Psychrometric position, 12-hour trail, and 24-hour trajectory for any U.S. location
(what… does… hygrothermal… mean?)Hygro (moisture) + thermal (heat): air has two properties your body
cares about, and this chart plots both at once. Left–right is temperature; up–down is the
actual amount of water in the air (the curved grey lines are relative humidity). Every possible state
of air is a single point — right now, your air is the orange dot. The thick line is its path:
grey trailing into the past, colored heading into the future (red = feeling warmer, blue = cooler),
fading as the forecast gets less certain. When the line runs through a tinted field, the air is
comfortable — bluish fields for indoors, green for outdoors. Engineers call this a psychrometric
chart; they've used it to size HVAC systems for a century. Here it answers smaller questions:
open the windows? wet grass in the park?