RealisticSeasons adds a few particle effects to your world for extra immersion. You can configure them in the config.yml. Here is a list of all the particles:
Shooting stars appear on summer nights and they're most visible around midnight. Players need their render distance set to at least 10 to be able to see them. Nights have never been this beautiful!
Big falling leaves can be spotted falling down during summer days under all trees. It makes your forests extra beautiful!
Small groups of fireflies can be found in forests all spring and summer nights. They can only spawn near birch and oak trees and a spawned group will be visible for around 3 minutes.
Each time a night starts in winter there's a 20% chance that the sky will be filled with white sparks. The effect will stay the entire night and it will blow away your players!
Small "leaf" particles will spawn under trees in fall/autumn for extra immersion.
There are currently 2 temperature-related particle effects in RealisticSeasons: one if you're cold and one if you're hot.
Your breath will become visible if the air temperature drops below 0°C. The player's own temperature doesn't matter here: only the temperature of the environment. Players can only see others' breath and not their own by default (it blocks part of your screen), but that can be changed in the config.
Players will sweat like above if their temperature is higher than 40°C.