In a broad sense, when you move from a humid cooler climate like the NE to a hot dry climate like the SW do you usually ending up tuning a gasoline engine richer or leaner. thanks.

When you have a hotter and/or more humid environment the molecules needed for combustion are spread further apart (less dense) so you need to lean your needles to get the optimum ratio again. The opposite for cold dry environments.
Temperature has the largest effect if I remember correctly, not humidity. Someone correct me if I am wrong, I haven't flown full scale for 20+ years or been in school for longer than that.
Your plane will also have to fly faster to create the same amount of lift. If you have ever flown full scale you can look at graphs/charts that show you take off/landing distances, mixture leaning etc. d/t temp and elevation changes.

Well that's a very good question. I have not made that move and tuned an engine afterwards but I can tell what effect things have on the engine mixture.

Low to high elevation, everything else the same: the engine runs rich because the air is thinner. You have to lean it.
Cold to hot air, everything else the same: The engine runs rich because the air is thinner, just like the elevation change above. You have to lean it.
Dry to humid air everything else the same: The engine runs rich because each unit of volume of air has less oxygen due to the water vapor displacing it. You have to lean it.

Reverse the above situations and you reverse the tuning of the needles (engine runs lean, you have to richen it).

I thnk you can find a needle setting on a gasser that will satisfy almost all conditions except very extreme (like sea level to 15,000 feet, or deep northern winter to hot/humid northern summer).

that is A-OK
now going from cold and humid to hot and dry is not that easy, because cold air cannot contain much water.

Some reasoning may be in order.

If you heat cold humid air to hot, it will be hot dry air.

Since water content is the same, I think only the temperature increase which reduces the oxigen amount per unit of volume is important in this question, and thus the needle has to be leaned out, despite the fact that humidity is lower.

Er, I've seen it 90* and 80% humidity. I've also seen it 90* and 20% humidity.

CR

If you would make a three bar graph, with one of the outer bars the temperature (and/or barometric pressure) and the other the humidity (in % water vapour content), you could place the middle bar such as to reflect the mixture strength or needle setting. With a bit of juggling of the scales, it would even be quite accurate.
You would also need extra tables to retrieve water content from temperature and dew point though. Cold humid air contains very little moisture. At elevated temperatures the ability to contain moisture increases tremendously, until at 200F the moisture content can be 100%, eliminating all air content.

...sounds like a great addition to YOUR webiste PE (hint hint)

Chad

Too many things to do, besides coping with mail backlogs.