plants..even though they may have been yards apart or had a different altitude of 5 or 6 feet.