While it is true that moist air is less dense than dry air at the same temperature, the phase change from liquid to gas removes some energy from the air. This loss of energy reduces the air temperature, thus increasing its density. The cooler/humid air will then sink. My downdraft dehydrator heats up to about 130 deg. F in the winter, and can dry cherry tomatoes cut in half in 2 days. I have yet to try it out in the summer months. I do have it sealed very tight with latches that pull the door shut. I get less air movement when it is empty or when the food has dried out.