Clarify WildFIRE-FM validity mask channels

data_sources/DATA_SOURCES.md

@@ -40,8 +40,8 @@ channels 10 and 11.
 | 7 | `vis` | NOAA HRRR | surface diagnostic, instant | m | none | Visibility |
 | 8 | `prate` | NOAA HRRR | surface, instant | kg m^-2 s^-1 | none | Precipitation rate |
 | 9 | `tp` | NOAA HRRR | surface, accumulated | kg m^-2 | none | Accumulated precipitation |
-| 10 | `firewx_valid` | Cache validity channel | input mask | mask | none | Dynamic/input validity mask for this regional cache |
-| 11 | `static_valid` | Static reprojection mask | static mask | mask | none | Fraction of static layers valid at the grid cell |
+| 10 | `firewx_valid` | Cache validity channel | input presence | binary mask | none | Cache-level dynamic input presence mask; 1.0 everywhere in this release |
+| 11 | `static_valid` | Static reprojection mask | static layers | fraction | none | Fraction of static layers valid at the grid cell after reprojection |
 | 12 | `fuel_fbfm40` | LANDFIRE FBFM40 | static layer | categorical code | none | Fire-behavior fuel model |
 | 13 | `canopy_cover` | LANDFIRE CC | static layer | provider-native value | none | Canopy cover |
 | 14 | `housing_density` | Wildfire Risk to Communities | static layer | provider-native value | none | Housing-unit density |
@@ -55,6 +55,13 @@ The `cape` channel is the HRRR surface instantaneous CAPE field selected with
 `typeOfLevel=surface` and `stepType=instant`. It is not the most-unstable CAPE
 or a layer CAPE variable.
 
+The two validity channels have different definitions. `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, with possible values 0.0, 0.25, 0.5, 0.75, and
+1.0.
+
 ## Notes
 
 - The paper places gridded resources on a projected 5 km EPSG:5070 grid.