Probably the higher humidity usually found in summer conditions but that is an unscientific guess on my part. Check out the following abstract (underlines and bold added by me):
Michael Kuehn1, Heiko Welsch1, Thomas Zahnert2 and Thomas Hummel3
(1) Division Aviation Physiology, German Air Force Institute of Aviation Medicine, Königsbrück, Germany
(2) Department of Otorhinolaryngology, University of Dresden Medical School, Fetscherstrasse 74, 01307 Dresden, Germany
(3) Smell and Taste Clinic, Department of Otorhinolaryngology, University of Dresden Medical School, Fetscherstrasse 74, 01307 Dresden, Germany
Received: 31 January 2007 Accepted: 5 September 2007 Published online: 25 September 2007
Abstract The present study aimed at investigating the question whether olfactory function changes in relation to barometric pressure and humidity. Using climate chambers, odor threshold and discrimination for butanol were tested in 75 healthy volunteers under hypobaric and hyperbaric, and different humidity conditions. Among other effects, olfactory sensitivity at threshold level, but not suprathreshold odor discrimination, was impaired in a hypobaric compared to a hyperbaric milieu, and thresholds were lower in humid, compared to relatively dry conditions. In conclusion, environmental conditions modulate the sense of smell, and may, consecutively, influence results from olfactory tests.