Now many news agencys are reporting that the mexican ufo sighting may
very well be ball lightning.  This should not disapoint those that
truly want to believe.  Ball lightning has been linked to UFOs now for
a very long time.  Ball lightning is also linked to spirit lights and
other ghostly phenomina.  Many do believe it often to exhibit signs of
intelligence, and its behavior and constitution are totally
unexplained to modern science.  You may also be interested in the
govornment project known as HAARP among others of its kind, which deal
exclusively with working in the upper part of our atmosphere called
the ionosphere, that is almost exclusively composed of the same
properties that ball lightning is usually thought to be composed of. 
There are many other barely understood and unusual natural phenomina
in the atmosphere,  Like:

    * Afterglow
    * Airglow
    * Alexander's band
    * Alpenglow
    * anticrepuscular rays
    * Auroral light 
    * crepuscular rays
    * Elves 
    * Glories
    * the Green ray
    * Halos, of Sun or Moon
    * Rainbows
    * Sprites 
    * Sun dogs 
    * Fata Morgana
    * Ball Lightning

Many times aliens from another world, may perhaps just be long
misunderstood friends we already have on earth.