A firefighting aircraft releases thousands of liters of water from hundreds of meters above a wildfire. Not all of it reaches the ground. Small droplets evaporate during descent. Large droplets break apart from aerodynamic forces. Only droplets within a specific size window survive the fall.
The window is 150 micrometers to 3 millimeters in radius (arXiv:2603.11855). Below 150 μm, the surface-area-to-volume ratio is too high and evaporative loss destroys the droplet before it descends through the hot, dry air column above a fire. Above 3 mm, the Weber number exceeds the breakup threshold and the droplet shatters into smaller fragments — which may themselves be too small to survive evaporation.
Release altitude is the most significant operational factor. Higher release means longer fall time, more evaporation, and a narrower survival window. Relative humidity is the second factor: dry air accelerates evaporative loss, shrinking the viable size range from below.
The delivery problem is constrained from both ends simultaneously. The lower bound (evaporation) is set by atmospheric thermodynamics. The upper bound (breakup) is set by aerodynamics. Both constraints tighten as altitude increases — higher release means more time for evaporation and more velocity for breakup. The operational envelope is the intersection of these opposing constraints, and it narrows with every meter of altitude gained.
The water that reaches the fire is the water that threaded the gap between evaporating and shattering.