Clarify WildFIRE-FM validity mask channels

models/wildfire_fm/README.md

@@ -31,14 +31,21 @@ zero-filled, with validity-mask channels carried in the input tensor.
 | Channels | Names | Source |
 |---:|---|---|
 | 0-9 | `t2m`, `d2m`, `u10`, `v10`, `cape`, `sp`, `blh`, `vis`, `prate`, `tp` | NOAA HRRR |
-| 10 | `firewx_valid` | Dynamic/input validity mask for this regional cache |
-| 11 | `static_valid` | Static reprojection validity mask |
+| 10 | `firewx_valid` | Cache-level dynamic input presence mask; 1.0 everywhere in this release |
+| 11 | `static_valid` | Fraction of static layers valid after reprojection |
 | 12-15 | `fuel_fbfm40`, `canopy_cover`, `housing_density`, `population` | LANDFIRE, WRC housing density, LandScan |
 
 FIRMS detections are used to derive the occupancy target, not as an input
 channel. WFIGS and MTBS are supporting event-level resources and are not part of
 the pretrained occupancy input.
 
+The two validity channels have different meanings. `firewx_valid` is a
+cache-level dynamic input presence mask; in the released California regional
+cache it is 1.0 everywhere and is not a per-variable HRRR missing-data mask.
+`static_valid` is the fraction of the four static layers that are valid after
+reprojection at each grid cell, so its possible values are 0.0, 0.25, 0.5, 0.75,
+and 1.0.
+
 For HRRR, `t2m` and `d2m` are 2 m fields in Kelvin; `u10` and `v10` are 10 m
 wind components in m/s. The `cape` channel is selected from the HRRR surface
 instantaneous group with `typeOfLevel=surface` and `stepType=instant`; it is not