Complex sentence: If seen, parachutist and light aircraft should avoid dust devils, which can collapse parachute canopies; near the ground, re-inflation or use of reserve parachutes is not possible.
Split into simpler sentences: If seen, parachutist and light aircraft should avoid dust devils. They can collapse parachute canopies, and if near the ground, re-inflation, or using the reserve parachute is not possible.