I've tried many photo fads since the 1970s, but I had never tried out a graduated neutral-density filter. These simple lenses are half-grey half-clear and rotate into position to make ultra-bright portions of the image less so. I found one online for less than $20 so figured it would be a rather inexpensive learning experience. One can achieve very similar effects in post-processing, but learning how light behaves and how computers work are separate learning experiences - so I bought it.
The shot shown here are not perfect examples, as I was in manual-exposure mode for the unfiltered (left) shot. I then added the filter, dialed down the cloud and shot again, so the foreground is exposed identically. Since one cannot pick on the cloud without the entire upper half being affected, the dark trees suffer in this shot. As I recall the meter showed this 2nd shot to be about 2/3 stop underexposed. Several levels of darkening are available using different filters, as is the abruptness of the dark-light transition. Mine shows why it cost so little, as the transition area is irregular; that really doesn't show up since it's practically in contact with the lens' front element, so for the money this performs just fine.