Why do Contrails (cirrus aviaticus) form into milky opaque sheets (cirrostratus aviaticus)?

Never in history have we had so many airline flights.  37,000,000 scheduled flights last year and who knows how many unscheduled?  Aircraft manufactures are swamped with orders.  Contrails (aviaticus clouds) visible from the ground are common but only represent a fraction of the invisible aerosol trails (aerotrail) left by all combustion aircraft composed of a mix of small nuclei of combustion residue called Cloud Condensation Nuclei (CCN) .  If we could somehow see each aerotrail  as we can contrails our entire view of the sky would change especially along heavily used flyways.  There we would see hundreds of persistent spreading trails and multiple layers of milky sheets everyday.

But we only see high altitude persistent contrails and milky sheets at certain times when meteorological conditions are right.

Imagine a full blue out day along a flyway, you have the aircraft but not a whiff or needle of a contrail is to be seen and it is difficult to spot the passing aircraft that you can hear.  At this time they are all laying a wide persistent aerotrail and if they were visible the sky would already be heavily streaked with other trails that had drifted into your area from flights outside your viewing area.  But back to what you are actually seeing. Later,  as the day progressed the high altitude  airliners begin  sporting short non-persistent trails and several hours later longer trails. By after noon you see segments of persistent contrails in the far horizon starting to drift into you area.  Several hours later they are closer and still intact and now some of the airliners in your flyway begin to spew thick persistent trails and even the approaching aviaticus clouds now look like natural cirrus in a lighter blue, whitish sky.  Shortly these milky clouds are covering your flyway and still the passing jets are laying thick persistent trails. From the ground view at a fixed point it appears like these aircraft are deliberately spraying some type of chemical to cause this change.  But you track the aircraft and they are regularly scheduled flights from all over the country (world in some cases) so what is going on? 

What you have observed is a high altitude boundary line passing over your area.  On one side of the line the upper atmosphere was relatively warm and dry and or the other side of the boundary line it is colder and wetter.  But it is not really a line, that is a thin line, but the line is more like a wide sheet.  Imagine  the a line in the sky that is 10-20 miles wide so it may be better to call this a boundary sheet.  This boundary sheet was first visible to you when you began to see the contrail segments drifting toward you from a far horizon.  As the leading edge of this sheet began to approach your area the atmospheric conditions began to slowly change from dry/warm to moist/cooler and the contrails in your flyway began appear and then become longer.  The closer this boundary sheet came the longer your trails form. At the same time that boundary sheet is being intersected with more and more aerotrails that are forming more visible spreading persistent contrails and that section of sky is taking on a lighter shade of blue and becoming increasingly milky white.  The moist air coming into contact with the dry air causes natural cirrus clouds to form into cirrostratus clouds and the same process is taking place with the cirrus aviaticus clouds.  By the time the boundary sheet reaches your overhead flyway the sky is already milky white.  Additional aircraft passed through this sheet during its slow progress to your area while it was reforming into a stratus from contact with the warmer air.   Once the sheet passes over and out of your area it may have been followed by natural thicker clouds as the original boundary sheet was the atmosphere that precede other natural cloud formations.  Or it may be followed by blue skies and a shrinking or disappearance of the contrails.  I have viewed many of the former but here is an example of the latter:

At the time of the event this is the a screen shot of the infrared loop: 

A screen shot of sky cover loop shows no cloud cover: (From 

From the moisture loop a swath of dry air is swiping across Wisconsin from the Southwest: Map from NOAA site

As the boundary line (sheet) moves in to my Central Wisconsin flyway, contrails begin to lengthen and segments of persistent trails can be seen in the the southwest.

As the boundary sheet approaches more jet traffic add additional exhaust nuclei to the sheet and the milky skies form. Photo of approaching moist air:

When the sheet passed between me and the sun I thought it was going to be thick enough to form a sun dog but it did not and after the sheet passed, an hour or two the boundary sheet and original persistent contrails could still be seen probably past Green Bay and over Lake Michigan.  At this time the contrails in my viewing flyway had resumed to whiffs and needles in a full blown blue out of sky.

Afternoon Forecast:  Since dawn we have had clear skies with not a whiff of one trail appearing along the Flyway.  By 2PM CT we could have some contrail forming,  We do have a nice swath of upper atmosphere dry air right along the Flyway but it appears that will push Northeast and the blue skies could be replaced by an wide swath of upper clouds trailing alongside a approaching heavier cloud mass.  I would expect at minimum lengthening contrails as this brushes overhead and some accumulating persistent vestiges that may form higher milky cirrostratus aviaticus.

Update: By 2:00 PM CT those edge clouds came rolling through as cirrocumulus, lower altitude than the cirrus and only one short trail was observed.  By 4 PM a few persistent segments drifted into the flyway and by 6 PM several long lasting trails (5-10 minutes) were laid overhead afterwards it reverted back to needles and whiffs through sunset.

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