Clarify WildFIRE-FM validity mask channels

README.md

@@ -222,8 +222,8 @@ masks included as channels 10 and 11.
 | 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 provides the occupancy target, not an input channel. WFIGS and MTBS are
@@ -233,6 +233,10 @@ pretrained occupancy input. Full channel-level details are in
 `models/wildfire_fm/input_channels.json`.
 The `cape` channel is the HRRR surface instantaneous CAPE field selected with
 `typeOfLevel=surface` and `stepType=instant`.
+For validity channels, `firewx_valid` is 1.0 everywhere in the released
+California regional cache and is not a per-variable HRRR missing-data mask;
+`static_valid` is the fraction of the four static layers valid after
+reprojection at each grid cell.
 
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