As a teenager we would park on the road just off the end of the runways at the old John Connally AFB out side of Waco. I don't remember what airline, but nightly we would watch 707s doing touch and goes. I mean the road was just off the end of the runway. If we went through the 4 string barbed wire fence we would be on the warning track. Usually about 1.5 to 2 minutes after the plane departed, we would hear a slight whistling noise and the tree tops would just start whipping around. We never felt a bit of air movement right under the path of the jets. I mean seriously whipping around. We always thought it was from the jet engines when applying power for the go part of the T&G. Later when I started flight training I realized it was the vortices coming off the wings as the plane flew over. Really impressive strength.