I've created a new Weather Type (based on Clear) and a new Climate which uses only this Weather type. I've then set my interior Cell to use this new Climate as its lighting template. However, I want to make the night-time REALLY dark, but at the moment it is hardly dark at all, and everything I do seems to produce no real effect... How do I alter how bright a Climate/Weather is?

I believe the lighting is adjusted in the weather type(for time of day) you can change the color of the sky the overall lighting. Tinkering with the weather setting definitely helps by adjusting the ambient lighting.

It's all in the weather settings. There are settings for day, sunset, night and sunrise. These settings control the amount of light/the color for the various parts that make up the weather. There is one for fog, for ambient light, for sunlight, for sun, for cloud textures, for the sky, the horizon, the stars etc. If you set ambient light and sunlight to really dark (RGB settings around 20 or lower) the player will need a torch to see anything.

If you have an interior where you can't see the sky all you need to change is ambient and sunlight (and maybe fog if it's a very large interior with a low fog setting). Ambient light is the overall light level. It doesn't look very nice as it will light up everything by the same degree. Sunlight is a directional light that comes from the direction where the sun (or moon(s) at night) is.

If you can see the sky you need to test a lot to make everything fit together (bright horizon with dark lower sky layer will look weird for example). But you can watch weather in the CS render window by pressing the sky button so you don't need to go in game to see what it looks like.

Thanks guys-- I totally missed the "Type" box Yeah, it's a closed interior (a few windows that you can't actually see out of), so it's just the light levels I need, to make it seem like the day is passing

Merci

There is OC Darker Nights that does this for the default weather system. I've used this as a template and then tinkered with the values so it's not as dark as OC, as on a clear night you don't spend your whole time bumping into trees - especially with 2 giant moons in the sky!