There are a lot of planes in the air, several thousand at any given time over the US. This animation shows all the tracked air traffic in the air at once. Each dot is a jet. Click on the image to see a 24 hour animation.
The animation is interesting since it shows there are broad flight paths that a lot of planes follow. This (along with wind) explains the parallel trails, and the grids.
If you have Google Earth installed, then click here for near-real time flight tracking of ALL the tracked flights in the US. It looks like this (click for a closer look, there’s more than there initially seems):
Given all these jets flying along regular airways, it’s hardly surprising that when the weather is right, that parallel persistent contrails will show up and then grids where the paths cross. In fact it’s surprising we don’t see more!
And here are some more Flight Pattern Visualizations. Pretty stuff.
See FBOWeb for more info on flight tracking: