The three factors are layers of the atmosphere with different density, humidity and wind speed.<br><br>The different density will cause the layers to remain intact and not mix with each other much. The different wind speeds (in this case higher speeds with height) will cause a wave to form because of friction. If the layers have just the right amount of moisture, then that moisture will condense as it is drawn into the wave and thus we can see it. There are also waves in the atmosphere that do not have any condensation (or clouds) in them and thus are not seen. This is what causes clear air turbulence for aircraft. <br><br><br>