Particles systems are using to define behavior of object groups and many special effects like snow, rain, moving fog, clouds, fire, can be effectively implemented by them. Unity particles system “Shuriken” of is very powerful, especially in new “Unity 5.5”
revision. Snow in “The Old Bridge”
scene, lighting emanations and stars in “The Fetch”
scene, clouds in “Floating Islands”
for example was done with power of particles.