hi all,
How can I modify (if it’s possible) the sky? the day, the clouds, precipitations?
thanks in advance 
Each of these elements is unique unto themselves.
Skies are normally created through custom material creation that is then applied to a type of mesh. If you would like to alter parameters such as color or zenith values, then the material would most likely be your best bet.
The time of day is a combination of several elements. At the very basics, it is derived from the material effects, and the orientation of the dominate light source representing the sun (relative to the sun source in the material).There should be an example of a day and night cycle in the UE4 example maps.
Clouds are a bit tricky as there are several ways to create them. Normally they are generated through the sky material by the means of textures masks or noise algorithms. Depending on project requirements, sometimes clouds will be created on camera facing sprites or fixed geometry planes (normally facing the ground). Some rare cases (Example: Skydiving game) will require actual meshes to be developed that contain vertex information for coloring and blending.
Weather systems are normally particle or mesh based. However, there are a few elements of weather that can be generated through screen particles/materials, local player volumes, or post process.
I would recommend studying these independently and then marrying them together as you go. Time of day is normally a combination of all these elements (And more - Example: Building lights turn on, street lights turn on), and I would look into this last.
-Jeremy-
yep an example of the day & night cycle would be awesome as I don’t know how to deal with lightmass
If you open the content examples then:
- Open the map called “Level_Scripting”
- Navigate to the Level Blueprint
- In Figure 2.1 (Matinee Cinematic)
Here you can gain access to the parameters being changed for the cycle to take place.
-Jeremy-