Lines in The Sky

I imagine what I am about to write about this topic is accurate. An expert would be able to correct what I write, but it makes some sense to me. Los Angeles is a metropolis where the Los Angeles International Airport (LAX) is located. Overhead on any given day there is a good chance that lines are visible in the sky. Those lines are called chemtrails or contrails. Chemtrails are visible traces of chemicals in the sky that are left after airplanes pass through the sky. Contrails are visible lines that are left created by moisture in the air that condenses into visible mist as airplanes pass. From the ground I imagine one way to determine the difference between chemtrails and contrails is the amount of time that the lines remain in the sky. Chemtrails will remain longer. Contrails will disappear as quickly as the air temperature rises enough to return the water droplets to vapor.

Lines in the sky over LA

In the pictures above, I assume these lines were left behind by airplanes because there are lots of airplanes overhead Los Angeles daily. I cannot tell whether the lines are chem or con trails. I know that these lines were left by airplanes. However, in the picture below, I have no idea how the lines were created. In fact, the lines are slightly visible and intercept to form what looks luke a cross. Taken together these details are curious. The fact that I lived in the city for nearly seven months over which no airplane ever flew overhead at the elevation where those lines appear to be, makes the sight of these lines even more curious. How did they get there? Why did I see them on that particular day? Why do they intersect to form a cross? Why? Why? Why? There are so many questions. The final question is: why do the lines appear to have been created by an erasure of clouds, when the other lines appear to be drawn by chalk?