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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 (https://worldview.earthdata.nasa.gov/?p=geographic&l=VIIRS_SNPP_CorrectedReflectance_TrueColor(hidden),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 (http://weather.uwyo.edu/cgi-bin/sounding?region=seasia&TYPE=PDF%3ASKEWT&YEAR=2017&MONTH=12&FROM=3100&TO=0100&STNM=42369), 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.