Complex sentence: When flying at night ash clouds are not visible to pilots, in addition they do not show up on radar, the small size of the ash particles do not return an echo to the onboard weather radars of commercial airliners.
Split into simpler sentences: When flying at night ash clouds are not visible to pilots. In addition, they do not show up on radar--the ash particles are too small to return an echo to the onboard weather radars of commercial airliners.