Wood is hygroscopic...it will take up moisture and give it up repeatedly. As a result of this characteristic, some gapping will happen and some cupping on occasion unless the indoor temperature and humidity levels remain constant...not many homes maintain these levels.

Do NOT acclimate your wood to the ambient temperature and humidity conditions of the outside air, especially if that is what you perceive as hot and humid...that will guarantee that you will have more gapping than is normal to expect. Get yourself a combination temperature and humidity instrument (about $25 at Radio Shack) and monitor temperature and humidity levels in the room where the wood will be installed. Professionals have moisture meters that read the level of both the wood and the subfloor that the wood is to be installed over and then they install when moisture readings are within 2% to 4% of each other. You don't have that instrument, so common sense will dictate. You want to acclimate the wood to the indoor environment that it will live in most of the time. If that is let's say 68 F and 50% humidity, then that is the conditions you what the wood to acclimate in. Turn on the HVAC system and adjust temperature and humidity levels to what you consider to be the 'typical' levels in your home. Acclimate for the time required...I acclimate wood for at least a week.

Since you are DIYing, pay attention to how you 'rack' the floor...avoid obvious 'step' patterns and keep end seams at least 8" from each other to avoid that visual insult.

Search the internet for some other professional opinions. Here is a link to NOFMA that can get you started.

Here is a link that might be useful: NOFMA

It is actually better to install during the hot humid summers, when your A/C may be pulling some of the humidity out. It is better to have the floor shrink during the winter and gap from the dryness of heating the home, then to install it tight during the winter when the flooring is shrunk to it's smallest dimensions, and then come summer the floor swells and cupps and buckles/tents/heaves.

Average moisture content is best. That way you may get minor cupping in the summers and minor gapping in the winter, if you like to keep windows open a lot, it is best to go by the average.

Thanks to both of you for the tips. Hopefully our late spring/early summer install will get us in that "sweet spot" of humidity so we won't get too much gapping or cupping. We've got a humidity gauge, so we'll use that to monitor the humidity and install when it's average.