i have a new level a dark one and i want to put like a sun and weather ?
Let's get rid of the darkness.
Add a Sky
Add some Atmosphere
Now you'll be able to see some things. From there, add geometric (like a cube for a floor), and add a Directional Light to light up the geometry (find the lights in Window > Modes).
Adding weather, like Rain/Snow/Etc will require visual effects. I reccomend exploring Epic's Particle Effects example. https://docs.unrealengine.com/latest/INT/Resources/Showcases/Effects/index.html
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