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  1. How to identify whether the data showing cloud or fog?

    From the MODIS L2 Cloud Products, Cloud Top Pressure over Lucknow, India showing CTP around 750-800 hpa (,MODIS_Aqua_CorrectedReflectance_TrueColor(hidden),MODIS_Terra_CorrectedReflectance_TrueColor,Reference_Labels,MODIS_Terra_Cloud_Top_Pressure_Day,Reference_Features(hidden),Coastlines&t=2017-12-31&z=3&v=70.85507903437916,18.838635004582,93.86484465937916,30.281994379582). Same time radiosonde data from Wyoming upper soundings at Lucknow station (, it showing closer ambient and dew point temperature at surface. Skew-T diagram showing almost 17-18 degree Celsius difference in temperature and dew point temperature at 750-800 hpa level. So could someone tell how to interpret these differences between satellite data and radiosonde observation? Thanks in advance

  2. Thank you for your question.

    Here are a few different answers to your question. We hope this helps you out. If you have additional questions please contact us further as we would be happy to help you out. 

    Thank you.

    1) how to determine fog from stratus in the imagery?

    In the daytime, it is difficult to determine the difference between fog and stratus in static images. In a succession of geostationary images one can distinguish fog because it dissipates from the outside inwards. At night, one can look at the 11.1 - 3.75 micron brightness temperature difference. Fog is often indicated by a positive difference (up to +5K) while most stratus clouds over land yield a negative difference.

    2) how to reconcile skew-t and CTP data?

    This is difficult because the satellite algorithm measures brightness temperatures (radiative quantities) which can be significantly different than the ambient temperatures from a radiosonde. Many times over stratus and fog, CTPs from the MODIS algorithm are biased too high. The uncertainty grows with thinner clouds and lower cloud altitudes as radiative properties of the surface begin to play a role. Also, the algorithm uses gridded NWP model output for surface and air temperatures which can differ in time from both the satellite overpass and the radiosonde.

  3. MISMATCH in catalogue of V10M with information given in the description file

    Dear collegues,

    From MERRA IAU 2d atmospheric single-level diagnostics tavg1_2d_slv_Nx in catalogue v10m description is Northward wind at 10 m above the displacement height, however in file description it is is Northward wind at 50 m above the displacement height. Could You say what is true? Is it possible to download v10m with no doubt?
    In attached, real example of what is inside the v10m.

    Your's Sincerely,
    Vitali Sharmar 

    1. Hello Vitali Sharmar,

      Thank you for bringing this to our attention. The discrepancy is in the long name in the data file for the metadata of the variable, however you will be downloading the Northward wind at 10m. I would like to strongly suggest that you use the MERRA-2 data set that is comparable to the MERRA data set. We offer the same subsetting capabilities for MERRA-2 (, you can also use the following URL to access the product directly in the dropdown:

      Here is the product Data Set Landing page where you can get more information about the product:

      Please let me know if you have further questions on the access or data. You can email me at

      Look forward to hearing from you,

      Dana Ostrenga

  4. Hello Dana Ostrenga,

    Thank You for helping me. 

    Vitali Sharmar

  5. Corrupted .hdf file in AIRS3STM?

    I'm getting errors when trying to load a data file I downloaded from the AIRS3STM dataset. Specifically, this file:

    I'm using pandas in Python, and this is the error I get:

    df = pd.read_hdf("AIRS.2017.01.01.L3.RetStd_IR031.v6.0.31.0.G17038155115.hdf")
    OSError: HDF5 error back trace
      File "C:\ci\hdf5_1525883595717\work\src\H5F.c", line 511, in H5Fopen
        unable to open file
      File "C:\ci\hdf5_1525883595717\work\src\H5Fint.c", line 1604, in H5F_open
        unable to read superblock
      File "C:\ci\hdf5_1525883595717\work\src\H5Fsuper.c", line 413, in H5F__super_read
        file signature not found
    End of HDF5 error back trace
    Unable to open/create file 'AIRS.2017.01.01.L3.RetStd_IR031.v6.0.31.0.G17038155115.hdf'

    A similar error occurs when I use the aug_eos5 tool:

    $./aug_eos5 AIRS.2017.01.01.L3.RetStd_IR031.v6.0.31.0.G17038155115.hdf
    HDF5-DIAG: Error detected in HDF5 (1.8.19) thread 0:
    #000: EHapi.c line 1493 in HE5_EHopen(): File "AIRS.2017.01.01.L3.RetStd_IR031.v6.0.31.0.G17038155115.hdf" cannot be opened.
    major: File accessibilty
    minor: Unable to open file
    #001: H5F.c line 602 in H5Fopen(): unable to open file
    major: File accessibilty
    minor: Unable to open file
    #002: H5Fint.c line 1085 in H5F_open(): unable to read superblock
    major: File accessibilty
    minor: Read failed
    #003: H5Fsuper.c line 275 in H5F_super_read(): file signature not found
    major: File accessibilty
    minor: Not an HDF5 file
    HDF5-DIAG: Error detected in HDF5 (1.8.19) thread 0:
    #000: GDapi.c line 238 in HE5_GDopen(): Cannot open the file "AIRS.2017.01.01.L3.RetStd_IR031.v6.0.31.0.G17038155115.hdf". Check the file name.
    major: File accessibilty
    minor: Unable to open file
    error(1) at read.c:995 | HE5_GDopen
    error(1) at run.c:2658 | open hdfeos5 file

    Any thoughts?