Select the sky layer and press Ctrl+D/Cmd+D to duplicate it. Rename the layer to something like Sky Flipped. To do this you select the layer on the time line and hit Return on the keyboard to rename it. As you might have figured out by now, we'll be flipping this layer. We're doing this to conceal the fact that we're using the same sky map twice. Expand the layer again, uncheck the little link symbol next to Scale and set the first value to -40% instead of 40%. Then check the link symbol again and, in the viewer, move the sky so the cloud formations don't overlap with the same ones in the layer underneath (Fig.23).
Now for the fun part: Still using the Sky Flipped layer, play around with different blending modes until you get a dramatic effect. I ended up with Linear Light. Don't think about matching the colour to the rest of the image yet, we'll look at that next. You now have two sky layers you can animate at different speeds, making it look like some clouds are moving faster than others across the screen. Don't animate anything just yet though (Fig.24).
To make the sky and the 3D clash less, we'll create what's called an Adjustment Layer. An Adjustment Layer doesn't contain graphics, but will let you apply effects to it that also applies to all layers below it. To create one, right-click any empty space in the time line and choose New and Adjustment Layer (Fig.25).
In this case, we only want the Adjustment Layer to affect the sky, so we will drag it to right above our two sky layers. If you like, you can rename the layer to something like Sky Adjustments. Now that we have the Sky Adjustments layer in place, right-click it, choose Effect, Color Correction and Hue/Saturation (Fig.26).
Now, dial Master Hue back to round -10 and Master Saturation to about -50 and the sky and city should begin to match each other (Fig.27).
Step 8: Tracking
The sky replacement is now almost finished, but one major thing is missing. While the rendered footage is constantly moving, the sky maps are stationary, making the composite painfully obvious as soon as you hit the play button. In other words, we'll have to animate the sky, making it look like it follows the city horizon. Doing this by hand can be very time consuming and painful, though luckily there is a better way called tracking. When tracking a shot, you're basically making After Effects watch a few selected areas of the image, frame by frame, and recording their movements. You can then apply the same movement to other objects in the scene. Areas used for tracking are preferably points of high contrast that don't deform too much during the clip, making it easy for After Effects to recognize through a long sequence of frames. In this respect, the shot we've just created is rather difficult to track. Nearly every part of the rendered sequence deforms over time. In addition the sky needs to move with the horizon, confining the area of tracking to that fine line which is almost constantly deforming. In other words, tracking this shot, although possible, will probably be difficult and time consuming.
Luckily, we're talking about a 3D render here, and so we have the means to fix this pretty easily. Go back to 3ds Max, create two pyramid shapes and place them close to the horizon, looking through the camera. Make sure to space them a little apart. Now, hide all other geometry in your scene, give the pyramids a white, self-illuminated texture and render the sequence against a black background. Instant tracking markers! (Fig.28)
This makes tracking a breeze. Import the pyramid pass and drag it to the bottom of the time line (Fig.29).