It's getting colder and drier here in the East Coast. It seems that two opposing forces are at work as far as paint drying time is concerned.

1. Low temperature --> less evaporation --> So, add less thinner to paint.
2. Dry air --> more evaporation --> So, add more thinner to paint.

So, in general should I add more thinner or less thinner to paint in the winter than in the summer? I want to minimize the chance that the sprayed paint dries out before hitting the target surface. FYI, I only use Tamiya acrylics.

The answer is Yes-No-Maybe.
Every day is a new day with airbrushes. TEST EVERYTHING before you commit to painting the model. Your ratios will change according to temperature and humidity always, even in summer. It is generally a bad idea to paint at all below 70F.
The key is testing before painting. Spray something else first, adjust accordingly.

If you are using acrylics, ad a few drops of glass cleaner. This will keep the paint from drying, before it hits the surface. INO I get better results when it is drier. I've run my airbrush at 56deg with no problem. I thin 1:1. I experience more problems when it is more humid and/or hotter. Enamel acts quite differently than acrylic, when airbrushing. Avoid enamels on high humidity days, as it may react poorly to the moister. Your drying time and perhaps entire paint session will suffer or fail. Once again, this is just my opinion based on my experiences. Of course, if you paint inside you shouldn't need concern yourself with the weather.

I spray inside at my basement. In winter, the basement temperature is about 66-68F and the humidity is about 40-45%.

I just don't think there is that much science to it,I work in my basement,sounds like the same conditions as you,I just eyeball my mix and spray,no problems ever.

Chrisk-k Temperature and humidity have no bearing on your mixing ratio.That should remain the same regardless of the conditions. What you need to do is change the evaporation rate of your mix to match the temperature and humidity.
You basically have it right, Hot= fast drying and Cold= slow drying.
So taking that into account, what you need to do is when it's hot, add a retarder to your mix to slow the evaporation rate down and give the paint a chance to flow out, settle and not dry before it hits the model, and when it's cooler, since that causes the evaporation rate to slow down, you probably will not have to add a retarder to the mix to get good results.
In a nutshell, mixing ratio should stay the same, and the evaporation rate should change, slower for hotter, faster for cooler. Hope that makes sense. Jeff T.

It is always dry here in Arizona. It only gets hot or hotter here also. I have a major evap issue when using acrylics (I usually am an enamel person for obvious reasons). I do the couple drops of window cleaner as Retiredyank suggests. I usually paint in the garage and I try to do it early morning when it is cooler. Summer time is the toughest where it mostly only drops into the 90's. I have not even attempted airbrushing during the day when it is in the 110's. I have recently purchased a paint booth and have switched to painting inside where the temps are in the high 70's (the air conditioner is up there as one of the greatest inventions of mankind second only to canned beer )

Cheers