Although residents of central Arizona are just now beginning to enjoy cooler fall temperatures,  fall allergy season started a bit earlier, sometime around mid to late September.  At this time, following a (hopefully) rainy monsoon season, weeds begin to proliferate and then pollinate in response to the shorter days and cooler nights.  As the monsoon gives way to endless days of snow-bird pleasantness, the dry, calm air slowly fills with dust, air pollutants, and pollen particles.  These chemicals and particles stay airborne longer and travel greater distances, producing the brown haze that we see hovering over the valley.  The meteorological phenomenon know as temperature inversion, where a layer of cool air is trapped beneath a lid of warm air,  adds to the buildup of particles close to ground level.   Add to this growing soup of organic and inorganic nastiness a steady plume of allergenic mold spores released during lawn scalping and other harvesting activities and you have the making of our typical fall allergy season.