Weak flashes of light in the sky On a recent, late-evening flight from New York City to San Fransisco, I was seated on the right side of the plane. We were flying over lake Michigan - about 2 hours into the flight. When we were over the mid-west I looked out of the window and could see these continuous flashes in the sky. At first I thought it was lightning, however this continued for almost 30 minutes or so and seemed to be silent too. What is this phenomenon?
You were most likely seeing lightning (to be sure, a date/time and your location would be necessary).  Lightning can be visible from quite a distance and storms can be incredibly active with lightning within the clouds and from cloud-to-cloud.  Given a large distance and being inside a (relatively) loud airplane cabin it is no surprise that you couldn't hear the thunder associated with the lightning flashes.