If you want to know, they're here. At any given time, it shows every plane in the sky, in the whole world. I'm surprised this info is available to the general public. Every day,we see planes in the sky, sometimes very high.. with or without con-trails. If you want to know about them,you can see all the information instantly...on your screen. Click on the link, FlightRadar24.com.
Some tricks: On the map you will see all the planes in the air. To view your region or town, you can zoom in by tapping with your mouse. Click on a airplane, and on the left screen you will get all the info; airline, type, speed, altitude in real time, re-calculated, every 10 seconds. On some you can even get a view from the cockpit!
Isn't technology wonderful?