Based on a current fog detection theory,a multiband threshold method for MODIS data was put forward to detect daytime fog in the South China Sea.It used Bands 1,2,18,20 and 31 of MODIS data to separate fog from the cloud and the sea surface.The digital detection indexes were as follows.If RB1<20%,RB2<20% and RB1>RB2,the pixel was identified to be the sea surface.If RB1>55%,RB2>55% and TB31<273 K,the pixel was identified to be a middle-and high-level cloud.If IFC>20,the pixel was classified to be sea fog.The method was verified with sea fog data observed from the coastal region of Guangdong during January-April 2011.Out of the 13 samples of satellite detection,nine were consistent with the surface observations,three were identified to be low-level the cloud according to the satellite detection but fog according to the surface observations,and only one sample was identified to be the ocean surface by the satellite detection but fog by the surface observations.Because the MODIS data cannot penetrate the cloud or fog,the model was designed for a single field of view which had only one layer of cloud or fog.It can accurately distinguish fog which is not covered by the cloud,but it identifies fog as cloud if the former is covered by a cloud.Generally speaking,the model is effective and feasible.
