Because most people spend essentially all their time in the lower 6 feet or so of the atmosphere, the human weather experience is largely two dimensional. But for the weather forecaster, the air above us is important because of what we don't know about it.
During winter, in particular, the air a few hundred feet off the ground can be significantly warmer than the air at the surface. But the vertical temperature profile is usually estimated, rather than measured, which makes forecasting mixed precipitation difficult.
Last Tuesday, freezing rain was falling over eastern North Dakota that was expected to turn to snow as the atmosphere cooled at night, but we had to guess when that would happen.
Rain had been reported just west of Fargo at Mapleton, but the air cooled just enough that West Fargo-Fargo-Moorhead-Dilworth received all snow, which made a huge difference for drivers. There was no way to have known this in advance.