After photographing nature scenics for a while, we develop a need for filters that don't exist! Luckily, I'm affiliated with Singh Ray filters and can just call Dr. Bob Singh and request a special filter be manufactured if the need occurs enough. In the following examples, such a filter was created by Dr. Singh from a request from Daryl Benson.
In the following scene, the horizon area is brighter than the sky, where the clouds are. A normal graduated neutral density filter will darken the top of the frame and graduate down to clear. The first example illustrates the use of this filter in this specific situation. As you can see, the foreground is bright, but the sky is very dark to the point of making the clouds too dark.
This occurs because the horizon is the brightest part of the image and the sky at the top of the frame is already a bit dark. What we need is a filter that is graduated up from the horizon where it's the darkest to the clear part at the top of the frame, where it is naturally dark anyway. This filter is called a Reverse Graduated Neutral Density. I find that I'm using this filter more and more in my work.
Here's an example of the use of reverse grad. Notice how the exposure at the horizon is exactly the same on both images. However, the skies are very different. The brighter image is much more appealing, using the reverse grad.