Can anyone tell me what the 4 horizontal lines represent in the forecast data. I can understand clouds, sunny, partially sunny but not sure what the one means. I'm assuming fog? But I'd like to know for sure. Thanks!

Can anyone tell me what the 4 horizontal lines represent in the forecast data. I can understand clouds, sunny, partially sunny but not sure what the one means. I'm assuming fog? But I'd like to know for sure. Thanks!