Upcast floats to at most float32, not float64

Author: Zac-HD

doc/whats-new.rst

@@ -52,7 +52,8 @@ Enhancements
   2D variable.
   By `Deepak Cherian <https://github.com/dcherian>`_.
 - Reduce memory usage when decoding a variable with a scale_factor, by
-  converting 8-bit and 16-bit integers to float32 instead of float64 (:pull:`1840`).
+  converting 8-bit and 16-bit integers to float32 instead of float64
+  (:pull:`1840`), and keeping float16 and float32 as float32 (:issue:`1842`).
   By `Zac Hatfield-Dodds <https://github.com/Zac-HD>`_.
 
 .. _Zarr: http://zarr.readthedocs.io/

xarray/coding/variables.py

@@ -215,6 +215,10 @@ class CFScaleOffsetCoder(VariableCoder):
     @staticmethod
     def _choose_float_dtype(data, has_offset):
         """Return a float dtype sufficient to losslessly represent `data`."""
+        # Keep float32 as-is.  Upcast half-precision to single-precision,
+        # because float16 is "intended for storage but not computation"
+        if data.dtype.itemsize <= 4 and np.issubdtype(data.dtype, np.floating):
+            return np.float32
         # float32 can exactly represent all integers up to 24 bits
         if data.dtype.itemsize <= 2 and np.issubdtype(data.dtype, np.integer):
             # A scale factor is entirely safe (vanishing into the mantissa),