Verify results of vapor_pressure results

[Marston]

from metpy.calc import vapor_pressure
vapor_pressure(988 * units.hPa, 18 * units('g/kg')).to('hPa')

The above example works but when pressure and mr are passed as dataArrays, the .to('hPa') causes an error which claims the dataArray as no method called to(). Removing this method works but the results seem to be in hPa. Curious if this is intentional behavior for scalars vs dataArrays?

[dcamron]

MetPy functions' outputs, where possible, preserve the type of the inputs. So if you provide a Pint Quantity (which is what 988 * units.hPa returns), that is what you'll get back. With that comes all of the functionality associated with Quantitys, like the Quantity.to() method for converting units. If instead you provide a DataArray, you will get one back, along with the methods accessible to those.

To help, we do provide our own helpers to work with units inside of DataArrays, eg DataArray.metpy.convert_units(),

vapor_pressure(my_dataset["pressure"], my_dataset["mixing_ratio"]).metpy.convert_units('hPa')

Check out the xarray with MetPy Tutorial on the docs for more info!

[dcamron]

From this we will probably remove the confusing .to('hPa') in the vapor_pressure documentation, thanks for highlighting that!

[Marston]

Thank you. Now I understand :-)