Bringing "night feeling" into a normal plain daylight picture is a bit hard as there still should be some color left but not too much, shadows change but still are there, highlights and midtones go down...
Here is a neat technique to achieve a nice, quite realistic and slightly "dramatic" light situation for whatever outdoor-pic you have.
1.
Open your source image.
Duplicate the background layer and apply Image- Adjust- Desaturate.
2.
Bring up the Curves by CTRL+ M and apply the setting as shown below. Depending on your source image and the light situation in it you will have to play a little - it should look like the pic shown, no hard black or white spots, not too much contrast... all a bit ugly grey - it's okay if ya loose some contrast.
3.
For the same greyscale Layer bring up the Levels by CTRL+ L and apply a setting as shown.
Mainly the whole layer should get darker so use just the sliders
i marked.
4.
Now the whole thing gets interesting. Change the Layer mode of the greyscale Layer to "Multiply" (in the Layers Palette). et voila!
You might use Levels or Curves again to tweak the darkness of the scene but you are basically ready.
5.
Looks pretty realistic but there is no logic in the light source :)
I'd suggest to add a source like i used the moon - but now you have a nice basement to play around with you could think of street lights, carlights and so on... :)
