Weather in DayZ Standalone is ever-changing. This affects the Temperature of your character.
Lightning (and Thunder)
Rain
Template:Effects Causes Treatment/s
Rendering
[2] Your Shadow settings affect how the Rain is rendered.
If it is set to Disabled, Very Low or Low you will likely see Rain inside buildings.
If it is set to Normal and above you should not see Rain inside buildings.
Notes and References
1. https://twitter.com/rocket2guns/status/434346873948622848
2. https://twitter.com/rocket2guns/status/434019818874753024