id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
386268842,MDU6SXNzdWUzODYyNjg4NDI=,2583,decode_cf not scaling and off-setting correctly,42973457,closed,0,,,8,2018-11-30T16:54:31Z,2018-12-15T23:53:19Z,2018-12-15T23:53:19Z,NONE,,,,"#### Problem description

I am importing atmospheric cloud fraction data from the PATMOSx OpenDap server (https://www.ncei.noaa.gov/thredds/satellite/avhrr-cloudprops-patmos-x.html). Cloud fraction is the percent of the pixel that the instruments flag as cloud, thus the output should be between 0 and 1. The data is stored unscaled as an int8. In order to retrieve the cloud fraction data, the data should be decoded as unscaled_data*scaling_factor + add_offset in order to be in the proper format. decode_cf does not seem to be using the algorithm in this way.

```python
import xarray as xr
url = 'http://www.ncei.noaa.gov/thredds/dodsC/avhrr-patmos-x-cloudprops-noaa-asc-fc/files/2003/patmosx_v05r03_NOAA-17_asc_d20030101_c20140314.nc'
# Decoding using decode_cf
ds=xr.open_dataset(url, decode_cf=True)
ds.cloud_fraction
<xarray.DataArray 'cloud_fraction' (time: 1, latitude: 1800, longitude: 3600)>
[6480000 values with dtype=float32]
Coordinates:
  * latitude   (latitude) float32 -89.947815 -89.84893 -89.74731 -89.64843 ...
  * longitude  (longitude) float32 -179.94507 -179.84619 -179.7473 ...
  * time       (time) datetime64[ns] 2003-01-01
Attributes:
    standard_name:  cloud_area_fraction
    long_name:      cloud fraction computed over a 3x3 pixel array at the nat...
    actual_range:   [0. 1.]
    valid_range:    [-127  127]
    scaled:         1
    _ChunkSizes:    [  1 361 361]

ds.cloud_fraction.plot()
#The cloud fraction data should be scaled to be a percentage between 0 and 1, but it is not. 

#Manually decoding cloud_fraction
ds=xr.open_dataset(url, decode_cf=False)

ds.cloud_fraction
<xarray.DataArray 'cloud_fraction' (time: 1, latitude: 1800, longitude: 3600)>
[6480000 values with dtype=int8]
Coordinates:
  * latitude   (latitude) int16 -32748 -32712 -32675 -32639 -32603 -32566 ...
  * longitude  (longitude) int16 -32757 -32739 -32721 -32703 -32685 -32666 ...
  * time       (time) float32 1041379200.0
Attributes:
    _Unsigned:      false
    standard_name:  cloud_area_fraction
    long_name:      cloud fraction computed over a 3x3 pixel array at the nat...
    coordinates:    latitude longitude
    actual_range:   [0. 1.]
    valid_range:    [-127  127]
    scale_factor:   0.003937008
    add_offset:     0.5
    _FillValue:     -128
    scaled:         1
    _ChunkSizes:    [  1 361 361]

cloud_fract_manual = ds.cloud_fraction * ds.cloud_fraction.scale_factor + ds.cloud_fraction.add_offset
cloud_fract_manual.plot()
```
decoded using decode_cf
![image](https://user-images.githubusercontent.com/42973457/49302288-b0011c00-f494-11e8-8223-dacd185f48f9.png)




#### Expected Output
![image](https://user-images.githubusercontent.com/42973457/49302242-952ea780-f494-11e8-8222-1ec1ba4c8bc6.png)

Decoded manually ` ds.cloud_fraction * ds.cloud_fraction.scale_factor + ds.cloud_fraction.add_offset`
#### Output of ``xr.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.111+
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.10.8
pandas: 0.23.2
numpy: 1.15.1
scipy: 1.1.0
netCDF4: 1.4.2
h5netcdf: 0.6.2
h5py: 2.8.0
Nio: None
zarr: 2.2.0
bottleneck: 1.2.1
cyordereddict: None
dask: 0.18.2
distributed: 1.22.1
matplotlib: 2.2.3
cartopy: 0.17.0
seaborn: None
setuptools: 39.2.0
pip: 18.1
conda: 4.5.4
pytest: 3.9.1
IPython: 6.4.0
sphinx: None
</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2583/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue