Just the temperature. As a general rule of thumb: if you need to wear gloves/keep your hands in your pocket to keep your hands warm when outside for a long period of time, it's too cold to get the best results with airbrushing. Humidity shouldn't be a factor if you are using a moisture trap unless you want to airbrush outside without cover when it's raining. Otherwise, you want the humidity to be as low as possible.
I did this as well. But there always seemed to be some residue form over spray. That took a bit of cleaning up.
Most all spray applications the ideal temperature is a minimum of 70 deg. F. That is basically so that the carrier for the paint can evaporate in a timely manner to get the best results. Same thing applies to some glues in that the ideal minimum temperature is again 70 deg. F. for the solvents to evaporate in the advertised time period for the product. High humidity can slow the drying process for both glues and paints in that the higher levels of humidity slow the evaporation rate of the respective solvents or carriers. Higher temperatures and lower humidity generally mean that the product will most likely set up well in a little less time that the manufacturers specify. I had a good friend who modeled in the horribly oversized scale a number of years ago while I modeled in N. He used to save up his paint projects for days when his wife would be out of town for a business trip or visiting because even the curing paint odors were enough to trigger her asthma.
A cheap portable paint booth can be made from a good double strength cardboard box in the range of 2ft x 2ft. Get an inexpensive bathroom vent fan with a few feet of hose, attach it to the top of the box, my favorite tool there is ducky tape! Pick up the box, decide what size access hole you need in the side and cut that out. Hopefully you remembered to wire in an extension cord to the fan, then plug it in and test it. Next thing is to find a suitable location, run the hose out the window, and paint. The shorter your vent hose, the better the evacuation of fumes, as the flexible hoses re really more of a flow restrictor than help! It's definitely time for me to build one. Have the fan and hose, just got to go get a box at my local office supply shop, and build it. I use Floquil paints, ( curses upon testors....) and just came in from doing a bit of painting on some dpm buildings and am truly feeling the fumes! And all the pretty colors!
I built mine out of an inexpensive 18 gallon tote. Works great. When done, I snap the lid on and it stays neat, clean, ready to go next time.
That would definitely be more durable than a box! Will have to work on that instead since I didn't pick up a box today.
Then again, you could build a paint booth using a wooden frame and a cardboard liner. It would be light and have a decent amount of durability with bonus points available if you want to make it collapsible.
