I've heard this before. But during fog I've tested using headlamps, headlamps + fog, and fog only and (maybe it's just me) I couldn't tell the difference. I actually thought I could see better with the lamps over the fog lights.
The headlamp + fog added some light, but I wouldn't say that I could see farther or clearer.
What do you think? Or am I thinking about the context of how to use fog lights wrong?
You make valid points there, and let me shed a little light (haha) :
The Cherokee fog lamps have terrible optics. The bulbs in them are just typical halogen fog light bulbs, but the optics being so bad, the beam pattern is not what a fog light pattern should be : low and wide. These lamps throw too much light up and not enough wide. In fog, you need all the light to be low and wide, and any bleeding up causes discomfort for your eyes as light going up hits water dropplets and makes it very difficult for us to see anything ahead.
The stock halogen headlights are also pretty terrible on the Cherokee in terms of beam pattern and usable output, but the cutoff is very sharp so there is very little bleeding up ; this means that in fog, the headlights would actually do better than the stock fog lights for foreword vision. But the headlight beam is also very narrow, so not so great in fog in that sense.
In an ideal world with great lamps, yellow works best in fog, heavy rain and snow because no matter how low and wide your beam is, you will have light hitting a lot of little water particles, which causes eye discomfort, and yellow light is the least offending to the human eye, Yelllow light allows our eyes to see better in such conditions, if you will.
With that all said, if you want useful fog lights on the Cherokee, you have to replace the lamps, as stated above by another member. There are some good ones out there, though not cheap : Rigid, JW Speaker, KC Hilites Gravity, and there could be more quality ones (edit to add : yes, Morimoto makes some good ones as well)... And if you want true fog light capability, get a model with amber lenses.
Replacing the bulb in the existing factory fog lamp wouldn't do much good...