Ah, the great question of our times. . . H2O, in the form of water vapor, easily attaches to methane molecules by the chlorine atom, or the sulfur released from bacteria, making the foul smell more readily attach to the nostril lining, subsequently sustaining the odor longer than in a dry climate. In addition, warm vapors will cause any crusted mucus to soften and disengage, providing more surface area within the sinuses. And lastly, (if you are male) the qualitive factor of "worse" is a perception of foreign gases - as to our own emissions, the qualitive factor is "better."